Keywords
Computer Science and Digital Science
 A1.1.11. Quantum architectures
 A4.2. Correcting codes
 A6. Modeling, simulation and control
 A6.1. Methods in mathematical modeling
 A6.1.1. Continuous Modeling (PDE, ODE)
 A6.1.2. Stochastic Modeling
 A6.1.3. Discrete Modeling (multiagent, people centered)
 A6.1.4. Multiscale modeling
 A6.2. Scientific computing, Numerical Analysis & Optimization
 A6.2.1. Numerical analysis of PDE and ODE
 A6.2.3. Probabilistic methods
 A6.2.6. Optimization
 A6.3.1. Inverse problems
 A6.3.2. Data assimilation
 A6.3.3. Data processing
 A6.3.4. Model reduction
 A6.4. Automatic control
 A6.4.1. Deterministic control
 A6.4.2. Stochastic control
 A6.4.3. Observability and Controlability
 A6.4.4. Stability and Stabilization
Other Research Topics and Application Domains
 B5.3. Nanotechnology
 B5.4. Microelectronics
 B6.5. Information systems
 B9.10. Privacy
1 Team members, visitors, external collaborators
Research Scientists
 Mazyar Mirrahimi [Team leader, Inria, Senior Researcher]
 Philippe Campagne Ibarcq [Inria, Researcher]
 Alain Sarlette [Inria, Researcher]
Faculty Members
 Zaki Leghtas [École Nationale Supérieure des Mines de Paris , Associate Professor]
 Pierre Rouchon [École Nationale Supérieure des Mines de Paris , Professor, HDR]
PostDoctoral Fellows
 Cyril Elouard [Inria, from Oct 2020]
 William Smith [Inria]
PhD Students
 Michiel Burgelman [Inria]
 Ronan Gautier [Inria, from Mar 2020]
 Jeremie Guillaud [Inria, until Aug 2020]
 Francois Marie Le Regent [Alice et Bob, from Sep 2020]
 Vincent Martin [Inria]
 Lev Arcady Sellem [Ecole normale supérieure ParisSaclay, from Sep 2020]
 Christian Siegele [Inria]
 Aron Vanselow [Inria, from Sep 2020]
Interns and Apprentices
 Francois Marie Le Regent [Inria, from Apr 2020 until Jun 2020]
 Adrien Rospars [Universié de Lyon, from Jun 2020 until Jul 2020]
 Lev Arcady Sellem [Ecole normale supérieure ParisSaclay, from Apr 2020 until Aug 2020]
 Donatas Simeliunas [École polytechnique, until Mar 2020]
Administrative Assistant
 Derya Gök [Inria]
External Collaborator
 Jeremie Guillaud [Alice et Bob, from Sep 2020]
2 Overall objectives
2.1 Overall objectives
The research activities of QUANTIC team lie at the border between theoretical and experimental efforts in the emerging field of quantum systems engineering. Our research topics are in direct continuation of a historic research theme of INRIA, classical automatic control, while opening completely new perspectives toward quantum control: by developing a new mathematical system theory for quantum circuits, we will realize the components of a future quantum information processing unit.
One of the unique features of our team concerns the large spectrum of our subjects going from the mathematical analysis of the physical systems (development of systematic mathematical methods for control and estimation of quantum systems), and the numerical analysis of the proposed solutions, to the experimental implementation of the quantum circuits based on these solutions. This is made possible by the constant and profound interaction between the applied mathematicians and the physicists in the group. Indeed, this close collaboration has already brought a significant acceleration in our research efforts. In a long run, this synergy should lead to a deeper understanding of the physical phenomena behind these emerging technologies and the development of new research directions within the field of quantum information processing.
Towards this ultimate task of practical quantum digital systems, the approach of the QUANTIC team is complementary to the one taken by teams with expertise in quantum algorithms. Indeed, we start from the specific controls that can be realistically applied on physical systems, to propose designs which combine them into hardware shortcuts implementing robust behaviors useful for quantum information processing. Whenever a significant new element of quantum engineering architecture is developed, the initial motivation is to prove an enabling technology with major impact for the groups working one abstraction layer higher: on quantum algorithms but also on e.g. secure communication and metrology applications.
3 Research program
3.1 Hardwareefficient quantum information processing
In this scientific program, we will explore various theoretical and experimental issues concerning protection and manipulation of quantum information. Indeed, the next, critical stage in the development of Quantum Information Processing (QIP) is most certainly the active quantum error correction (QEC). Through this stage one designs, possibly using many physical qubits, an encoded logical qubit which is protected against major decoherence channels and hence admits a significantly longer effective coherence time than a physical qubit. Reliable (faulttolerant) computation with protected logical qubits usually comes at the expense of a significant overhead in the hardware (up to thousands of physical qubits per logical qubit). Each of the involved physical qubits still needs to satisfy the best achievable properties (coherence times, coupling strengths and tunability). More remarkably, one needs to avoid undesired interactions between various subsystems. This is going to be a major difficulty for qubits on a single chip.
The usual approach for the realization of QEC is to use many qubits to obtain a larger Hilbert space of the qubit register 87, 90. By redundantly encoding quantum information in this Hilbert space of larger dimension one make the QEC tractable: different error channels lead to distinguishable error syndromes. There are two major drawbacks in using multiqubit registers. The first, fundamental, drawback is that with each added physical qubit, several new decoherence channels are added. Because of the exponential increase of the Hilbert's space dimension versus the linear increase in the number of decay channels, using enough qubits, one is able to eventually protect quantum information against decoherence. However, multiplying the number of possible errors, this requires measuring more error syndromes. Note furthermore that, in general, some of these new decoherence channels can lead to correlated action on many qubits and this needs to be taken into account with extra care: in particular, such kind of nonlocal error channels are problematic for surface codes. The second, more practical, drawback is that it is still extremely challenging to build a register of more than on the order of 10 qubits where each of the qubits is required to satisfy near the best achieved properties: these properties include the coherence time, the coupling strengths and the tunability. Indeed, building such a register is not merely only a fabrication task but rather, one requirers to look for architectures such that, each individual qubit can be addressed and controlled independently from the others. One is also required to make sure that all the noise channels are wellcontrolled and uncorrelated for the QEC to be effective.
We have recently introduced a new paradigm for encoding and protecting quantum information in a quantum harmonic oscillator (e.g. a highQ mode of a 3D superconducting cavity) instead of a multiqubit register 64. The infinite dimensional Hilbert space of such a system can be used to redundantly encode quantum information. The power of this idea lies in the fact that the dominant decoherence channel in a cavity is photon damping, and no more decay channels are added if we increase the number of photons we insert in the cavity. Hence, only a single error syndrome needs to be measured to identify if an error has occurred or not. Indeed, we are convinced that most early proposals on continuous variable QIP 61, 55 could be revisited taking into account the design flexibilities of Quantum Superconducting Circuits (QSC) and the new coupling regimes that are provided by these systems. In particular, we have illustrated that coupling a qubit to the cavity mode in the strong dispersive regime provides an important controllability over the Hilbert space of the cavity mode 63. Through a recent experimental work 95, we benefit from this controllability to prepare superpositions of quasiorthogonal coherent states, also known as Schrödinger cat states.
In this Scheme, the logical qubit is encoded in a fourcomponent Schrödinger cat state. Continuous quantum nondemolition (QND) monitoring of a single physical observable, consisting of photon number parity, enables then the tractability of single photon jumps. We obtain therefore a firstorder quantum error correcting code using only a single highQ cavity mode (for the storage of quantum information), a single qubit (providing the nonlinearity needed for controllability) and a single lowQ cavity mode (for reading out the error syndrome). An earlier experiment on such QND photonnumber parity measurements 91 has recently led to a first experimental realization of a full quantum error correcting code improving the coherence time of quantum information 8. As shown in Figure 1, this leads to a significant hardware economy for realization of a protected logical qubit. Our goal here is to push these ideas towards a reliable and hardwareefficient paradigm for universal quantum computation.
3.2 Reservoir (dissipation) engineering and autonomous stabilization of quantum systems
Being at the heart of any QEC protocol, the concept of feedback is central for the protection of quantum information, enabling manyqubit quantum computation or longdistance quantum communication. However, such a closedloop control which requires a realtime and continuous measurement of the quantum system has been for long considered as counterintuitive or even impossible. This thought was mainly caused by properties of quantum measurements: any measurement implies an instantaneous strong perturbation to the system's state. The concept of quantum nondemolition (QND) measurement has played a crucial role in understanding and resolving this difficulty 37. In the context of cavity quantum electrodynamics (cavity QED) with Rydberg atoms 57, a first experiment on continuous QND measurements of the number of microwave photons was performed by the group at Laboratoire KastlerBrossel (ENS) 56. Later on, this ability of performing continuous measurements allowed the same group to realize the first continuous quantum feedback protocol stabilizing highly nonclassical states of the microwave field in the cavity, the socalled photon number states 10 (this groundbreaking work was mentioned in the Nobel prize attributed to Serge Haroche). The QUANTIC team contributed to the theoretical work behind this experiment 47, 27, 89, 29. These contributions include the development and optimization of the quantum filters taking into account the quantum measurement backaction and various measurement noises and uncertainties, the development of a feedback law based on control Lyapunov techniques, and the compensation of the feedback delay.
In the context of circuit quantum electrodynamics (circuit QED) 45, recent advances in quantumlimited amplifiers 80, 93 have opened doors to highfidelity nondemolition measurements and realtime feedback for superconducting qubits 58. This ability to perform highfidelity nondemolition measurements of a quantum signal has very recently led to quantum feedback experiments with quantum superconducting circuits 93, 79, 39. Here again, the QUANTIC team has participated to one of the first experiments in the field where the control objective is to track a dynamical trajectory of a single qubit rather than stabilizing a stationary state. Such quantum trajectory tracking could be further explored to achieve metrological goals such as the stabilization of the amplitude of a microwave drive 71.
While all this progress has led to a strong optimism about the possibility to perform active protection of quantum information against decoherence, the rather short dynamical time scales of these systems limit, to a great amount, the complexity of the feedback strategies that could be employed. Indeed, in such measurementbased feedback protocols, the timeconsuming data acquisition and posttreatment of the output signal leads to an important latency in the feedback procedure.
The reservoir (dissipation) engineering 77 and the closely related coherent feedback 69 are considered as alternative approaches circumventing the necessity of a realtime data acquisition, signal processing and feedback calculations. In the context of quantum information, the decoherence, caused by the coupling of a system to uncontrolled external degrees of freedom, is generally considered as the main obstacle to synthesize quantum states and to observe quantum effects. Paradoxically, it is possible to intentionally engineer a particular coupling to a reservoir in the aim of maintaining the coherence of some particular quantum states. In a general viewpoint, these approaches could be understood in the following manner: by coupling the quantum system to be stabilized to a strongly dissipative ancillary quantum system, one evacuates the entropy of the main system through the dissipation of the ancillary one. By building the feedback loop into the Hamiltonian, this type of autonomous feedback obviates the need for a complicated external control loop to correct errors. On the experimental side, such autonomous feedback techniques have been used for qubit reset 54, singlequbit state stabilization 72, and the creation 32 and stabilization 62, 68, 86 of states of multipartite quantum systems.
Such reservoir engineering techniques could be widely revisited exploring the flexibility in the Hamiltonian design for QSC. We have recently developed theoretical proposals leading to extremely efficient, and simple to implement, stabilization schemes for systems consisting of a single, two or three qubits 54, 66, 43, 46. The experimental results based on these protocols have illustrated the efficiency of the approach 54, 86. Through these experiments, we exploit the strong dispersive interaction 84 between superconducting qubits and a single lowQ cavity mode playing the role of a dissipative reservoir. Applying continuouswave (cw) microwave drives with wellchosen fixed frequencies, amplitudes, and phases, we engineer an effective interaction Hamiltonian which evacuates the entropy of the system interacting with a noisy environment: by driving the qubits and cavity with continuouswave drives, we induce an autonomous feedback loop which corrects the state of the qubits every time it decays out of the desired target state. The schemes are robust against small variations of the control parameters (drives amplitudes and phase) and require only some basic calibration. Finally, by avoiding resonant interactions between the qubits and the lowQ cavity mode, the qubits remain protected against the Purcell effect, which would reduce the coherence times. We have also investigated both theoretically and experimentally the autonomous stabilization of nonclassical states (such as Schrodinger cat states and Fock states) of microwave field confined in a highQ cavity mode 82, 597, 5.
3.3 System theory for quantum information processing
In parallel and in strong interactions with the above experimental goals, we develop systematic mathematical methods for dynamical analysis, control and estimation of composite and open quantum systems. These systems are built with several quantum subsystems whose irreversible dynamics results from measurements and/or decoherence. A special attention is given to spin/spring systems made with qubits and harmonic oscillators. These developments are done in the spirit of our recent contributions 81, 27, 88, 83, 89, 299 resulting from collaborations with the cavity quantum electrodynamics group of Laboratoire Kastler Brossel.
3.4 Stabilization by measurementbased feedback
The protection of quantum information via efficient QEC is a combination of (i) tailored dynamics of a quantum system in order to protect an informational qubit from certain decoherence channels, and (ii) controlled reaction to measurements that efficiently detect and correct the dominating disturbances that are not rejected by the tailored quantum dynamics.
In such feedback scheme, the system and its measurement are quantum objects whereas the controller and the control input are classical. The stabilizing control law is based on the past values of the measurement outcomes. During our work on the LKB photon box, we have developed, for single input systems subject to quantum nondemolition measurement, a systematic stabilization method 29: it is based on a discretetime formulation of the dynamics, on the construction of a strict control Lyapunov function and on an explicit compensation of the feedbackloop delay. Keeping the QND measurement assumptions, extensions of such stabilization schemes will be investigated in the following directions: finite set of values for the control input with application to the convergence analysis of the atomic feedback scheme experimentally tested in 96; multiinput case where the construction by inversion of a Metzler matrix of the strict Lyapunov function is not straightforward; continuoustime systems governed by diffusive master equations; stabilization towards a set of density operators included in a target subspace; adaptive measurement by feedback to accelerate the convergence towards a stationary state as experimentally tested in 75. Without the QND measurement assumptions, we will also address the stabilization of nonstationary states and trajectory tracking, with applications to systems similar to those considered in 58, 39.
3.5 Filtering, quantum state and parameter estimations
The performance of every feedback controller crucially depends on its online estimation of the current situation. This becomes even more important for quantum systems, where full state measurements are physically impossible. Therefore the ultimate performance of feedback correction depends on fast, efficient and optimally accurate state and parameter estimations.
A quantum filter takes into account imperfection and decoherence and provides the quantum state at time $t\ge 0$ from an initial value at $t=0$ and the measurement outcomes between 0 and $t$. Quantum filtering goes back to the work of Belavkin 33 and is related to quantum trajectories 40, 44. A modern and mathematical exposure of the diffusive models is given in 31. In 97 a first convergence analysis of diffusive filters is proposed. Nevertheless the convergence characterization and estimation of convergence rate remain open and difficult problems. For discrete time filters, a general stability result based on fidelity is proven in 81, 88. This stability result is extended to a large class of continuoustime filters in 28. Further efforts are required to characterize asymptotic and exponential stability. Estimations of convergence rates are available only for quantum nondemolition measurements 34. Parameter estimations based on measurement data of quantum trajectories can be formulated within such quantum filtering framework 49, 73.
We will continue to investigate stability and convergence of quantum filtering. We will also exploit our fidelitybased stability result to justify maximum likelihood estimation and to propose, for open quantum system, parameter estimation algorithms inspired of existing estimation algorithms for classical systems. We will also investigate a more specific quantum approach: it is noticed in 38 that postselection statistics and “past quantum” state analysis 50 enhance sensitivity to parameters and could be interesting towards increasing the precision of an estimation.
3.6 Stabilization by interconnections
In such stabilization schemes, the controller is also a quantum object: it is coupled to the system of interest and is subject to decoherence and thus admits an irreversible evolution. These stabilization schemes are closely related to reservoir engineering and coherent feedback 77, 69. The closedloop system is then a composite system built with the original system and its controller. In fact, and given our particular recent expertise in this domain 986, 54, this subsection is dedicated to further developing such stabilization techniques, both experimentally and theoretically.
The main analysis issues are to prove the closedloop convergence and to estimate the convergence rates. Since these systems are governed by Lindblad differential equations (continuoustime case) or Kraus maps (discretetime case), their stability is automatically guaranteed: such dynamics are contractions for a large set of metrics (see 76). Convergence and asymptotic stability is less well understood. In particular most of the convergence results consider the case where the target steadystate is a density operator of maximum rank (see, e.g., 30[chapter 4, section 6]). When the goal steadystate is not full rank very few convergence results are available.
We will focus on this geometric situation where the goal steadystate is on the boundary of the cone of positive Hermitian operators of finite trace. A specific attention will be given to adapt standard tools (Lyapunov function, passivity, contraction and Lasalle's invariance principle) for infinite dimensional systems to spin/spring structures inspired of 9, 786, 54 and their associated FokkerPlanck equations for the Wigner functions.
We will also explore the Heisenberg point of view in connection with recent results of the INRIA projectteam MAXPLUS (algorithms and applications of algebras of maxplus type) relative to PerronFrobenius theory 53, 52. We will start with 85 and 78 where, based on a theorem due to Birkhoff 35, dual Lindblad equations and dual Kraus maps governing the Heisenberg evolution of any operator are shown to be contractions on the cone of Hermitian operators equipped with Hilbert's projective metric. As the Heisenberg picture is characterized by convergence of all operators to a multiple of the identity, it might provide a mean to circumvent the rank issues. We hope that such contraction tools will be especially well adapted to analyzing quantum systems composed of multiple components, motivated by the facts that the same geometry describes the contraction of classical systems undergoing synchronizing interactions 92 and by our recent generalized extension of the latter synchronizing interactions to quantum systems 70.
Besides these analysis tasks, the major challenge in stabilization by interconnections is to provide systematic methods for the design, from typical building blocks, of control systems that stabilize a specific quantum goal (state, set of states, operation) when coupled to the target system. While constructions exist for socalled linear quantum systems 74, this does not cover the states that are more interesting for quantum applications. Various strategies have been proposed that concatenate iterative control steps for openloop steering 94, 67 with experimental limitations. The characterization of Kraus maps to stabilize any types of states has also been established 36, but without considering experimental implementations. A viable stabilization by interaction has to combine the capabilities of these various approaches, and this is a missing piece that we want to address.
3.6.1 Perturbation methods
With this subsection we turn towards more fundamental developments that are necessary in order to address the complexity of quantum networks with efficient reduction techniques. This should yield both efficient mathematical methods, as well as insights towards unravelling dominant physical phenomena/mechanisms in multipartite quantum dynamical systems.
In the Schrödinger point of view, the dynamics of open quantum systems are governed by master equations, either deterministic or stochastic 57, 51. Dynamical models of composite systems are based on tensor products of Hilbert spaces and operators attached to the constitutive subsystems. Generally, a hierarchy of different timescales is present. Perturbation techniques can be very useful to construct reliable models adapted to the timescale of interest.
To eliminate high frequency oscillations possibly induced by quasiresonant classical drives, averaging techniques are used (rotating wave approximation). These techniques are well established for closed systems without any dissipation nor irreversible effect due to measurement or decoherence. We will consider in a first step the adaptation of these averaging techniques to deterministic Lindblad master equations governing the quantum state, i.e. the system density operator. Emphasis will be put on first order and higher order corrections based on noncommutative computations with the different operators appearing in the Lindblad equations. Higher order terms could be of some interest for the protected logical qubit of figure 1b. In future steps, we intend to explore the possibility to explicitly exploit averaging or singular perturbation properties in the design of coherent quantum feedback systems; this should be an opensystems counterpart of works like 65.
To eliminate subsystems subject to fast convergence induced by decoherence, singular perturbation techniques can be used. They provide reduced models of smaller dimension via the adiabatic elimination of the rapidly converging subsystems. The derivation of the slow dynamics is far from being obvious (see, e.g., the computations of page 142 in 41 for the adiabatic elimination of lowQ cavity). Conversely to the classical composite systems where we have to eliminate one component in a Cartesian product, we here have to eliminate one component in a tensor product. We will adapt geometric singular perturbations 48 and invariant manifold techniques 42 to such tensor product computations to derive reduced slow approximations of any order. Such adaptations will be very useful in the context of quantum Zeno dynamics to obtain approximations of the slow dynamics on the decoherencefree subspace corresponding to the slow attractive manifold.
Perturbation methods are also precious to analyze convergence rates. Deriving the spectrum attached to the Lindblad differential equation is not obvious. We will focus on the situation where the decoherence terms of the form $L\rho {L}^{\u2020}({L}^{\u2020}L\rho +\rho {L}^{\u2020}L)/2$ are small compared to the conservative terms $i[H/\hslash ,\rho ]$. The difficulty to overcome here is the degeneracy of the unperturbed spectrum attached to the conservative evolution $\frac{d}{dt}\rho =i[H/\hslash ,\rho ]$. The degree of degeneracy of the zero eigenvalue always exceeds the dimension of the Hilbert space. Adaptations of usual perturbation techniques 60 will be investigated. They will provide estimates of convergence rates for slightly open quantum systems. We expect that such estimates will help to understand the dependence on the experimental parameters of the convergence rates observed in 54, 86, 66.
As particular outcomes for the other subsections, we expect that these developments towards simpler dominant dynamics will guide the search for optimal control strategies, both in openloop microwave networks and in autonomous stabilization schemes such as reservoir engineering. It will further help to efficiently compute explicit convergence rates and quantitative performances for all the intended experiments.
4 Application domains
4.1 Quantum engineering
A new field of quantum systems engineering has emerged during the last few decades. This field englobes a wide range of applications including nanoelectromechanical devices, nuclear magnetic resonance applications, quantum chemical synthesis, high resolution measurement devices and finally quantum information processing devices for implementing quantum computation and quantum communication. Recent theoretical and experimental achievements have shown that the quantum dynamics can be studied within the framework of estimation and control theory, but give rise to new models that have not been fully explored yet.
The QUANTIC team's activities are defined at the border between theoretical and experimental efforts of this emerging field with an emphasis on the applications in quantum information, computation and communication. The main objective of this interdisciplinary team is to develop quantum devices ensuring a robust processing of quantum information.
On the theory side, this is done by following a system theory approach: we develop estimation and control tools adapted to particular features of quantum systems. The most important features, requiring the development of new engineering methods, are related to the concept of measurement and feedback for composite quantum systems. The destructive and partial 1 nature of measurements for quantum systems lead to major difficulties in extending classical control theory tools. Indeed, design of appropriate measurement protocols and, in the sequel, the corresponding quantum filters estimating the state of the system from the partial measurement record, are themselves building blocks of the quantum system theory to be developed.
On the experimental side, we develop new quantum information processing devices based on quantum superconducting circuits. Indeed, by realizing superconducting circuits at low temperatures and using microwave measurement techniques, the macroscopic and collective degrees of freedom such as the voltage and the current are forced to behave according to the laws of quantum mechanics. Our quantum devices are aimed to protect and process quantum information through these integrated circuits.
5 Highlights of the year
 Pierre Rouchon has obtained an ERC advanced grant in pannel PE7 entitled QFeedback (Quantum feedback Engineering).
 Philippe CampagneIbarcq has obtained an ANR JCJC grant entitile SYNCAMIL “Synthetic NonLocal Hamiltonians for the protection of quantum information”.
 Philippe CampagneIbarcq has obtained a regional DIM SIRTEQ grant for experimental equipment.
 Successful PhD defense of Raphaël Lescanne under the supervision of Zaki Leghtas and Takis Kontos.
 Successful HDR defense of Zaki Leghtas and Alain Sarlette.
 Creation of startup Alice and Bob. Philippe CampagneIbarcq, Zaki Leghtas and Mazyar Mirrahimi are members of the scientific board.
 Mazyar Mirrahimi was a plenary speaker at the European Control Conference.
6 New results
6.1 Exponential suppression of bitflips in a qubit encoded in an oscillator
Participants: R. Lescanne, M. Villiers, A. Sarlette, M. Mirrahimi, Z. Leghtas.
A quantum system interacts with its environment—if ever so slightly—no matter how much care is put into isolating it. Therefore, quantum bits undergo errors, putting dauntingly difficult constraints on the hardware suitable for quantum computation. New strategies are emerging to circumvent this problem by encoding a quantum bit nonlocally across the phase space of a physical system. Because most sources of decoherence result from local fluctuations, the foundational promise is to exponentially suppress errors by increasing a measure of this nonlocality. Prominent examples are topological quantum bits, which delocalize information over real space and where spatial extent measures nonlocality. Here, we encode a quantum bit in the field quadrature space of a superconducting resonator endowed with a special mechanism that dissipates photons in pairs. This process pins down two computational states to separate locations in phase space. By increasing this separation, we measure an exponential decrease of the bitflip rate while only linearly increasing the phaseflip rate. Because bitflips are autonomously corrected, only phaseflips remain to be corrected via a onedimensional quantum error correction code. This exponential scaling demonstrates that resonators with nonlinear dissipation are promising building blocks for quantum computation with drastically reduced hardware overhead. This experimental work was published in 16.
6.2 Irreversible QubitPhoton Coupling for the Detection of Itinerant Microwave Photons
Participants: R. Lescanne, Z. Leghtas
Single photon detection is a key resource for sensing at the quantum limit and the enabling technology for measurementbased quantum computing. Photon detection at optical frequencies relies on irreversible photoassisted ionization of various natural materials. However, microwave photons have energies 5 orders of magnitude lower than optical photons, and are therefore ineffective at triggering measurable phenomena at macroscopic scales. Here, we report the observation of a new type of interaction between a single twolevel system (qubit) and a microwave resonator. These two quantum systems do not interact coherently; instead, they share a common dissipative mechanism to a cold bath: the qubit irreversibly switches to its excited state if and only if a photon enters the resonator. We have used this highly correlated dissipation mechanism to detect itinerant photons impinging on the resonator. This scheme does not require any prior knowledge of the photon waveform nor its arrival time, and dominant decoherence mechanisms do not trigger spurious detection events (dark counts). We demonstrate a detection efficiency of $58\%$ and a record low dark count rate of 1.4 per millisecond. This work establishes engineered nonlinear dissipation as a key enabling resource for a new class of lownoise nonlinear microwave detectors. This experimental work was published in 15.
6.3 Theory of interactions between cavity photons induced by a mesoscopic circuit
Participants: Z. Leghtas
We use a quantum pathintegral approach to describe the behavior of a microwave cavity coupled to a dissipative mesoscopic circuit. We integrate out the mesoscopic electronic degrees of freedom to obtain a cavity effective action at fourth order in the light/matter coupling. By studying the structure of this action, we establish sufficient conditions in which the cavity dynamics can be described with a Lindblad equation. This equation depends on effective parameters set by electronic correlation functions. It reveals that the mesoscopic circuit induces an effective Kerr interaction and twophoton dissipative processes. We use our method to study the effective dynamics of a cavity coupled to a double quantum dot with normal metal reservoirs. If the cavity is driven at twice its frequency, the doubledot circuit generates photonic squeezing and nonclassicalities visible in the cavity Wigner function. In particular, we find a counterintuitive situation where mesoscopic dissipation enables the production of photonic Schrödinger cats. These effects can occur for realistic circuit parameters. Our method can be generalized straightforwardly to more complex circuit geometries with, for instance, multiple quantum dots and other types of fermionic reservoirs such as superconductors and ferromagnets. This theoretical work was published in 12.
6.4 Quantum error correction of a qubit encoded in grid states of an oscillator
Participants: Philippe CampagneIbarcq and Mazyar Mirrahimi
Quantum Error Correction (QEC) derives from the reasonable assumption that noise is local, that is, it does not act in a coordinated way on different parts of the physical system. Therefore, if a logical qubit is encoded nonlocally, we can detect and correct noiseinduced evolution before it corrupts the encoded information . In 2001, Gottesman, Kitaev and Preskill (GKP) proposed a hardwareefficient instance of such a nonlocal qubit: a superposition of position eigenstates that forms grid states of a single oscillator.
In this work in collaboration with the group of Michel Devoret at Yale university, we experimentally prepare square and hexagonal GKP code states through a feedback protocol that incorporates nondestructive measurements that are implemented with a superconducting microwave cavity having the role of the oscillator. We demonstrate QEC of an encoded qubit with suppression of all logical errors, in quantitative agreement with a theoretical estimate based on the measured imperfections of the experiment. This protocol is applicable to other continuousvariable systems and, in contrast to previous implementations of QEC , can mitigate all logical errors generated by a wide variety of noise processes and facilitate faulttolerant quantum computation.
This work was published in 11.
6.5 Scaling up reservoir engineering for errorcorrecting codes
Participants: Alain Sarlette and Vincent Martin
Errorcorrecting codes are usually envisioned to counter errors by operating unitary corrections depending on the projective measurement results of some syndrome observables. We here propose a way to use them in a more integrated way, where the error correction is applied continuously and autonomously by an engineered environment. We focus on a proposal for the repetition code that counters bitflip errors, and how to scale up the network encoding a logical quantum bit, towards stronger information protection. This proposal is motivated by the catqubits, which are strongly protected against one type of error and only need a repetitioncode type protection against the other type. The challenge has been to devise a network architecture which allows to autonomously correct higherorder errors, while remaining realistic towards experimental realization by avoiding alltoall or alltoone coupling. This work 22 submitted to MTNS 2020 (pandemic postponed as MTNS 2021) was proposing a first architecture and analysis tools.
6.6 Numberresolved photocounter for propagating microwave mode
Participants: Pierre Rouchon
Detectors of propagating microwave photons have recently been realized using superconducting circuits. However a numberresolved photocounter is still missing. In this work, we demonstrate a singleshot counter for propagating microwave photons that can resolve up to 3 photons. It is based on a pumped Josephson Ring Modulator that can catch an arbitrary propagating mode by frequency conversion and store its quantum state in a stationary memory mode. A transmon qubit then counts the number of photons in the memory mode using a series of binary questions. Using measurement based feedback, the number of questions is minimal and scales logarithmically with the maximal number of photons. The detector features a detection efficiency of $0.96\pm 0.04$, and a dark count probability of $0.030\pm 0.002$ for an average dead time of $4.5\mu $s. To maximize its performance, the device is first used as an in situ waveform detector from which an optimal pump is computed and applied. Depending on the number of incoming photons, the detector succeeds with a probability that ranges from $(54\pm 2)\%$ to $99\%$. This experimental result in collaboration with the team of Benjamin Huard at ENS Lyon was published in 13.
6.7 Stabilization and operation of a Kerrcat qubit
Participants: Mazyar Mirrahimi
Quantum superpositions of macroscopically distinct classical states, socalled Schrödinger cat states, are a resource for quantum metrology, quantum communication, and quantum computation. In particular, the superpositions of two oppositephase coherent states in an oscillator encode a qubit protected against phaseflip errors. However, several challenges have to be overcome in order for this concept to become a practical way to encode and manipulate errorprotected quantum information. The protection must be maintained by stabilizing these highly excited states and, at the same time, the system has to be compatible with fast gates on the encoded qubit and a quantum nondemolition readout of the encoded information. Here, we experimentally demonstrate a novel method for the generation and stabilization of Schrödinger cat states based on the interplay between Kerr nonlinearity and singlemode squeezing in a superconducting microwave resonator. We show an increase in transverse relaxation time of the stabilized, errorprotected qubit over the singlephoton Fockstate encoding by more than one order of magnitude. We perform all singlequbit gate operations on timescales more than sixty times faster than the shortest coherence time and demonstrate singleshot readout of the protected qubit under stabilization. Our results showcase the combination of fast quantum control with the robustness against errors intrinsic to stabilized macroscopic states and open up the possibility of using these states as resources in quantum information processing. This experiment performed by the group of Michel Devoret at Yale university was published in 14.
7 Bilateral contracts and grants with industry
7.1 Bilateral contracts with industry
 New contracts with the startup Alice and Bob, funding the PhD thesis of FrançoisMarie Le Regent and Vincent Martin.
8 Partnerships and cooperations
8.1 International initiatives
8.1.1 Inria International Labs
TAQUILLA
 Title: Tailored quantum information protocols for quantum superconducting circuits
 Duration: 2016 2020
 Coordinator: Mazyar Mirrahimi

Partners:
 Department of Applied Physics, Yale University (United States)
 Inria contact: Mazyar Mirrahimi
 Summary: In this collaboration, we were interested to establish an alternative approach to quantum error correction (QEC) for superconducting qubits. This approach, developed through the InriaYale collaboration, is known under the name of cat codes, because it employs multiple interconnected high coherence cavity modes with nonlinear dissipation to encode a qubit in superpositions of Schrödinger cat states. We aimed to implement error protected qubits, fault tolerant operations, and demonstrate the scalability of this approach.
8.1.2 Participation in other international programs
 DARPA: Alain Sarlette is international key personnel on the DARPA project “The Quantum Computing Revolution and Optimization: Challenges and Opportunities” led by optimization researchers at Lehigh Universty. This project of about 2M dollars can fund some exchanges during the coming years.
 YaleARO subaward: In the framework of the collaborations with Yale university, Quantic team has received a subaward from Yale university. This subaward is part of an ARO (Army Research Office) grant received by our collaborators at Yale and covers the expenses related to our collaborations (hiring of new PhD students and postdocs at Inria and travels between Inria and Yale). This subaward was terminated in August 2020.
 Berkeley exchange initiative: P. Rouchon and A. Sarlette have set up an exchange initiative with Birgitta Whaley about quantum control and error correction based on continuous measurements. Due to the pandemic conditions, this initiative has been delayed of oneyear until 2021.
8.2 European initiatives
8.2.1 FP7 & H2020 Projects
ERC Starting Grant ECLIPSE
 Program: H2020
 Type: ERC
 Project acronym: ECLIPSE
 Project title: Exotic superconducting circuits to probe and protect quantum states of light and matter
 Duration: 20192024
 Coordinator: Zaki Leghtas, Mines Paristech
 Abstract: Quantum systems can occupy peculiar states, such as superposition or entangled states. These states are intrinsically fragile and eventually get wiped out by inevitable interactions with the environment. Protecting quantum states against decoherence is a formidable and fundamental problem in physics, which is pivotal for the future of quantum computing. The theory of quantum error correction provides a solution, but its current envisioned implementations require daunting resources: a single bit of information is protected by encoding it across tens of thousands of physical qubits. This project intend to encode quantum information in an entirely new type of qubit with two key specificities. First, it will be encoded in a single superconducting circuit resonator whose infinite dimensional Hilbert space can replace large registers of physical qubits. Second, this qubit will be rfpowered, continuously exchanging photons with a reservoir. This approach challenges the intuition that a qubit must be isolated from its environment. Instead, the reservoir acts as a feedback loop which continuously and autonomously corrects against errors. This correction takes place at the level of the quantum hardware, and reduces the need for error syndrome measurements which are resource intensive. The circuits I will develop manipulate quantum states of light, whose utility transcends the long term goal of quantum computing, and can readily be used to probe fundamental properties of matter. In mesoscopic physics where a large number of particles exhibit collective quantum phenomena, the measurement tools to characterize subtle quantum effects are often lacking. Here, the project proposes to measure the spin entanglement of a single Cooper pair, by coupling a superconductor to a circuit composed of microwave resonators and a carbon nanotube. The spin entanglement can be swapped into microwave photons, which can be detected by deploying the arsenal of quantum limited microwave measurement devices.
ERC Advanced Grant QFeedback
 Program: H2020
 Type: ERC
 Project acronym: QFeedback
 Project title: Quantum feedback Engineering
 Duration: 20202025
 Coordinator: Pierre Rouchon, Mines Paristech

Abstract : Quantum technologies, such as quantum computers and simulators, have the potential of revolutionizing our computational speed, communication security and measurement precision. The power of the quantum relies on two key but fragile resources: quantum coherence and entanglement. This promising field is facing a major open question: how to design machines which exploit quantum properties on a large scale, and efficiently protect them from external perturbations (decoherence), which tend to suppress the quantum advantage?
Making a system robust and stable to the influence of external perturbations is one of the core problems in control engineering. The goal of this project is to address the above question from the angle of control systems. The fundamental and scientific ambition is to elaborate theoretical control methods to analyse and design feedback schemes for protecting and stabilizing quantum information. QFeedback develops mathematical methods to harness the inherently stochastic aspects of quantum measurements. Relying on the development of original mathematical perturbation techniques specific to open quantum systems, QFeedback proposes a new hierarchical strategy for quantum feedback modeling, design and analysis.
The building block of a quantum machine is the quantum bit (qubit), a system which can adopt two quantum states. Despite major progress, qubits remain fragile and lose their quantum properties before a meaningful task can be accomplished. For this reason, a qubit must be both protected against external perturbations, and manipulated to perform a task. Today, no such qubit has been built. In collaboration with experimentalists, the practical ambition is to design, relying on the control tools developed here, qubits readily integrable in a quantum processing unit. The physical platform will be Josephson superconducting circuits. QFeedback is expected to demonstrate the crucial role of control engineering in emerging quantum technologies.
Quantera Grant QuCos
 Program: H2020
 Type: Quantera
 Project acronym: QuCos
 Project title: Quantum Computation with Schrödinger cat states
 Duration: 20192023
 Coordinator: Gerhard Kirchmair, University of Innsbruck, Austria.
 Inria contacts: Zaki Leghtas and Mazyar Mirrahimi
 Other partners: ENS Lyon (France), Karlsruhe Institut of Technology (Germany), Quantum Machines (Israel), National Institute for Research and Development of Isotopic and Molecular Technologies, Romania.
 Abstract: This project seeks to establish a radically new, alternative approach to realizing the fundamental building blocks of quantum computers with superconducting qubits. In the next 3 years, we plan to employ only a handful of realistic components to realize robust errorcorrected logical quantum bits. We aim to demonstrate the same level of protection provided by a few hundreds of qubits (with properties beyond the state of the art) in today’s mainstream approach of the socalled surface code architecture. Our alternative approach is known as cat codes, because it employs multiple interconnected high coherence cavity modes with nonlinear dissipation, to encode a qubit in superpositions of Schrödinger cat states. Our project combines realizing the quantum processor architecture as well as the control system and the protocols that drive it, building towards a fullstack errorcorrected quantum computer. The partners in our collaboration form a strong synergetic group that has the full range of expertise needed to design and realize these systems, and to obtain these challenging goals. Furthermore, all partners of our project, including both industry and academia, have worked together and published works in the fields of quantum computing and quantum information processing. We aim to implement error protected qubits, fault tolerant operations, and demonstrate the scalability of this approach by realizing a repetition code. Our project will enable quantum experiments towards the ambitious and welldefined goal of constructing a logical qubit, on which we can perform gates, and most importantly, quantum errorcorrection (QEC).
8.2.2 Collaborations with major European organizations
 Partner 1: ENS Lyon
 We are pursuing our interdisciplinary work about quantum control from theoretical aspects in direct collaboration with existing experiments (ENS Lyon) with the group of Benjamin Huard, former member of the QUANTIC team. Joint papers are published and underway. The ANRJCJC project HAMROQS by Alain Sarlette has Benjamin Huard as external supporting collaborator.
 Partner 2: Laboratoire Kastler Brossel
 We have been continuing collaborations with the teams of Samuel Deleglise and Igor Dotsenko from Laboratoire Kastler Brossel on the theoretical analysis of their experiments.
 Partner 4: Ghent University.
 Alain Sarlette is participating in a Belgian (Flanders) initiative on a BoseEinstein condensate experiment. The experiment setup is being finalized at UGent where Alain Sarlette has been teaching since 2011; he has a standby consulting role with openings for suggestions.
8.3 National initiatives
 ANR project HAMROQS: In the framework of the ANR program JCJC, Alain Sarlette has received a funding for his research program "Highaccuracy model reduction for open quantum systems". This grant of 212k euros started on april 2019 and will run for 4 years.
 ANR project SYNCAMIL: In the framework of the ANR program JCJC, Philippe CampagneIbarcq has received a funding for his research program "Synthetic NonLocal Hamiltonians for the protection of quantum information". This grant of 380k euros has allowed us to purchase the experimental equipment to perform an experiment on stabilization of GKP grid states at ENS Paris, and to hire a postdoctoral associate, Vincent Lienhard, for 2 years. The project started in December 2020 for 48 months.
8.4 Regional initiatives
 DIM SIRTEQ In the framework of the DIM program AAP PME, the DIM Sirteq is cofinancing for 50 keuros the purchase of a highsampling rate Arbitrary Waveform Generator which will be used for experiments in the project SYNCAMIL.
 DIM SIRTEQ PhD fellowship: We have received in 2017 a funding from DIM SIRTEQ to cover half of the PhD of Jérémie Guillaud under supervision of Mazyar Mirrahimi.
 DIM SIRTEQ project SCOOP:Half a PhD grant for Marius Villiers, supervised by Zaki Leghtas and Audrey Cottet (ENS Paris). The project is to use quantum circuits to detect the entanglement of a single Cooper pair.
 EDPIF PhD fellowship: Ecole Doctorale de Physique en Ile de France has funded half a PhD grant for Marius Villiers.
 DGA PhD fellowship: Direction Générale de l'Armement has funded half a PhD grant for Camille Berdou supervised by Zaki Leghtas. The project is to build a repetition code of catqubits.
 Mines Paristech PhD Fellowship: Ecole des Mines Paristech has funded half a PhD grant for Camille Berdou.
9 Dissemination
9.1 Promoting scientific activities
9.1.1 Journal
Member of the editorial boards
 Pierre Rouchon is member of the editorial board of Annual Reviews in Control.
Reviewer  reviewing activities
 Philippe CampagneIbarcq was a reviewer for Physical Review X and Nature.
 Zaki Leghtas was a reviewer for Physical Review Letters, Physical Review X and Nature Physics.
 Mazyar Mirrahimi was a reviewer for Physical Review X Quantum, Physical Review Letters, Reviews of Modern Physics.
 Pierre Rouchon is a reviewer for IEEE TAC, Automatica and Physical Review X.
 Alain Sarlette was a reviewer for several automatic control and dynamical systems journals and conferences, as well as for Physical Review Journals and for Quantum specialized journals.
9.1.2 Invited talks
 Zaki Leghtas: Neuromorphic meets quantum. C2N Palaiseau France. Invited by Danijela Markovic.
 Zaki Leghtas: Ecole Polytechnique, France. Invited by Timothée Nicolas .
 Mazyar Mirrahimi: Plenary Speaker, European Control Conference, SaintPetersbourgh.
 Mazyar Mirrahimi: Colloquium, Inria Sophia Antipolis.
 Alain Sarlette: IFAC world congress, July 2020 (speaker in invited session – virtual meeting): Quantum adiabatic elimination at arbitrary order for photon number measurement.
9.1.3 Scientific expertise
 Pierre Rouchon is a member of the scientific committee of LAGEP (Laboratoire d'Automatique et de Génie des Procédés) since 2017.
 Pierre Rouchon is a member of the “Conseil Scientifique du DIM Math Innov” since 2017.
 Mazyar Mirrahimi is the copresident of Inria's comité des emplois scientifiques.
9.2 Teaching  Supervision  Juries
9.2.1 Teaching
 Cycle Ingénieur : Zaki Leghtas, Quantum Mechanics and Statistical Physics, Mines ParisTech, 12 hours, France.
 Cycle Ingénieur : Zaki Leghtas, Quantum Computing, Mines ParisTech, 20 hours, France.
 Cycle Ingénieur : Philippe CampagneIbarcq, Quantum Mechanics and Statistical Physics, Mines ParisTech, 12 hours.
 Cycle Ingénieur : Philippe CampagneIbarcq, Electromagnetism, ESPCI, 15 hours.
 Cycle Ingénieur : Mazyar Mirrahimi, Automatic control with Applications in Robotics and in Quantum engineering, 8 hour amphi and 16 hours TD, 3rd year, Ecole Polytechnique, France.
 Cycle Ingénieur : Mazyar Mirrahimi, Control of dynamical models, 40 hours TD, 2nd year, Ecole Polytechnique, France.
 Cycle Ingénieur : Mazyar Mirrahimi, Module algorithmique quantum control, 20 hours TD, 2nd year, Ecole Polytechnique, France.
 Master: Alain Sarlette, Robotics, 24 hours, Ghent University, Belgium.
 Master: Alain Sarlette and Pierre Rouchon, Dynamics and control of quantum systems, 18 hours amphi, M2, Sorbonne Université, France.
9.2.2 Supervision
 PhD in progress: Christian Siegele, Quantum error correction with grid states of light, advisors: Philippe CampagneIbarcq and Mazyar Mirrahimi, starting date: Sept 2019.
 PhD in progress: Aron Vanselow, Autonomous stabilization of GKP grid states in high impedance circuits, advisors: Philippe CampagneIbarcq and Mazyar Mirrahimi, starting date: Sept 2020.
 PhD in progress : Michiel Burgelman, A systematic study of strongly driven and dissipative quantum systems towards highaccuracy quantum control designs, advisors: Pierre Rouchon and Alain Sarlette, starting date: Nov 2018.
 PhD to be defended : Jérémie Guillaud, Faulttolerant quantum computation with catqubits, advisor: Mazyar Mirrahimi, starting date: Nov 2017.
 PhD in progress : Vincent Martin, Faulttolerance of quantum systems under continuoustime feedback stabilization, advisor: Mazyar Mirrahimi, starting date: Oct 2018.
 PhD defended: Raphaël Lescanne, Engineering MultiPhoton Dissipation In Superconducting Circuits For Quantum Error Correction, advisors: Zaki Leghtas and Takis Kontos, Defended on February 20th, 2020.
 PhD in progress: Marius Villiers, Probing the spin entanglement of single Cooper pair, advisors: Zaki Leghtas and Takis Kontos, starting date: September 2019.
 PhD in progress: Camille Berdou, A catqubit repetition code, advisors: Zaki Leghtas and Pierre Rouchon, starting date Sept 2019.
 PhD in progress: Alvise Borgognoni. Topologically protected qubits. Supervision of Zaki Leghtas.
 PhD in progress: Ulysse Réglade. A 3D catqubit. Supervision of Zaki Leghtas and Raphaël Lescanne.
 PhD in progress: FrançoisMarie Le Régent, Quantum computing architecture with catqubits, advisor: Mazyar Mirrahimi and Jérémie Guillaud, Starting date: Sept 2020.
 PhD in progress: Ronan Gautier, Efficient protection and fast gates in bosonic quantum codes. advisor: Alain Sarlette, Starting date: September 2020.
 PhD in progress: LevArcady Sellem. Mathematical methods for simulation and control of open quantum systems, advisors: Claude Le Bris and Pierre Rouchon, Starting date: October 2020.
9.2.3 Juries
 Philippe CampagneIbarcq was a PhD opponent of Daniel Weigand, TU Delft, Netherlands.
 Zaki Leghtas was in the PhD committee of Mario Gely, TU Delft, Netherlands.
 Pierre Rouchon was a reviewer of the HDR Defense of Marcin Nowicky, Poznan University of Technology.
 Mazyar Mirrahimi was in the PhD committee of Rodrigo Cortinas, Ecole Normale Supérieure.
9.3 Popularization
9.3.1 Interventions
 Philippe CampagneIbarcq: Telecom ParisTalks “Quantum computing, from research to engineering”
10 Scientific production
10.1 Major publications
 1 article Quantum FastForwarding: Markov chains and graph property testing Quantum Information & Computation April 2019
 2 article Quantum error correction of a qubit encoded in grid states of an oscillator Nature 584 Text and figures edited for clarity. The claims of the paper remain the same. Author list fixed August 2020
 3 article Exponential stabilization of quantum systems under continuous nondemolition measurements Automatica December 2019
 4 article Repetition Cat Qubits for FaultTolerant Quantum Computation Physical Review X https://arxiv.org/abs/1904.09474  22 pages, 11 figures December 2019
 5 articleConfining the state of light to a quantum manifold by engineered twophoton lossScience3476224February 2015, 853857
 6 article Exponential suppression of bitflips in a qubit encoded in an oscillator Nature Physics March 2020
 7 articleDynamically protected catqubits: a new paradigm for universal quantum computationNew Journal of Physics164apr 2014, 045014
 8 articleExtending the lifetime of a quantum bit with error correction in superconducting circuitsNature5362016, 5
 9 article Stabilization of nonclassical states of the radiation field in a cavity by reservoir engineering Phys. Rev. Lett. 107 010402 2011
 10 articleRealtime quantum feedback prepares and stabilizes photon number statesNature4772011, 7377
10.2 Publications of the year
International journals
 11 article Quantum error correction of a qubit encoded in grid states of an oscillator Nature 584 August 2020
 12 articleTheory of interactions between cavity photons induced by a mesoscopic circuitPhysical Review B10215October 2020, 155105
 13 article NumberResolved Photocounter for Propagating Microwave Mode Physical Review Applied 14 4 October 2020
 14 articleStabilization and operation of a Kerrcat qubitNature584August 2020, 205209
 15 article Irreversible QubitPhoton Coupling for the Detection of Itinerant Microwave Photons Physical Review X 10 2 May 2020
 16 article Exponential suppression of bitflips in a qubit encoded in an oscillator Nature Physics March 2020
International peerreviewed conferences
Doctoral dissertations and habilitation theses
 17 thesis Exotic superconducting circuits to probe and protect quantum states of light and matter ENS Paris  Ecole Normale Supérieure de Paris November 2020
 18 thesis Habilitationà Diriger des Recherches (HDR) Sorbonne Universites, UPMC University of Paris 6 October 2020
Reports & preprints
 19 misc Lifting Markov Chains To Mix Faster: Limits and Opportunities January 2021
 20 misc Multiplexed photon number measurement December 2020
 21 misc Error Rates and Resource Overheads of Repetition Cat Qubits December 2020
 22 misc Scaling up reservoir engineering for errorcorrecting codes December 2020
 23 misc Energyparticipation quantization of Josephson circuits December 2020
 24 misc Quantum adiabatic elimination at arbitrary order for photon number measurement December 2020
 25 misc Magnifying quantum phase fluctuations with Cooperpair pairing December 2020
 26 misc Determining the position of a single spin relative to a metallic nanowire December 2020
10.3 Cited publications
 27 articleStabilization of a delayed quantum system: the Photon Box casestudyIEEE Trans. Automatic Control5782012, 19181930
 28 articleStability of continuoustime quantum filters with measurement imperfectionsRussian Journal of Mathematical Physics212014, 297315
 29 articleFeedback stabilization of discretetime quantum systems subject to nondemolition measurements with imperfections and delaysAutomatica4992013, 26832692
 30 book S. Attal A. Joye C.A. Pillet Open Quantum Systems III: Recent Developments Springer, Lecture notes in Mathematics 1880 2006
 31 book Quantum Trajectories and Measurements in Continuous Time: the Diffusive Case Springer Verlag 2009
 32 article An opensystem quantum simulator with trapped ions Nature 470 486 2011
 33 articleQuantum stochastic calculus and quantum nonlinear filteringJournal of Multivariate Analysis4221992, 171201
 34 articleLarge Time Behavior and Convergence Rate for Quantum Filters Under Standard Non Demolition ConditionsCommunications in Mathematical Physics2014, 121URL: http://dx.doi.org/10.1007/s0022001420296
 35 articleExtensions of Jentzch's theoremTrans. Amer. Math. Soc.851957, 219227
 36 article Engineering stable discretetime quantum dynamics via a canonical QR decomposition IEEE Trans. Autom. Control 55 2010
 37 book Quantum Measurements Cambridge University Press 1992
 38 articleObserving Interferences between Past and Future Quantum States in Resonance FluorescencePhys. Rev. Lett.11218040218May 2014, URL: http://link.aps.org/doi/10.1103/PhysRevLett.112.180402
 39 article Persistent Control of a Superconducting Qubit by Stroboscopic Measurement Feedback Phys. Rev. X 3 021008 2013
 40 book An Open Systems Approach to Quantum Optics SpringerVerlag 1993
 41 book Statistical Methods in Quantum Optics 2: NonClassical Fields Spinger 2007
 42 book Application of Center Manifold Theory Springer 1981
 43 articleDissipationinduced continuous quantum error correction for superconducting circuitsPhys. Rev. A902014, 062344
 44 articleWavefunction approach to dissipative processes in quantum opticsPhys. Rev. Lett.6851992, 580583
 45 misc Superconducting Qubits: A Short Review arXiv:condmat/0411174 2004
 46 article Remote entanglement stabilization and concentration by quantum reservoir engineering Physical Review A https://arxiv.org/abs/1703.03379  5 pages, 4 figures July 2018
 47 article Quantum feedback by discrete quantum nondemolition measurements: towards ondemand generation of photonnumber states Physical Review A 80: 013805013813 2009
 48 articleGeometric singular perturbation theory for ordinary differential equationsJ. Diff. Equations311979, 5398
 49 articleState and dynamical parameter estimation for open quantum systemsPhys. Rev. A644042105September 2001, URL: http://link.aps.org/doi/10.1103/PhysRevA.64.042105
 50 articlePast Quantum States of a Monitored SystemPhys. Rev. Lett.11116160401October 2013, URL: http://link.aps.org/doi/10.1103/PhysRevLett.111.160401
 51 book Quantum Noise Springer 2010
 52 article Checking the strict positivity of Kraus maps is NPhard arXiv:1402.1429 2014
 53 articleThe contraction rate in Thompson's part metric of orderpreserving flows on a cone  Application to generalized Riccati equationsJournal of Differential Equations2568April 2014, 29022948URL: http://www.sciencedirect.com/science/article/pii/S0022039614000424
 54 article Demonstrating a Driven Reset Protocol of a Superconducting Qubit Phys. Rev. Lett. 110 120501 2013
 55 article Encoding a qubit in an oscillator Phys. Rev. A 64 012310 2001
 56 articleProgressive fieldstate collapse and quantum nondemolition photon countingNature4482007, 889893
 57 book Exploring the Quantum: Atoms, Cavities and Photons Oxford University Press 2006
 58 articleQuantum backaction of an individual variablestrength measurementScience3392013, 178181
 59 articleSinglephotonresolved crossKerr interaction for autonomous stabilization of photonnumber statesPhys. Rev. Lett.1152015, 180501
 60 book Perturbation Theory for Linear Operators Springer 1966
 61 article A scheme for efficient quantum computation with linear optics Nature 409 46 2001
 62 article Entanglement Generated by Dissipation and Steady State Entanglement of Two Macroscopic Objects Phys. Rev. Lett. 107 080503 2011
 63 article Deterministic protocol for mapping a qubit to coherent state superpositions in a cavity Phys. Rev. A 87 042315 2013
 64 article Hardwareefficient autonomous quantum memory protection Phys. Rev. Lett. 111 120501 2013
 65 article Adiabatic passage and ensemble control of quantum systems J. Phys. B 44 154017 2011
 66 article Stabilizing a Bell state of two superconducting qubits by dissipation engineering Phys. Rev. A 88 023849 2013
 67 articleEnsemble control of Bloch equationsIEEE Trans. Autom. Control542009, 528536
 68 articleDissipative production of a maximally entangled steady state of two quantum bitsNature5042013, 415418
 69 article Coherent quantum feedback Phys. Rev. A 62 022108 2000
 70 article Consensus for quantum networks: from symmetry to gossip iterations IEEE Trans. Automat. Control in press 2014
 71 conference Strong measurement and quantum feedback for persistent Rabi oscillations in circuit QED experiments IEEE Conference on Decision and Control IEEE Conference on Decision and Control 2012
 72 article Cavityassisted quantum bath engineering Phys. Rev. Lett. 109 183602 2012
 73 articleEstimation of classical parameters via continuous probing of complementary quantum observablesNew Journal of Physics15121250022013, URL: http://stacks.iop.org/13672630/15/i=12/a=125002
 74 articleCoherent quantum LQG controlAutomatica452009, 18371846
 75 articleAdaptive Quantum Nondemolition Measurement of a Photon NumberPhys. Rev. Lett.1128080401Feb 2014, URL: http://link.aps.org/doi/10.1103/PhysRevLett.112.080401
 76 articleMonotone Metrics on matrix spacesLinear Algebra and its Applications2441996, 8196
 77 articleQuantum Reservoir Engineering with Laser Cooled Trapped IonsPhys. Rev. Lett.77231996, 47284731
 78 articleHilbert's projective metric in quantum information theoryJournal of Mathematical Physics528082201August 2011, URL: http://dx.doi.org/10.1063/1.3615729
 79 article Initialization by measurement of a superconducting quantum bit circuit Phys. Rev. Lett. 109 050507 2012
 80 article Widely tunable, nondegenerate threewave mixing microwave device operating near the quantum limit Phys. Rev. Lett. 108 147701 2012
 81 articleFidelity is a SubMartingale for DiscreteTime Quantum FiltersIEEE Transactions on Automatic Control56112011, 27432747
 82 articleContinuous generation and stabilization of mesoscopic field superposition states in a quantum circuitPhys. Rev. A912015, 013810
 83 article Stabilization of nonclassical states of one and twomode radiation fields by reservoir engineering Phys. Rev. A 86 012114 2012
 84 articleResolving photon number states in a superconducting circuitNature4452007, 515518
 85 inproceedingsConsensus in noncommutative spacesDecision and Control (CDC), 2010 49th IEEE Conference on2010, 65966601
 86 articleAutonomously stabilized entanglement between two superconducting quantum bitsNature5042013, 419422
 87 articleScheme for reducing decoherence in quantum memoryPhys. Rev. A521995, 24932496
 88 inproceedingsDesign and Stability of DiscreteTime Quantum Filters with Measurement ImperfectionsAmerican Control Conference2012, 50845089
 89 article Approximate stabilization of infinite dimensional quantum stochastic system Reviews in Mathematical Physics 25 1350001 2013
 90 article Error Correcting Codes in Quantum Theory Phys. Rev. Lett 77 5 1996
 91 articleTracking photon jumps with repeated quantum nondemolition parity measurementsNature5112014, 444448
 92 article Problems in decentralized decision making and computation PhD Thesis, MIT 1984
 93 articleStabilizing Rabi oscillations in a superconducting qubit using quantum feedbackNature4902012, 7780
 94 articleDynamical decoupling of open quantum systemPhys. Rev. Lett.821999, 24172421
 95 articleDeterministically encoding quantum information using 100photon Schrödinger cat statesScience3422013, 607610
 96 article Field locked to Fock state by quantum feedback with single photon corrections Physical Review Letter 108 243602 2012
 97 articleThe stability of quantum Markov filtersInfin. Dimens. Anal. Quantum Probab. Relat. Top.122009, 153172