Section: Application Domains

Quantum control

The issue of designing efficient transfers between different atomic or molecular levels is crucial in atomic and molecular physics, in particular because of its importance in those fields such as photochemistry (control by laser pulses of chemical reactions), nuclear magnetic resonance (NMR, control by a magnetic field of spin dynamics) and, on a more distant time horizon, the strategic domain of quantum computing. This last application explicitly relies on the design of quantum gates, each of them being, in essence, an open loop control law devoted to a prescribed simultaneous control action. NMR is one of the most promising techniques for the implementation of a quantum computer.

Physically, the control action is realized by exciting the quantum system by means of one or several external fields, being them magnetic or electric fields. The resulting control problem has attracted increasing attention, especially among quantum physicists and chemists (see, for instance, [91] , [96] ). The rapid evolution of the domain is driven by a multitude of experiments getting more and more precise and complex (see the recent review [52] ). Control strategies have been proposed and implemented, both on numerical simulations and on physical systems, but there is still a large gap to fill before getting a complete picture of the control properties of quantum systems. Control techniques should necessarily be innovative, in order to take into account the physical peculiarities of the model and the specific experimental constraints.

The area where the picture got clearer is given by finite dimensional linear closed models.

• Finite dimensional refers to the dimension of the space of wave functions, and, accordingly, to the finite number of energy levels.

• Linear means that the evolution of the system for a fixed (constant in time) value of the control is determined by a linear vector field.

• Closed refers to the fact that the systems are assumed to be totally disconnected from the environment, resulting in the conservation of the norm of the wave function.

The resulting model is well suited for describing spin systems and also arises naturally when infinite dimensional quantum systems of the type discussed below are replaced by their finite dimensional Galerkin approximations. Without seeking exhaustiveness, let us mention some of the issues that have been tackled for finite dimensional linear closed quantum systems:

• controllability [34] ,

• bounds on the controllability time [30] ,

• STIRAP processes [101] ,

• simultaneous control [74] ,

• optimal control ( [70] , [43] , [54] ),

• numerical simulations [80] .

Several of these results use suitable transformations or approximations (for instance the so-called rotating wave) to reformulate the finite-dimensional Schrödinger equation as a sub-Riemannian system. Open systems have also been the object of an intensive research activity (see, for instance, [35] , [71] , [92] , [49] ).

In the case where the state space is infinite dimensional, some optimal control results are known (see, for instance, [39] , [50] , [67] , [40] ). The controllability issue is less understood than in the finite dimensional setting, but several advances should be mentioned. First of all, it is known that one cannot expect exact controllability on the whole Hilbert sphere [100] . Moreover, it has been shown that a relevant model, the quantum oscillator, is not even approximately controllable [93] , [83] . These negative results have been more recently completed by positive ones. In [41] , [42] Beauchard and Coron obtained the first positive controllability result for a quantum particle in a 1D potential well. The result is highly nontrivial and is based on Coron's return method (see [56] ). Exact controllability is proven to hold among regular enough wave functions. In particular, exact controllability among eigenfunctions of the uncontrolled Schrödinger operator can be achieved. Other important approximate controllability results have then been proved using Lyapunov methods [82] , [87] , [68] . While [82] studies a controlled Schrödinger equation in $ℝ$ for which the uncontrolled Schrödinger operator has mixed spectrum, [87] , [68] deal mainly with general discrete-spectrum Schrödinger operators.

In all the positive results recalled in the previous paragraph, the quantum system is steered by a single external field. Different techniques can be applied in the case of two or more external fields, leading to additional controllability results [59] , [46] .

The picture is even less clear for nonlinear models, such as Gross–Pitaevski and Hartree–Fock equations. The obstructions to exact controllability, similar to the ones mentioned in the linear case, have been discussed in [65] . Optimal control approaches have also been considered [38] , [51] . A comprehensive controllability analysis of such models is probably a long way away.