Section: New Results

Tracking photon jumps with repeated quantum non-demolition parity measurements

Participant : Mazyar Mirrahimi.

Quantum error correction (QEC) is required for a practical quantum computer because of the fragile nature of quantum information. In quantum error correction, information is redundantly stored in a large quantum state space and one or more observables must be monitored to reveal the occurrence of an error, without disturbing the information encoded in an unknown quantum state. Such observables, typically multi-quantum-bit parities, must correspond to a special symmetry property inherent in the encoding scheme. Measurements of these observables, or error syndromes, must also be performed in a quantum non-demolition way (projecting without further perturbing the state) and more quickly than errors occur. Previously, quantum non-demolition measurements of quantum jumps between states of well-defined energy have been performed in systems such as trapped ions, electrons, cavity quantum electrodynamics, nitrogen?vacancy centres and superconducting quantum bits. So far, however, no fast and repeated monitoring of an error syndrome had been achieved. Mazyar Mirrahimi has participated to an experiment performed by the group of Robert Schoelkopf (Department of Applied Physics, Yale University) where the quantum jumps of a possible error syndrome, namely the photon number parity of a microwave cavity, were tracked by mapping this property onto an ancilla quantum bit, whose only role is to facilitate quantum state manipulation and measurement. This quantity is just the error syndrome required in a QEC scheme proposed by Mazyar Mirrahimi and his former PhD student, Zaki Leghtas, and in a close collaboration with the teams of Michel Devoret and Robert Schoelkopf. This scheme should lead to a hardware-efficient protected quantum memory using Schrödinger cat states (quantum superpositions of different coherent states of light) in a harmonic oscillator [4] . We demonstrated the projective nature of this measurement onto a region of state space with well-defined parity by observing the collapse of a coherent state onto even or odd cat states. The measurement is fast compared with the cavity lifetime, has a high single-shot fidelity and has a 99.8 per cent probability per single measurement of leaving the parity unchanged. In combination with the deterministic encoding of quantum information in cat states realized earlier [10] , the quantum non-demolition parity tracking that we have demonstrated represents an important step towards implementing an active system that extends the lifetime of a quantum bit. This result was published in Nature [9] .