Coherent oscillations in a quantum manifold stabilized by dissipation

  1. S. Touzard,
  2. A. Grimm,
  3. Z. Leghtas,
  4. S. O. Mundhada,
  5. P. Reinhold,
  6. R. Heeres,
  7. C. Axline,
  8. M. Reagor,
  9. K. Chou,
  10. J. Blumoff,
  11. K. M. Sliwa,
  12. S. Shankar,
  13. L. Frunzio,
  14. R. J. Schoelkopf,
  15. M. Mirrahimi,
  16. and M.H. Devoret
The quantum Zeno effect (QZE) is the apparent freezing of a quantum system in one state under the influence of a continuous observation. It has been further generalized to the stabilization
of a manifold spanned by multiple quantum states. In that case, motion inside the manifold can subsist and can even be driven by the combination of a dissipative stabilization and an external force. A superconducting microwave cavity that exchanges pairs of photons with its environments constitutes an example of a system which displays a stabilized manifold spanned by Schr\“odinger cat states. For this driven-dissipative system, the quantum Zeno stabilization transforms a simple linear drive into photon number parity oscillations within the stable cat state manifold. Without this stabilization, the linear drive would trivially displace the oscillator state and push it outside of the manifold. However, the observation of this effect is experimentally challenging. On one hand, the adiabaticity condition requires the oscillations to be slow compared to the manifold stabilization rate. On the other hand, the oscillations have to be fast compared with the coherence timescales within the stabilized manifold. Here, we implement the stabilization of a manifold spanned by Schr\“odinger cat states at a rate that exceeds the main source of decoherence by two orders of magnitude, and we show Zeno-driven coherent oscillations within this manifold. While related driven manifold dynamics have been proposed and observed, the non-linear dissipation specific to our experiment adds a crucial element: any drift out of the cat state manifold is projected back into it. The coherent oscillations of parity observed in this work are analogous to the Rabi rotation of a qubit protected against phase-flips and are likely to become part of the toolbox in the construction of a fault-tolerant logical qubit.

Tracking Photon Jumps with Repeated Quantum Non-Demolition Parity Measurements

  1. L. Sun,
  2. A. Petrenko,
  3. Z. Leghtas,
  4. B. Vlastakis,
  5. G. Kirchmair,
  6. K. M. Sliwa,
  7. A. Narla,
  8. M. Hatridge,
  9. S. Shankar,
  10. J. Blumoff,
  11. L. Frunzio,
  12. M. Mirrahimi,
  13. M. H. Devoret,
  14. and R. J. Schoelkopf
Quantum error correction (QEC) is required for a practical quantum computer because of the fragile nature of quantum information. In QEC, information is redundantly stored in a large
Hilbert 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-qubit parities such as , must correspond to a special symmetry property inherent to the encoding scheme. Measurements of these observables, or error syndromes, must also be performed in a quantum non-demolition (QND) way and faster than the rate at which errors occur. Previously, QND measurements of quantum jumps between energy eigenstates have been performed in systems such as trapped ions, electrons, cavity quantum electrodynamics (QED), nitrogen-vacancy (NV) centers, and superconducting qubits. So far, however, no fast and repeated monitoring of an error syndrome has been realized. Here, we track the quantum jumps of a possible error syndrome, the photon number parity of a microwave cavity, by mapping this property onto an ancilla qubit. This quantity is just the error syndrome required in a recently proposed scheme for a hardware-efficient protected quantum memory using Schr\“{o}dinger cat states in a harmonic oscillator. We demonstrate the projective nature of this measurement onto a parity eigenspace by observing the collapse of a coherent state onto even or odd cat states. The measurement is fast compared to the cavity lifetime, has a high single-shot fidelity, and has a 99.8% probability per single measurement of leaving the parity unchanged. In combination with the deterministic encoding of quantum information in cat states realized earlier, our demonstrated QND parity tracking represents a significant step towards implementing an active system that extends the lifetime of a quantum bit.