A. Petrenko

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.
arxiv pdf

Photon Shot Noise Dephasing in the Strong-Dispersive Limit of Circuit QED

  1. A. P. Sears,
  2. A. Petrenko,
  3. G. Catelani,
  4. L. Sun,
  5. Hanhee Paik,
  6. G. Kirchmair,
  7. L. Frunzio,
  8. L. I. Glazman,
  9. S. M. Girvin,
  10. and R. J. Schoelkopf
We study the photon shot noise dephasing of a superconducting transmon qubit in the strong-dispersive limit, due to the coupling of the qubit to its readout cavity. As each random arrival
or departure of a photon is expected to completely dephase the qubit, we can control the rate at which the qubit experiences dephasing events by varying textit{in situ} the cavity mode population and decay rate. This allows us to verify a pure dephasing mechanism that matches theoretical predictions, and in fact explains the increased dephasing seen in recent transmon experiments as a function of cryostat temperature. We investigate photon dynamics in this limit and observe large increases in coherence times as the cavity is decoupled from the environment. Our experiments suggest that the intrinsic coherence of small Josephson junctions, when corrected with a single Hahn echo, is greater than several hundred microseconds.
arxiv pdf