Recovering quantum coherence of a cavity qubit through environment monitoring and active feedback

  1. Uri Goldblatt,
  2. Nitzan Kahn,
  3. Sergey Hazanov,
  4. Ofir Milul,
  5. Barkay Guttel,
  6. Lalit M. Joshi,
  7. Daniel Chausovsky,
  8. Fabien Lafont,
  9. and Serge Rosenblum
Decoherence in qubits, caused by their interaction with a noisy environment, poses a significant challenge to developing reliable quantum processors. Monitoring the qubit’s environment
enables not only to flag decoherence events but also to reverse these errors, thereby restoring the qubit coherence. This approach is particularly beneficial for superconducting cavity qubits, whose unavoidable interaction with auxiliary transmons impacts their coherence. In this work, we uncover the intricate dynamics of cavity qubit decoherence by tracking the noisy trajectory of a transmon acting as the cavity’s environment. Using real-time feedback, we successfully recover the lost coherence of the cavity qubit, achieving a fivefold increase in its dephasing time. Alternatively, by detecting transmon errors and converting them into erasures, we improve the cavity phase coherence by more than an order of magnitude. These advances are essential for using cavity qubits with low photon loss rates as long-lived quantum memories with high-fidelity gates and can enable more efficient bosonic quantum error correction codes.

A superconducting quantum memory with tens of milliseconds coherence time

  1. Ofir Milul,
  2. Barkay Guttel,
  3. Uri Goldblatt,
  4. Sergey Hazanov,
  5. Lalit M. Joshi,
  6. Daniel Chausovsky,
  7. Nitzan Kahn,
  8. Engin Çiftyürek,
  9. Fabien Lafont,
  10. and Serge Rosenblum
Storing quantum information for an extended period of time is essential for running quantum algorithms with low errors. Currently, superconducting quantum memories have coherence times
of a few milliseconds, and surpassing this performance has remained an outstanding challenge. In this work, we report a qubit encoded in a novel superconducting cavity with a coherence time of 34 ms, an improvement of over an order of magnitude compared to previous demonstrations. We use this long-lived quantum memory to store a Schrödinger cat state with a record size of 1024 photons, indicating the cavity’s potential for bosonic quantum error correction.

Path-Independent Quantum Gates with Noisy Ancilla

  1. Wen-Long Ma,
  2. Mengzhen Zhang,
  3. Yat Wong,
  4. Kyungjoo Noh,
  5. Serge Rosenblum,
  6. Philip Reinhold,
  7. Robert J. Schoelkopf,
  8. and Liang Jiang
Ancilla systems are often indispensable to universal control of a nearly isolated quantum system. However, ancilla systems are typically more vulnerable to environmental noise, which
limits the performance of such ancilla-assisted quantum control. To address this challenge of ancilla-induced decoherence, we propose a general framework that integrates quantum control and quantum error correction, so that we can achieve robust quantum gates resilient to ancilla noise. We introduce the path independence criterion for fault-tolerant quantum gates against ancilla errors. As an example, a path-independent gate is provided for superconducting circuits with a hardware-efficient design.

High-fidelity measurement of qubits encoded in multilevel superconducting circuits

  1. Salvatore S. Elder,
  2. Christopher S. Wang,
  3. Philip Reinhold,
  4. Connor T. Hann,
  5. Kevin S. Chou,
  6. Brian J. Lester,
  7. Serge Rosenblum,
  8. Luigi Frunzio,
  9. Liang Jiang,
  10. and Robert J. Schoelkopf
Qubit measurements are central to quantum information processing. In the field of superconducting qubits, standard readout techniques are not only limited by the signal-to-noise ratio,
but also by state relaxation during the measurement. In this work, we demonstrate that the limitation due to relaxation can be suppressed by using the many-level Hilbert space of superconducting circuits: in a multilevel encoding, the measurement is only corrupted when multiple errors occur. Employing this technique, we show that we can directly resolve transmon gate errors at the level of one part in 103. Extending this idea, we apply the same principles to the measurement of a logical qubit encoded in a bosonic mode and detected with a transmon ancilla, implementing a proposal by Hann et al. [Phys. Rev. A \textbf{98} 022305 (2018)]. Qubit state assignments are made based on a sequence of repeated readouts, further reducing the overall infidelity. This approach is quite general and several encodings are studied; the codewords are more distinguishable when the distance between them is increased with respect to photon loss. The tradeoff between multiple readouts and state relaxation is explored and shown to be consistent with the photon-loss model. We report a logical assignment infidelity of 5.8×10−5 for a Fock-based encoding and 4.2×10−3 for a QEC code (the S=2,N=1 binomial code). Our results will not only improve the fidelity of quantum information applications, but also enable more precise characterization of process or gate errors.

Error-corrected gates on an encoded qubit

  1. Philip Reinhold,
  2. Serge Rosenblum,
  3. Wen-Long Ma,
  4. Luigi Frunzio,
  5. Liang Jiang,
  6. and Robert J. Schoelkopf
To solve classically hard problems, quantum computers need to be resilient to the influence of noise and decoherence. In such a fault-tolerant quantum computer, noise-induced errors
must be detected and corrected in real-time to prevent them from propagating between components. This requirement is especially pertinent while applying quantum gates, when the interaction between components can cause errors to quickly spread throughout the system. However, the large overhead involved in most fault-tolerant architectures makes implementing these systems a daunting task, which motivates the search for hardware-efficient alternatives. Here, we present a gate enacted by a multilevel ancilla transmon on a cavity-encoded logical qubit that is fault-tolerant with respect to decoherence in both the ancilla and the encoded qubit. We maintain the purity of the encoded qubit in the presence of ancilla errors by detecting those errors in real-time, and applying the appropriate corrections. We show a reduction of the logical gate error by a factor of two in the presence of naturally occurring decoherence, and demonstrate resilience against ancilla bit-flips and phase-flips by observing a sixfold suppression of the gate error with increased energy relaxation, and a fourfold suppression with increased dephasing noise. The results demonstrate that bosonic logical qubits can be controlled by error-prone ancilla qubits without inheriting the ancilla’s inferior performance. As such, error-corrected ancilla-enabled gates are an important step towards fully fault-tolerant processing of bosonic qubits.

A CNOT gate between multiphoton qubits encoded in two cavities

  1. Serge Rosenblum,
  2. Yvonne Gao,
  3. Philip Reinhold,
  4. Chen Wang,
  5. Christopher Axline,
  6. Luigi Frunzio,
  7. Steven Girvin,
  8. Liang Jiang,
  9. Mazyar Mirrahimi,
  10. Michel Devoret,
  11. and Robert Schoelkopf
Entangling gates between qubits are a crucial component for performing algorithms in quantum computers. However, any quantum algorithm will ultimately have to operate on error-protected
logical qubits, which are effective qubits encoded in a high-dimensional Hilbert space. A common approach is to encode logical qubits in collective states of multiple two-level systems, but algorithms operating on multiple logical qubits are highly complex and have not yet been demonstrated. Here, we experimentally realize a controlled NOT (CNOT) gate between two multiphoton qubits in two microwave cavities. In this approach, we encode a qubit in the large Hilbert space of a single cavity mode, rather than in multiple two-level systems. We couple two such encoded qubits together through a transmon, which is driven with an RF pump to apply the CNOT gate within 190 ns. This is two orders of magnitude shorter than the decoherence time of any part of the system, enabling high-fidelity operations comparable to state-of-the-art gates between two-level systems. These results are an important step towards universal algorithms on error-corrected logical qubits.