Readout-induced suppression and enhancement of superconducting qubit lifetimes

  1. Ted Thorbeck,
  2. Zhihao Xiao,
  3. Archana Kamal,
  4. and Luke C.G. Govia
It has long been known that the lifetimes of superconducting qubits suffer during readout, increasing readout errors. We show that this degradation is due to the anti-Zeno effect, as
readout-induced dephasing broadens the qubit so that it overlaps ‚hot spots‘ of strong dissipation, likely due to two-level systems in the qubit’s bath. Using a flux-tunable qubit to probe the qubit’s frequency dependent loss, we accurately predict the change in lifetime during readout with a new self-consistent master equation that incorporates the modification to qubit relaxation due to measurement-induced dephasing. Moreover, we controllably demonstrate both the Zeno and anti-Zeno effects, which explain suppression and the rarer enhancement of qubit lifetimes during readout.

Miniaturizing transmon qubits using van der Waals materials

  1. Abhinandan Antony,
  2. Martin V. Gustafsson,
  3. Guilhem J. Ribeill,
  4. Matthew Ware,
  5. Anjaly Rajendran,
  6. Luke C.G. Govia,
  7. Thomas A. Ohki,
  8. Takashi Taniguchi,
  9. Kenji Watanabe,
  10. James Hone,
  11. and Kin Chung Fong
Quantum computers can potentially achieve an exponential speedup versus classical computers on certain computational tasks, as was recently demonstrated in systems of superconductingqubits. However, these qubits have large footprints due to the need of ultra low-loss capacitors. The large electric field volume of \textit{quantum compatible} capacitors stems from their distributed nature. This hinders scaling by increasing parasitic coupling in circuit designs, degrading individual qubit addressability, and limiting the minimum achievable circuit area. Here, we report the use of van der Waals (vdW) materials to reduce the qubit area by a factor of >1000. These qubit structures combine parallel-plate capacitors comprising crystalline layers of superconducting niobium diselenide (NbSe2) and insulating hexagonal-boron nitride (hBN) with conventional aluminum-based Josephson junctions. We measure a vdW transmon T1 relaxation time of 1.06 μs, demonstrating that a highly-compact capacitor can reach a loss-tangent of <2.83×10−5. Our results demonstrate a promising path towards breaking the paradigm of requiring large geometric capacitors for long quantum coherence in superconducting qubits, and illustrate the broad utility of layered heterostructures in low-loss, high-coherence quantum devices.[/expand]

Characterizing mid-circuit measurements on a superconducting qubit using gate set tomography

  1. Kenneth Rudinger,
  2. Guilhem J. Ribeill,
  3. Luke C.G. Govia,
  4. Matthew Ware,
  5. Erik Nielsen,
  6. Kevin Young,
  7. Thomas A. Ohki,
  8. Robin Blume-Kohout,
  9. and Timothy Proctor
Measurements that occur within the internal layers of a quantum circuit — mid-circuit measurements — are an important quantum computing primitive, most notably for quantum
error correction. Mid-circuit measurements have both classical and quantum outputs, so they can be subject to error modes that do not exist for measurements that terminate quantum circuits. Here we show how to characterize mid-circuit measurements, modelled by quantum instruments, using a technique that we call quantum instrument linear gate set tomography (QILGST). We then apply this technique to characterize a dispersive measurement on a superconducting transmon qubit within a multiqubit system. By varying the delay time between the measurement pulse and subsequent gates, we explore the impact of residual cavity photon population on measurement error. QILGST can resolve different error modes and quantify the total error from a measurement; in our experiment, for delay times above 1000 ns we measured a total error rate (i.e., half diamond distance) of ϵ⋄=8.1±1.4%, a readout fidelity of 97.0±0.3%, and output quantum state fidelities of 96.7±0.6% and 93.7±0.7% when measuring 0 and 1, respectively.

Deep Neural Network Discrimination of Multiplexed Superconducting Qubit States

  1. Benjamin Lienhard,
  2. Antti Vepsäläinen,
  3. Luke C.G. Govia,
  4. Cole R. Hoffer,
  5. Jack Y. Qiu,
  6. Diego Ristè,
  7. Matthew Ware,
  8. David Kim,
  9. Roni Winik,
  10. Alexander Melville,
  11. Bethany Niedzielski,
  12. Jonilyn Yoder,
  13. Guilhem J. Ribeill,
  14. Thomas A. Ohki,
  15. Hari K. Krovi,
  16. Terry P. Orlando,
  17. Simon Gustavsson,
  18. and William D. Oliver
Demonstrating the quantum computational advantage will require high-fidelity control and readout of multi-qubit systems. As system size increases, multiplexed qubit readout becomes
a practical necessity to limit the growth of resource overhead. Many contemporary qubit-state discriminators presume single-qubit operating conditions or require considerable computational effort, limiting their potential extensibility. Here, we present multi-qubit readout using neural networks as state discriminators. We compare our approach to contemporary methods employed on a quantum device with five superconducting qubits and frequency-multiplexed readout. We find that fully-connected feedforward neural networks increase the qubit-state-assignment fidelity for our system. Relative to contemporary discriminators, the assignment error rate is reduced by up to 25 % due to the compensation of system-dependent nonidealities such as readout crosstalk which is reduced by up to one order of magnitude. Our work demonstrates a potentially extensible building block for high-fidelity readout relevant to both near-term devices and future fault-tolerant systems.

Stroboscopic qubit measurement with squeezed illumination

  1. Andrew Eddins,
  2. Sydney Schreppler,
  3. David M. Toyli,
  4. Leigh S. Martin,
  5. Shay Hacohen-Gourgy,
  6. Luke C.G. Govia,
  7. Hugo Ribeiro,
  8. Aashish A. Clerk,
  9. and Irfan Siddiqi
Microwave squeezing represents the ultimate sensitivity frontier for superconducting qubit measurement. However, observation of enhancement has remained elusive, in part because integration
with conventional dispersive readout pollutes the signal channel with antisqueezed vacuum. Here we induce a stroboscopic light-matter coupling with superior squeezing compatibility, and observe an increase in the room-temperature signal-to-noise ratio of 24%. Squeezing the orthogonal phase controls measurement backaction, slowing dephasing by a factor of 1.8. This protocol enables the practical use of microwave squeezing for qubit state measurement.

Enhanced qubit readout using locally-generated squeezing and inbuilt Purcell-decay suppression

  1. Luke C.G. Govia,
  2. and Aashish A. Clerk
We introduce and analyze a dispersive qubit readout scheme where two-mode squeezing is generated directly in the measurement cavities. The resulting suppression of noise enables fast,
high- fidelity readout of naturally weakly coupled qubits, and the possibility to protect strongly coupled qubits from decoherence by weakening their coupling. Unlike other approaches exploiting squeezing, our setup avoids the difficult task of transporting and injecting with high fidelity an externally-generated squeezed state. Our setup is also surprisingly robust against unwanted non-QND backaction effects, as interference naturally suppresses Purcell decay: the system acts as its own Purcell filter. Our setup is compatible with the experimental state-of-the-art in circuit QED systems, but the basic idea could also be realized in other systems.

Coherent Feedback Improved Qubit Initialization in the Dispersive Regime

  1. Luke C.G. Govia,
  2. and Frank K. Wilhelm
Readout of the state of a superconducting qubit by homodyne detection of the output signal from a dispersively coupled microwave resonator is a common technique in circuit quantum electrodynamics,
and is often claimed to be quantum non-demolition (QND) up to the same order of approximation as in the dispersive approximation. However, in this work we show that only in the limit of infinite measurement time is this protocol QND, as the formation of a dressed coherent state in the qubit-cavity system applies an effective rotation to the qubit state. We show how this rotation can be corrected by a coherent operation, leading to improved qubit initialization by measurement and coherent feedback.

Scalable two- and four-qubit parity measurement with a threshold photon counter

  1. Luke C.G. Govia,
  2. Emily J. Pritchett,
  3. B. L. T. Plourde,
  4. Maxim G. Vavilov,
  5. R. McDermott,
  6. and Frank K. Wilhelm
Parity measurement is a central tool to many quantum information processing tasks. In this Letter, we propose a method to directly measure two- and four-qubit parity with low overhead
in hard- and software, while remaining robust to experimental imperfections. Our scheme relies on dispersive qubit-cavity coupling and photon counting that is sensitive only to intensity; both ingredients are widely realized in many different quantum computing modalities. For a leading technology in quantum computing, superconducting integrated circuits, we analyze the measurement contrast and the back action of the scheme and show that this measurement comes close enough to an ideal parity measurement to be applicable to quantum error correction.

High-fidelity qubit measurement with a microwave photon counter

  1. Luke C.G. Govia,
  2. Emily J. Pritchett,
  3. Canran Xu,
  4. B. L. T. Plourde,
  5. Maxim G. Vavilov,
  6. Frank K. Wilhelm,
  7. and R. McDermott
High-fidelity, efficient quantum nondemolition readout of quantum bits is integral to the goal of quantum computation. As superconducting circuits approach the requirements of scalable,
universal fault tolerance, qubit readout must also meet the demand of simplicity to scale with growing system size. Here we propose a fast, high-fidelity, scalable measurement scheme based on the state-selective ring-up of a cavity followed by photodetection with the recently introduced Josephson photomultiplier (JPM), a current-biased Josephson junction. This scheme maps qubit state information to the binary digital output of the JPM, circumventing the need for room-temperature heterodyne detection and offering the possibility of a cryogenic interface to superconducting digital control circuitry. Numerics show that measurement contrast in excess of 95% is achievable in a measurement time of 140 ns. We discuss perspectives to scale this scheme to enable readout of multiple qubit channels with a single JPM.

Theory of Josephson Photomultipliers: Optimal Working Conditions and Back Action

  1. Luke C.G. Govia,
  2. Emily J. Pritchett,
  3. Seth T. Merkel,
  4. Deanna Pineau,
  5. and Frank K. Wilhelm
We describe the back action of microwave-photon detection via a Josephson photomultiplier (JPM), a superconducting qubit coupled strongly to a high-quality microwave cavity. The back
action operator depends qualitatively on the duration of the measurement interval, resembling the regular photon annihilation operator at short interaction times and approaching a variant of the photon subtraction operator at long times. The optimal operating conditions of the JPM differ from those considered optimal for processing and storing of quantum information, in that a short $T_2$ of the JPM suppresses the cavity dephasing incurred during measurement. Understanding this back action opens the possibility to perform multiple JPM measurements on the same state, hence performing efficient state tomography.