Strategies and trade-offs for controllability and memory time of ultra-high-quality microwave cavities in circuit QED

  1. Iivari Pietikäinen,
  2. Ondřej Černotík,
  3. Alec Eickbusch,
  4. Aniket Maiti,
  5. John W.O. Garmon,
  6. Radim Filip,
  7. and Steven M. Girvin
Three-dimensional microwave cavity resonators have been shown to reach lifetimes of the order of a second by maximizing the cavity volume relative to its surface, using better materials,
and improving surface treatments. Such cavities represent an ideal platform for quantum computing with bosonic qubits, but their efficient control remains an outstanding problem since the large mode volume results in inefficient coupling to nonlinear elements used for their control. Moreover, this coupling induces additional cavity decay via the inverse Purcell effect which can easily destroy the advantage of {a} long intrinsic lifetime. Here, we discuss conditions on, and protocols for, efficient utilization of these ultra-high-quality microwave cavities as memories for conventional superconducting qubits. We show that, surprisingly, efficient write and read operations with ultra-high-quality cavities does not require similar quality factors for the qubits and other nonlinear elements used to control them. Through a combination of analytical and numerical calculations, we demonstrate that efficient coupling to cavities with second-scale lifetime is possible with state-of-the-art transmon and SNAIL devices and outline a route towards controlling cavities with even higher quality factors. Our work explores a potentially viable roadmap towards using ultra-high-quality microwave cavity resonators for storing and processing information encoded in bosonic qubits.

A high on-off ratio beamsplitter interaction for gates on bosonically encoded qubits

  1. Benjamin J. Chapman,
  2. Stijn J. de Graaf,
  3. Sophia H. Xue,
  4. Yaxing Zhang,
  5. James Teoh,
  6. Jacob C. Curtis,
  7. Takahiro Tsunoda,
  8. Alec Eickbusch,
  9. Alexander P. Read,
  10. Akshay Koottandavida,
  11. Shantanu O. Mundhada,
  12. Luigi Frunzio,
  13. M. H. Devoret,
  14. S. M. Girvin,
  15. and R. J. Schoelkopf
Encoding a qubit in a high quality superconducting microwave cavity offers the opportunity to perform the first layer of error correction in a single device, but presents a challenge:
how can quantum oscillators be controlled while introducing a minimal number of additional error channels? We focus on the two-qubit portion of this control problem by using a 3-wave mixing coupling element to engineer a programmable beamsplitter interaction between two bosonic modes separated by more than an octave in frequency, without introducing major additional sources of decoherence. Combining this with single-oscillator control provided by a dispersively coupled transmon provides a framework for quantum control of multiple encoded qubits. The beamsplitter interaction gbs is fast relative to the timescale of oscillator decoherence, enabling over 103 beamsplitter operations per coherence time, and approaching the typical rate of the dispersive coupling χ used for individual oscillator control. Further, the programmable coupling is engineered without adding unwanted interactions between the oscillators, as evidenced by the high on-off ratio of the operations, which can exceed 105. We then introduce a new protocol to realize a hybrid controlled-SWAP operation in the regime gbs≈χ, in which a transmon provides the control bit for the SWAP of two bosonic modes. Finally, we use this gate in a SWAP test to project a pair of bosonic qubits into a Bell state with measurement-corrected fidelity of 95.5%±0.2%.

Architectures for Multinode Superconducting Quantum Computers

  1. James Ang,
  2. Gabriella Carini,
  3. Yanzhu Chen,
  4. Isaac Chuang,
  5. Michael Austin DeMarco,
  6. Sophia E. Economou,
  7. Alec Eickbusch,
  8. Andrei Faraon,
  9. Kai-Mei Fu,
  10. Steven M. Girvin,
  11. Michael Hatridge,
  12. Andrew Houck,
  13. Paul Hilaire,
  14. Kevin Krsulich,
  15. Ang Li,
  16. Chenxu Liu,
  17. Yuan Liu,
  18. Margaret Martonosi,
  19. David C. McKay,
  20. James Misewich,
  21. Mark Ritter,
  22. Robert J. Schoelkopf,
  23. Samuel A. Stein,
  24. Sara Sussman,
  25. Hong X. Tang,
  26. Wei Tang,
  27. Teague Tomesh,
  28. Norm M. Tubman,
  29. Chen Wang,
  30. Nathan Wiebe,
  31. Yong-Xin Yao,
  32. Dillon C. Yost,
  33. and Yiyu Zhou
Many proposals to scale quantum technology rely on modular or distributed designs where individual quantum processors, called nodes, are linked together to form one large multinode
quantum computer (MNQC). One scalable method to construct an MNQC is using superconducting quantum systems with optical interconnects. However, a limiting factor of these machines will be internode gates, which may be two to three orders of magnitude noisier and slower than local operations. Surmounting the limitations of internode gates will require a range of techniques, including improvements in entanglement generation, the use of entanglement distillation, and optimized software and compilers, and it remains unclear how improvements to these components interact to affect overall system performance, what performance from each is required, or even how to quantify the performance of each. In this paper, we employ a `co-design‘ inspired approach to quantify overall MNQC performance in terms of hardware models of internode links, entanglement distillation, and local architecture. In the case of superconducting MNQCs with microwave-to-optical links, we uncover a tradeoff between entanglement generation and distillation that threatens to degrade performance. We show how to navigate this tradeoff, lay out how compilers should optimize between local and internode gates, and discuss when noisy quantum links have an advantage over purely classical links. Using these results, we introduce a roadmap for the realization of early MNQCs which illustrates potential improvements to the hardware and software of MNQCs and outlines criteria for evaluating the landscape, from progress in entanglement generation and quantum memory to dedicated algorithms such as distributed quantum phase estimation. While we focus on superconducting devices with optical interconnects, our approach is general across MNQC implementations.

Fast Universal Control of an Oscillator with Weak Dispersive Coupling to a Qubit

  1. Alec Eickbusch,
  2. Volodymyr Sivak,
  3. Andy Z. Ding,
  4. Salvatore S. Elder,
  5. Shantanu R. Jha,
  6. Jayameenakshi Venkatraman,
  7. Baptiste Royer,
  8. S. M. Girvin,
  9. Robert J. Schoelkopf,
  10. and Michel H. Devoret
Efficient quantum control of an oscillator is necessary for many bosonic applications including error-corrected computation, quantum-enhanced sensing, robust quantum communication,
and quantum simulation. For these applications, oscillator control is often realized through off-resonant hybridization to a qubit with dispersive shift χ where typical operation times of 2π/χ are routinely assumed. Here, we challenge this assumption by introducing and demonstrating a novel control method with typical operation times over an order of magnitude faster than 2π/χ. Using large auxiliary displacements of the oscillator to enhance gate speed, we introduce a universal gate set with built-in dynamical decoupling consisting of fast conditional displacements and qubit rotations. We demonstrate the method using a superconducting cavity weakly coupled to a transmon qubit in a regime where previously known methods would fail. Our demonstrations include preparation of a single-photon state 30 times faster than 2π/χ with 98±1(%) fidelity and preparation of squeezed vacuum with a squeezing level of 11.1 dB, the largest intracavity squeezing reported in the microwave regime. Finally, we demonstrate fast measurement-free preparation of logical states for the binomial and Gottesman-Kitaev-Preskill (GKP) code, and we identify possible fidelity limiting mechanisms including oscillator dephasing.

Stabilized Cat in Driven Nonlinear Cavity: A Fault-Tolerant Error Syndrome Detector

  1. Shruti Puri,
  2. Alexander Grimm,
  3. Philippe Campagne-Ibarcq,
  4. Alec Eickbusch,
  5. Kyungjoo Noh,
  6. Gabrielle Roberts,
  7. Liang Jiang,
  8. Mazyar Mirrahimi,
  9. Michel H. Devoret,
  10. and Steven M. Girvin
low-weight operations with an ancilla to extract information about errors without causing backaction on the encoded system. Essentially, ancilla errors must not propagate to the encoded
system and induce errors beyond those which can be corrected. The current schemes for achieving this fault-tolerance to ancilla errors come at the cost of increased overhead requirements. An efficient way to extract error syndromes in a fault-tolerant manner is by using a single ancilla with strongly biased noise channel. Typically, however, required elementary operations can become challenging when the noise is extremely biased. We propose to overcome this shortcoming by using a bosonic-cat ancilla in a parametrically driven nonlinear cavity. Such a cat-qubit experiences only bit-flip noise and is stabilized against phase-flips. To highlight the flexibility of this approach, we illustrate the syndrome extraction process in a variety of codes such as qubit-based toric codes, bosonic cat- and Gottesman-Kitaev-Preskill (GKP) codes. Our results open a path for realizing hardware-efficient, fault-tolerant error syndrome extraction.