Holographic Gaussian Boson Sampling with Matrix Product States on 3D cQED Processors

  1. Ningyi Lyu,
  2. Paul Bergold,
  3. Micheline B. Soley,
  4. Chen Wang,
  5. and Victor S. Batista
We introduce quantum circuits for simulations of multi-mode state-vectors on 3D cQED processors, using matrix product state representations. The circuits are demonstrated as applied
to simulations of molecular docking based on holographic Gaussian boson sampling, as illustrated for binding of a thiol-containing aryl sulfonamide ligand to the tumor necrosis factor-α converting enzyme receptor. We show that cQED devices with a modest number of modes could be employed to simulate multimode systems by re-purposing working modes through measurement and re-initialization. We anticipate a wide range of GBS applications could be implemented on compact 3D cQED processors analogously, using the holographic approach. Simulations on qubit-based quantum computers could be implemented analogously, using circuits that represent continuous variables in terms of truncated expansions of Fock states.

Observation of discrete charge states of a coherent two-level system in a superconducting qubit

  1. Bao-Jie Liu,
  2. Ying-Ying Wang,
  3. Tal Sheffer,
  4. and Chen Wang
We report observations of discrete charge states of a coherent dielectric two-level system (TLS) that is strongly coupled to an offset-charge-sensitive superconducting transmon qubit.
We measure an offset charge of 0.072e associated with the two TLS eigenstates, which have a transition frequency of 2.9 GHz and a relaxation time exceeding 3 ms. Combining measurements in the strong dispersive and resonant regime, we quantify both transverse and longitudinal couplings of the TLS-qubit interaction. We further perform joint tracking of TLS transitions and quasiparticle tunneling dynamics but find no intrinsic correlations. This study demonstrates microwave-frequency TLS as a source of low-frequency charge noise.

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.

Controlled-NOT gates for fluxonium qubits via selective darkening of transitions

  1. Konstantin N. Nesterov,
  2. Chen Wang,
  3. Vladimir E. Manucharyan,
  4. and Maxim G. Vavilov
We analyze the cross-resonance effect for fluxonium circuits and investigate a two-qubit gate scheme based on selective darkening of a transition. In this approach, two microwave pulses
at the frequency of the target qubit are applied simultaneously with a proper ratio between their amplitudes to achieve a controlled-NOT operation. We study in detail coherent gate dynamics and calculate gate error. With nonunitary effects accounted for, we demonstrate that gate error below 10−4 is possible for realistic hardware parameters. This number is facilitated by long coherence times of computational transitions and strong anharmonicity of fluxoniums, which easily prevents excitation to higher excited states during the gate microwave drive.

A practical guide for building superconducting quantum devices

  1. Yvonne Y. Gao,
  2. M. Adriaan Rol,
  3. Steven Touzard,
  4. and Chen Wang
Quantum computing offers a powerful new paradigm of information processing that has the potential to transform a wide range of industries. In the pursuit of the tantalizing promises
of a universal quantum computer, a multitude of new knowledge and expertise has been developed, enabling the construction of novel quantum algorithms as well as increasingly robust quantum hardware. In particular, we have witnessed rapid progress in the circuit quantum electrodynamics (cQED) technology, which has emerged as one of the most promising physical systems that is capable of addressing the key challenges in realizing full-stack quantum computing on a large scale. In this article, we present some of the most crucial building blocks developed by the cQED community in recent years and a précis of the latest achievements towards robust universal quantum computation. More importantly, we aim to provide a synoptic outline of the core techniques that underlie most cQED experiments and offer a practical guide for a novice experimentalist to design, construct, and characterize their first quantum device

Arbitrary controlled-phase gate on fluxonium qubits using differential ac-Stark shifts

  1. Haonan Xiong,
  2. Quentin Ficheux,
  3. Aaron Somoroff,
  4. Long B. Nguyen,
  5. Ebru Dogan,
  6. Dario Rosenstock,
  7. Chen Wang,
  8. Konstantin N. Nesterov,
  9. Maxim G. Vavilov,
  10. and Vladimir E. Manucharyan
Large scale quantum computing motivates the invention of two-qubit gate schemes that not only maximize the gate fidelity but also draw minimal resources. In the case of superconducting
qubits, the weak anharmonicity of transmons imposes profound constraints on the gate design, leading to increased complexity of devices and control protocols. Here we demonstrate a resource-efficient control over the interaction of strongly-anharmonic fluxonium qubits. Namely, applying an off-resonant drive to non-computational transitions in a pair of capacitively-coupled fluxoniums induces a ZZ-interaction due to unequal ac-Stark shifts of the computational levels. With a continuous choice of frequency and amplitude, the drive can either cancel the static ZZ-term or increase it by an order of magnitude to enable a controlled-phase (CP) gate with an arbitrary programmed phase shift. The cross-entropy benchmarking of these non-Clifford operations yields a sub 1% error, limited solely by incoherent processes. Our result demonstrates the advantages of strongly-anharmonic circuits over transmons in designing the next generation of quantum processors.

Microwave-Activated Controlled-Z Gate for Fixed-Frequency Fluxonium Qubits

  1. Konstantin N. Nesterov,
  2. Ivan V. Pechenezhskiy,
  3. Chen Wang,
  4. Vladimir E. Manucharyan,
  5. and Maxim G. Vavilov
The superconducting fluxonium circuit is an artificial atom with a strongly anharmonic spectrum: when biased at a half flux quantum, the lowest qubit transition is an order of magnitude
smaller in frequency than those to higher levels. Similar to conventional atomic systems, such a frequency separation between the computational and noncomputational subspaces allows independent optimizations of the qubit coherence and two-qubit interactions. Here we describe a controlled-Z gate for two fluxoniums connected either capacitively or inductively, with qubit transitions fixed near 500 MHz. The gate is activated by a microwave drive at a resonance involving the second excited state. We estimate intrinsic gate fidelities over 99.9% with gate times below 100 ns.

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.

A coaxial line architecture for integrating and scaling 3D cQED systems

  1. Christopher Axline,
  2. Matthew Reagor,
  3. Reinier W. Heeres,
  4. Philip Reinhold,
  5. Chen Wang,
  6. Kevin Shain,
  7. Wolfgang Pfaff,
  8. Yiwen Chu,
  9. Luigi Frunzio,
  10. and Robert J. Schoelkopf
Numerous loss mechanisms can limit coherence and scalability of planar and 3D-based circuit quantum electrodynamics (cQED) devices, particularly due to their packaging. The low loss
and natural isolation of 3D enclosures make them good candidates for coherent scaling. We introduce a coaxial transmission line device architecture with coherence similar to traditional 3D cQED systems. Measurements demonstrate well-controlled external and on-chip couplings, a spectrum absent of cross-talk or spurious modes, and excellent resonator and qubit lifetimes. We integrate a resonator-qubit system in this architecture with a seamless 3D cavity, and separately pattern a qubit, readout resonator, Purcell filter and high-Q stripline resonator on a single chip. Device coherence and its ease of integration make this a promising tool for complex experiments.

A Schrodinger Cat Living in Two Boxes

  1. Chen Wang,
  2. Yvonne Y. Gao,
  3. Philip Reinhold,
  4. R. W. Heeres,
  5. Nissim Ofek,
  6. Kevin Chou,
  7. Christopher Axline,
  8. Matthew Reagor,
  9. Jacob Blumoff,
  10. K. M. Sliwa,
  11. L. Frunzio,
  12. S. M. Girvin,
  13. Liang Jiang,
  14. M. Mirrahimi,
  15. M. H. Devoret,
  16. and R. J. Schoelkopf
Quantum superpositions of distinct coherent states in a single-mode harmonic oscillator, known as „cat states“, have been an elegant demonstration of Schrodinger’s
famous cat paradox. Here, we realize a two-mode cat state of electromagnetic fields in two microwave cavities bridged by a superconducting artificial atom, which can also be viewed as an entangled pair of single-cavity cat states. We present full quantum state tomography of this complex cat state over a Hilbert space exceeding 100 dimensions via quantum non-demolition measurements of the joint photon number parity. The ability to manipulate such multi-cavity quantum states paves the way for logical operations between redundantly encoded qubits for fault-tolerant quantum computation and communication.