Real-time quantum error correction beyond break-even

  1. V. V. Sivak,
  2. A. Eickbusch,
  3. B. Royer,
  4. S. Singh,
  5. I. Tsioutsios,
  6. S. Ganjam,
  7. A. Miano,
  8. B. L. Brock,
  9. A. Z. Ding,
  10. L. Frunzio,
  11. S. M. Girvin,
  12. R. J. Schoelkopf,
  13. and M. H. Devoret
The ambition of harnessing the quantum for computation is at odds with the fundamental phenomenon of decoherence. The purpose of quantum error correction (QEC) is to counteract the
natural tendency of a complex system to decohere. This cooperative process, which requires participation of multiple quantum and classical components, creates a special type of dissipation that removes the entropy caused by the errors faster than the rate at which these errors corrupt the stored quantum information. Previous experimental attempts to engineer such a process faced an excessive generation of errors that overwhelmed the error-correcting capability of the process itself. Whether it is practically possible to utilize QEC for extending quantum coherence thus remains an open question. We answer it by demonstrating a fully stabilized and error-corrected logical qubit whose quantum coherence is significantly longer than that of all the imperfect quantum components involved in the QEC process, beating the best of them with a coherence gain of G=2.27±0.07. We achieve this performance by combining innovations in several domains including the fabrication of superconducting quantum circuits and model-free reinforcement learning.

Frequency-tunable Kerr-free three-wave mixing with a gradiometric SNAIL

  1. A. Miano,
  2. G. Liu,
  3. V. V. Sivak,
  4. N. E. Frattini,
  5. V. R. Joshi,
  6. W. Dai,
  7. L. Frunzio,
  8. and M. H. Devoret
Three-wave mixing is a key process in superconducting quantum information processing, being involved in quantum-limited amplification and parametric coupling between superconducting
cavities. These operations can be implemented by SNAIL-based devices that present a Kerr-free flux-bias point where unwanted parasitic effects such as Stark shift are suppressed. However, with a single flux-bias parameter, these circuits can only host one Kerr-free point, limiting the range of their applications. In this Letter, we demonstrate how to overcome this constraint with a gradiometric SNAIL, a doubly-flux biased superconducting circuit for which both effective inductance and Kerr coefficient can be independently tuned. Experimental data show the capability of the gradiometric SNAIL to suppress Kerr effect in a three-wave mixing parametric amplifier over a continuum of flux bias points corresponding to a 1.7 GHz range of operating frequencies.

Ac losses in field-cooled type I superconducting cavities

  1. G. Catelani,
  2. K. Li,
  3. C. J. Axline,
  4. T. Brecht,
  5. L. Frunzio,
  6. R. J. Schoelkopf,
  7. and L. I. Glazman
As superconductors are cooled below their critical temperature, stray magnetic flux can become trapped in regions that remain normal. The presence of trapped flux facilitates dissipation
of ac current in a superconductor, leading to losses in superconducting elements of microwave devices. In type II superconductors, dissipation is well-understood in terms of the dynamics of vortices hosting a single flux quantum. In contrast, the ac response of type I superconductors with trapped flux has not received much attention. Building on Andreev’s early work [Sov. Phys. JETP 24, 1019 (1967)], here we show theoretically that the dominant dissipation mechanism is the absorption of the ac field at the exposed surfaces of the normal regions, while the deformation of the superconducting/normal interfaces is unimportant. We use the developed theory to estimate the degradation of the quality factors in field-cooled cavities, and we satisfactorily compare these theoretical estimates to the measured field dependence of the quality factors of two aluminum cavities.

Free-standing silicon shadow masks for transmon qubit fabrication

  1. I. Tsioutsios,
  2. K. Serniak,
  3. S. Diamond,
  4. Z. Wang,
  5. S. Shankar,
  6. L. Frunzio,
  7. R. J. Schoelkopf,
  8. and M. H. Devoret
Nanofabrication techniques for superconducting qubits rely on resist-based masks patterned by electron-beam or optical lithography. We have developed an alternative nanofabrication
technique based on free-standing silicon shadow masks fabricated from silicon-on-insulator wafers. These silicon shadow masks not only eliminate organic residues associated with resist-based lithography, but also provide a pathway to better understand and control surface-dielectric losses in superconducting qubits by decoupling mask fabrication from substrate preparation. We have successfully fabricated aluminum 3D transmon superconducting qubits with these shadow masks, and demonstrated energy relaxation times on par with state-of-the-art values.

A stabilized logical quantum bit encoded in grid states of a superconducting cavity

  1. P. Campagne-Ibarcq,
  2. A. Eickbusch,
  3. S. Touzard,
  4. E. Zalys-Geller,
  5. N. E. Frattini,
  6. V. V. Sivak,
  7. P. Reinhold,
  8. S. Puri,
  9. S. Shankar,
  10. R. J. Schoelkopf,
  11. L. Frunzio,
  12. M. Mirrahimi,
  13. and M.H. Devoret
The majority of quantum information tasks require error-corrected logical qubits whose coherence times are vastly longer than that of currently available physical qubits. Among the
many quantum error correction codes, bosonic codes are particularly attractive as they make use of a single quantum harmonic oscillator to encode a correctable qubit in a hardware-efficient manner. One such encoding, based on grid states of an oscillator, has the potential to protect a logical qubit against all major physical noise processes. By stroboscopically modulating the interaction of a superconducting microwave cavity with an ancillary transmon, we have successfully prepared and permanently stabilized these grid states. The lifetimes of the three Bloch vector components of the encoded qubit are enhanced by the application of this protocol, and agree with a theoretical estimate based on the measured imperfections of the experiment.

Quantum back-action of variable-strength measurement

  1. M. Hatridge,
  2. S. Shankar,
  3. M. Mirrahimi,
  4. F. Schackert,
  5. K. Geerlings,
  6. T. Brecht,
  7. K. M. Sliwa,
  8. B. Abdo,
  9. L. Frunzio,
  10. S. M. Girvin,
  11. R. J. Schoelkopf,
  12. and M. H. Devoret
Measuring a quantum system can randomly perturb its state. The strength and nature of this back-action depends on the quantity which is measured. In a partial measurement performed
by an ideal apparatus, quantum physics predicts that the system remains in a pure state whose evolution can be tracked perfectly from the measurement record. We demonstrate this property using a superconducting qubit dispersively coupled to a cavity traversed by a microwave signal. The back-action on the qubit state of a single measurement of both signal quadratures is observed and shown to produce a stochastic operation whose action is determined by the measurement result. This accurate monitoring of a qubit state is an essential prerequisite for measurement-based feedback control of quantum systems.

Gated conditional displacement readout of superconducting qubits

  1. S. Touzard,
  2. A. Kou,
  3. N. E. Frattini,
  4. V. V. Sivak,
  5. S. Puri,
  6. A. Grimm,
  7. L. Frunzio,
  8. S. Shankar,
  9. and M.H. Devoret
We have realized a new interaction between superconducting qubits and a readout cavity that results in the displacement of a coherent state in the cavity, conditioned on the state of
the qubit. This conditional state, when it reaches the cavity-following, phase-sensitive amplifier, matches its measured observable, namely the in-phase quadrature. In a setup where several qubits are coupled to the same readout resonator, we show it is possible to measure the state of a target qubit with minimal dephasing of the other qubits. Our results suggest novel directions for faster readout of superconducting qubits and implementations of bosonic quantum error-correcting codes.

Hot non-equilibrium quasiparticles in transmon qubits

  1. K. Serniak,
  2. M. Hays,
  3. G. de Lange,
  4. S. Diamond,
  5. S. Shankar,
  6. L. D. Burkhart,
  7. L. Frunzio,
  8. M. Houzet,
  9. and M. H. Devoret
Non-equilibrium quasiparticle excitations degrade the performance of a variety of superconducting circuits. Understanding the energy distribution of these quasiparticles will yield
insight into their generation mechanisms, the limitations they impose on superconducting devices, and how to efficiently mitigate quasiparticle-induced qubit decoherence. To probe this energy distribution, we directly correlate qubit transitions with charge-parity switches in an offset-charge-sensitive transmon qubit, and find that quasiparticle-induced excitation events are the dominant mechanism behind the residual excited-state population in our samples. The observed quasiparticle distribution would limit T1 to ≈200 μs, which indicates that quasiparticle loss in our devices is on equal footing with all other loss mechanisms. Furthermore, the measured rate of quasiparticle-induced excitation events is greater than that of relaxation events, which signifies that the quasiparticles are more energetic than would be predicted from a thermal distribution describing their apparent density.

Fault-tolerant measurement of a quantum error syndrome

  1. S. Rosenblum,
  2. P. Reinhold,
  3. M. Mirrahimi,
  4. Liang Jiang,
  5. L. Frunzio,
  6. and R. J. Schoelkopf
Quantum error correction can allow quantum computers to operate despite the presence of noise and imperfections. A critical component of any error correcting scheme is the mapping of
error syndromes onto an ancillary measurement system. However, errors occurring in the ancilla can propagate onto the logical qubit, and irreversibly corrupt the encoded information. Here, we demonstrate a fault-tolerant syndrome measurement scheme that dramatically suppresses forward propagation of ancilla errors. We achieve an eightfold reduction of the logical error probability per measurement, while maintaining the syndrome assignment fidelity. We use the same method to prevent the propagation of thermal ancilla excitations, increasing the logical qubit dephasing time by more than an order of magnitude. Our approach is hardware-efficient, as it uses a single multilevel transmon ancilla and a cavity-encoded logical qubit, whose interaction is engineered in situ using an off-resonant sideband drive. These results demonstrate that hardware-efficient approaches which exploit system-specific error models can yield practical advances towards fault-tolerant quantum computation.

Programmable interference between two microwave quantum memories

  1. Yvonne Y. Gao,
  2. B. J. Lester,
  3. Yaxing Zhang,
  4. C. Wang,
  5. S. Rosenblum,
  6. L. Frunzio,
  7. Liang Jiang,
  8. S. M. Girvin,
  9. and R. J. Schoelkopf
Interference experiments provide a simple yet powerful tool to unravel fundamental features of quantum physics. Here we engineer an RF-driven, time-dependent bilinear coupling that
can be tuned to implement a robust 50:50 beamsplitter between stationary states stored in two superconducting cavities in a three-dimensional architecture. With this, we realize high contrast Hong-Ou- Mandel (HOM) interference between two spectrally-detuned stationary modes. We demonstrate that this coupling provides an efficient method for measuring the quantum state overlap between arbitrary states of the two cavities. Finally, we showcase concatenated beamsplitters and differential phase shifters to implement cascaded Mach-Zehnder interferometers, which can control the signature of the two-photon interference on-demand. Our results pave the way toward implementation of scalable boson sampling, the application of linear optical quantum computing (LOQC) protocols in the microwave domain, and quantum algorithms between long-lived bosonic memories.