Coherent oscillations in a quantum manifold stabilized by dissipation

  1. S. Touzard,
  2. A. Grimm,
  3. Z. Leghtas,
  4. S. O. Mundhada,
  5. P. Reinhold,
  6. R. Heeres,
  7. C. Axline,
  8. M. Reagor,
  9. K. Chou,
  10. J. Blumoff,
  11. K. M. Sliwa,
  12. S. Shankar,
  13. L. Frunzio,
  14. R. J. Schoelkopf,
  15. M. Mirrahimi,
  16. and M.H. Devoret
The quantum Zeno effect (QZE) is the apparent freezing of a quantum system in one state under the influence of a continuous observation. It has been further generalized to the stabilization
of a manifold spanned by multiple quantum states. In that case, motion inside the manifold can subsist and can even be driven by the combination of a dissipative stabilization and an external force. A superconducting microwave cavity that exchanges pairs of photons with its environments constitutes an example of a system which displays a stabilized manifold spanned by Schr\“odinger cat states. For this driven-dissipative system, the quantum Zeno stabilization transforms a simple linear drive into photon number parity oscillations within the stable cat state manifold. Without this stabilization, the linear drive would trivially displace the oscillator state and push it outside of the manifold. However, the observation of this effect is experimentally challenging. On one hand, the adiabaticity condition requires the oscillations to be slow compared to the manifold stabilization rate. On the other hand, the oscillations have to be fast compared with the coherence timescales within the stabilized manifold. Here, we implement the stabilization of a manifold spanned by Schr\“odinger cat states at a rate that exceeds the main source of decoherence by two orders of magnitude, and we show Zeno-driven coherent oscillations within this manifold. While related driven manifold dynamics have been proposed and observed, the non-linear dissipation specific to our experiment adds a crucial element: any drift out of the cat state manifold is projected back into it. The coherent oscillations of parity observed in this work are analogous to the Rabi rotation of a qubit protected against phase-flips and are likely to become part of the toolbox in the construction of a fault-tolerant logical qubit.

Micromachined integrated quantum circuit containing a superconducting qubit

  1. T. Brecht,
  2. Y. Chu,
  3. C. Axline,
  4. W. Pfaff,
  5. J. Z. Blumoff,
  6. K. Chou,
  7. L. Krayzman,
  8. L. Frunzio,
  9. and R. J. Schoelkopf
We present a device demonstrating a lithographically patterned transmon integrated with a micromachined cavity resonator. Our two-cavity, one-qubit device is a multilayer microwave
integrated quantum circuit (MMIQC), comprising a basic unit capable of performing circuit-QED (cQED) operations. We describe the qubit-cavity coupling mechanism of a specialized geometry using an electric field picture and a circuit model, and finally obtain specific system parameters using simulations. Fabrication of the MMIQC includes lithography, etching, and metallic bonding of silicon wafers. Superconducting wafer bonding is a critical capability that is demonstrated by a micromachined storage cavity lifetime 34.3 μs, corresponding to a quality factor of 2 million at single-photon energies. The transmon coherence times are T1=6.4 μs, and TEcho2=11.7 μs. We measure qubit-cavity dispersive coupling with rate χqμ/2π=−1.17 MHz, constituting a Jaynes-Cummings system with an interaction strength g/2π=49 MHz. With these parameters we are able to demonstrate cQED operations in the strong dispersive regime with ease. Finally, we highlight several improvements and anticipated extensions of the technology to complex MMIQCs.

Suspending superconducting qubits by silicon micromachining

  1. Y. Chu,
  2. C. Axline,
  3. C. Wang,
  4. T. Brecht,
  5. Y. Y. Gao,
  6. L. Frunzio,
  7. and R. J. Schoelkopf
We present a method for relieving aluminum 3D transmon qubits from a silicon substrate using micromachining. Our technique is a high yield, one-step deep reactive ion etch that requires
no additional fabrication processes, and results in the suspension of the junction area and edges of the aluminum film. The drastic change in the device geometry affects both the dielectric and flux noise environment experienced by the qubit. In particular, the participation ratios of various dielectric interfaces are significantly modified, and suspended qubits exhibited longer T1’s than non-suspended ones. We also find that suspension increases the flux noise experienced by tunable SQUID-based qubits.

Implementing and characterizing precise multi-qubit measurements

  1. J. Z. Blumoff,
  2. K. Chou,
  3. C. Shen,
  4. M. Reagor,
  5. C. Axline,
  6. R. T. Brierley,
  7. M. P. Silveri,
  8. C. Wang,
  9. B. Vlastakis,
  10. S. E. Nigg,
  11. L. Frunzio,
  12. M. H. Devoret,
  13. L. Jiang,
  14. S. M. Girvin,
  15. and R. J. Schoelkopf
There are two general requirements to harness the computational power of quantum mechanics: the ability to manipulate the evolution of an isolated system and the ability to faithfully
extract information from it. Quantum error correction and simulation often make a more exacting demand: the ability to perform non-destructive measurements of specific correlations within that system. We realize such measurements by employing a protocol adapted from [S. Nigg and S. M. Girvin, Phys. Rev. Lett. 110, 243604 (2013)], enabling real-time selection of arbitrary register-wide Pauli operators. Our implementation consists of a simple circuit quantum electrodynamics (cQED) module of four highly-coherent 3D transmon qubits, collectively coupled to a high-Q superconducting microwave cavity. As a demonstration, we enact all seven nontrivial subset-parity measurements on our three-qubit register. For each we fully characterize the realized measurement by analyzing the detector (observable operators) via quantum detector tomography and by analyzing the quantum back-action via conditioned process tomography. No single quantity completely encapsulates the performance of a measurement, and standard figures of merit have not yet emerged. Accordingly, we consider several new fidelity measures for both the detector and the complete measurement process. We measure all of these quantities and report high fidelities, indicating that we are measuring the desired quantities precisely and that the measurements are highly non-demolition. We further show that both results are improved significantly by an additional error-heralding measurement. The analyses presented here form a useful basis for the future characterization and validation of quantum measurements, anticipating the demands of emerging quantum technologies.

Ten Milliseconds for Aluminum Cavities in the Quantum Regime

  1. M. Reagor,
  2. Hanhee Paik,
  3. G. Catelani,
  4. L. Sun,
  5. C. Axline,
  6. E. Holland,
  7. I.M. Pop,
  8. N.A. Masluk,
  9. T. Brecht,
  10. L. Frunzio,
  11. M.H. Devoret,
  12. L.I. Glazman,
  13. and R. J. Schoelkopf
A promising quantum computing architecture couples superconducting qubits to microwave resonators (circuit QED), a system in which three-dimensional microwave cavities have become a
valuable resource. Such cavities have surface-to-volume ratios, or participation ratios a thousandfold smaller than in planar devices, deemphasizing potentially lossy surface elements by an equal amount. Motivated by this principle, we have tested aluminum superconducting cavity resonators with internal quality factors greater than 0.5 billion and intrinsic lifetimes reaching 0.01 seconds at single photon power and millikelvin temperatures. These results are the first to explore the use of superconducting aluminum, a ubiquitous material in circuit QED, as the basis of highly coherent (Q~10^7-10^9) cavity resonators. Measurements confirm the cavities‘ predicted insensitivity to quasiparticles (kinetic inductance fraction-5ppm) and an absence of two level dielectric fluctuations.