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.

Surface participation and dielectric loss in superconducting qubits

  1. Chen Wang,
  2. Christopher Axline,
  3. Yvonne Y. Gao,
  4. Teresa Brecht,
  5. Luigi Frunzio,
  6. Michel H. Devoret,
  7. and Robert J. Schoelkopf
We study the energy relaxation times (T1) of superconducting transmon qubits in 3D cavities as a function of dielectric participation ratios of material surfaces. This surface participation
ratio, representing the fraction of electric field energy stored in a dissipative surface layer, is computed by a two-step finite-element simulation and experimentally varied by qubit geometry. With a clean electromagnetic environment and suppressed non-equilibrium quasiparticle density, we find an approximately proportional relation between the transmon relaxation rates and surface participation ratios. These results suggest dielectric dissipation arising from material interfaces is the major limiting factor for the T1 of transmons in 3D cQED architecture. Our analysis also supports the notion of spatial discreteness of surface dielectric dissipation.

A quantum memory with near-millisecond coherence in circuit QED

  1. Matthew Reagor,
  2. Wolfgang Pfaff,
  3. Christopher Axline,
  4. Reinier W. Heeres,
  5. Nissim Ofek,
  6. Katrina Sliwa,
  7. Eric Holland,
  8. Chen Wang,
  9. Jacob Blumoff,
  10. Kevin Chou,
  11. Michael J. Hatridge,
  12. Luigi Frunzio,
  13. Michel H. Devoret,
  14. Liang Jiang,
  15. and Robert J. Schoelkopf
Significant advances in coherence have made superconducting quantum circuits a viable platform for fault-tolerant quantum computing. To further extend capabilities, highly coherent
quantum systems could act as quantum memories for these circuits. A useful quantum memory must be rapidly addressable by qubits, while maintaining superior coherence. We demonstrate a novel superconducting microwave cavity architecture that is highly robust against major sources of loss that are encountered in the engineering of circuit QED systems. The architecture allows for near-millisecond storage of quantum states in a resonator while strong coupling between the resonator and a transmon qubit enables control, encoding, and readout at MHz rates. The observed coherence times constitute an improvement of almost an order of magnitude over those of the best available superconducting qubits. Our design is an ideal platform for studying coherent quantum optics and marks an important step towards hardware-efficient quantum computing with Josephson junction-based quantum circuits.

Measurement and Control of Quasiparticle Dynamics in a Superconducting Qubit

  1. Chen Wang,
  2. Yvonne Y. Gao,
  3. Ioan M. Pop,
  4. Uri Vool,
  5. Chris Axline,
  6. Teresa Brecht,
  7. Reinier W. Heeres,
  8. Luigi Frunzio,
  9. Michel H. Devoret,
  10. Gianluigi Catelani,
  11. Leonid I. Glazman,
  12. and Robert J. Schoelkopf
Superconducting circuits have attracted growing interest in recent years as a promising candidate for fault-tolerant quantum information processing. Extensive efforts have always been
taken to completely shield these circuits from external magnetic field to protect the integrity of superconductivity. Surprisingly, here we show vortices can dramatically improve the performance of superconducting qubits by reducing the lifetimes of detrimental single-electron-like excitations known as quasiparticles. Using a contactless injection technique with unprecedented dynamic range, we directly demonstrate the power-law decay characteristics of the canonical quasiparticle recombination process, and show quantization of quasiparticle trapping rate due to individual vortices. Each vortex in our aluminium film shows a quasiparticle „trapping power“ of 0.067±0.005 cm2/s, enough to dominate over the vanishingly weak recombination in a modern transmon qubit. These results highlight the prominent role of quasiparticle trapping in future development of quantum circuits, and provide a powerful characterization tool along the way.

Non-Poissonian Quantum Jumps of a Fluxonium Qubit due to Quasiparticle Excitations

  1. Uri Vool,
  2. Ioan M. Pop,
  3. Katrina Sliwa,
  4. Baleegh Abdo,
  5. Chen Wang,
  6. Teresa Brecht,
  7. Yvonne Y. Gao,
  8. Shyam Shankar,
  9. Michael Hatridge,
  10. Gianluigi Catelani,
  11. Mazyar Mirrahimi,
  12. Luigi Frunzio,
  13. Robert J. Schoelkopf,
  14. Leonid I. Glazman,
  15. and Michel H. Devoret
As the energy relaxation time of superconducting qubits steadily improves, non-equilibrium quasiparticle excitations above the superconducting gap emerge as an increasingly relevant
limit for qubit coherence. We measure fluctuations in the number of quasiparticle excitations by continuously monitoring the spontaneous quantum jumps between the states of a fluxonium qubit, in conditions where relaxation is dominated by quasiparticle loss. Resolution on the scale of a single quasiparticle is obtained by performing quantum non-demolition projective measurements within a time interval much shorter than T1, using a quantum limited amplifier (Josephson Parametric Converter). The quantum jumps statistics switches between the expected Poisson distribution and a non-Poissonian one, indicating large relative fluctuations in the quasiparticle population, on time scales varying from seconds to hours. This dynamics can be modified controllably by injecting quasiparticles or by seeding quasiparticle-trapping vortices by cooling down in magnetic field.