Using bi-fluxon tunneling to protect the Fluxonium qubit

  1. Waël Ardati,
  2. Sébastien Léger,
  3. Shelender Kumar,
  4. Vishnu Narayanan Suresh,
  5. Dorian Nicolas,
  6. Cyril Mori,
  7. Francesca D'Esposito,
  8. Tereza Vakhtel,
  9. Olivier Buisson,
  10. Quentin Ficheux,
  11. and Nicolas Roch
Encoding quantum information in quantum states with disjoint wave-function support and noise insensitive energies is the key behind the idea of qubit protection. While fully protected
qubits are expected to offer exponential protection against both energy relaxation and pure dephasing, simpler circuits may grant partial protection with currently achievable parameters. Here, we study a fluxonium circuit in which the wave-functions are engineered to minimize their overlap while benefiting from a first-order-insensitive flux sweet spot. Taking advantage of a large superinductance (L∼1 μH), our circuit incorporates a resonant tunneling mechanism at zero external flux that couples states with the same fluxon parity, thus enabling bifluxon tunneling. The states |0⟩ and |1⟩ are encoded in wave-functions with parities 0 and 1, respectively, ensuring a minimal form of protection against relaxation. Two-tone spectroscopy reveals the energy level structure of the circuit and the presence of 4π quantum-phase slips between different potential wells corresponding to m=±1 fluxons, which can be precisely described by a simple fluxonium Hamiltonian or by an effective bifluxon Hamiltonian. Despite suboptimal fabrication, the measured relaxation (T1=177±3 μs) and dephasing (TE2=75±5 μs) times not only demonstrate the relevance of our approach but also opens an alternative direction towards quantum computing using partially-protected fluxonium qubits.

Enhancing Dispersive Readout of Superconducting Qubits Through Dynamic Control of the Dispersive Shift: Experiment and Theory

  1. François Swiadek,
  2. Ross Shillito,
  3. Paul Magnard,
  4. Ants Remm,
  5. Christoph Hellings,
  6. Nathan Lacroix,
  7. Quentin Ficheux,
  8. Dante Colao Zanuz,
  9. Graham J. Norris,
  10. Alexandre Blais,
  11. Sebastian Krinner,
  12. and Andreas Wallraff
The performance of a wide range of quantum computing algorithms and protocols depends critically on the fidelity and speed of the employed qubit readout. Examples include gate sequences
benefiting from mid-circuit, real-time, measurement-based feedback, such as qubit initialization, entanglement generation, teleportation, and perhaps most importantly, quantum error correction. A prominent and widely-used readout approach is based on the dispersive interaction of a superconducting qubit strongly coupled to a large-bandwidth readout resonator, frequently combined with a dedicated or shared Purcell filter protecting qubits from decay. By dynamically reducing the qubit-resonator detuning and thus increasing the dispersive shift, we demonstrate a beyond-state-of-the-art two-state-readout error of only 0.25% in 100 ns integration time. Maintaining low readout-drive strength, we nearly quadruple the signal-to-noise ratio of the readout by doubling the readout mode linewidth, which we quantify by considering the hybridization of the readout-resonator and its dedicated Purcell-filter. We find excellent agreement between our experimental data and our theoretical model. The presented results are expected to further boost the performance of new and existing algorithms and protocols critically depending on high-fidelity, fast, mid-circuit measurements.

Calibration of Drive Non-Linearity for Arbitrary-Angle Single-Qubit Gates Using Error Amplification

  1. Stefania Lazăr,
  2. Quentin Ficheux,
  3. Johannes Herrmann,
  4. Ants Remm,
  5. Nathan Lacroix,
  6. Christoph Hellings,
  7. Francois Swiadek,
  8. Dante Colao Zanuz,
  9. Graham J. Norris,
  10. Mohsen Bahrami Panah,
  11. Alexander Flasby,
  12. Michael Kerschbaum,
  13. Jean-Claude Besse,
  14. Christopher Eichler,
  15. and Andreas Wallraff
The ability to execute high-fidelity operations is crucial to scaling up quantum devices to large numbers of qubits. However, signal distortions originating from non-linear components
in the control lines can limit the performance of single-qubit gates. In this work, we use a measurement based on error amplification to characterize and correct the small single-qubit rotation errors originating from the non-linear scaling of the qubit drive rate with the amplitude of the programmed pulse. With our hardware, and for a 15-ns pulse, the rotation angles deviate by up to several degrees from a linear model. Using purity benchmarking, we find that control errors reach 2×10−4, which accounts for half of the total gate error. Using cross-entropy benchmarking, we demonstrate arbitrary-angle single-qubit gates with coherence-limited errors of 2×10−4 and leakage below 6×10−5. While the exact magnitude of these errors is specific to our setup, the presented method is applicable to any source of non-linearity. Our work shows that the non-linearity of qubit drive line components imposes a limit on the fidelity of single-qubit gates, independent of improvements in coherence times, circuit design, or leakage mitigation when not corrected for.

Energetics of a Single Qubit Gate

  1. Jeremy Stevens,
  2. Daniel Szombati,
  3. Maria Maffei,
  4. Cyril Elouard,
  5. Réouven Assouly,
  6. Nathanaël Cottet,
  7. Rémy Dassonneville,
  8. Quentin Ficheux,
  9. Stefan Zeppetzauer,
  10. Audrey Bienfait,
  11. Andrew N. Jordan,
  12. Alexia Auffèves,
  13. and Benjamin Huard
Qubits are physical, a quantum gate thus not only acts on the information carried by the qubit but also on its energy. What is then the corresponding flow of energy between the qubit
and the controller that implements the gate? Here we exploit a superconducting platform to answer this question in the case of a quantum gate realized by a resonant drive field. During the gate, the superconducting qubit becomes entangled with the microwave drive pulse so that there is a quantum superposition between energy flows. We measure the energy change in the drive field conditioned on the outcome of a projective qubit measurement. We demonstrate that the drive’s energy change associated with the measurement backaction can exceed by far the energy that can be extracted by the qubit. This can be understood by considering the qubit as a weak measurement apparatus of the driving field.

Millisecond coherence in a superconducting qubit

  1. Aaron Somoroff,
  2. Quentin Ficheux,
  3. Raymond A. Mencia,
  4. Haonan Xiong,
  5. Roman V. Kuzmin,
  6. and Vladimir E. Manucharyan
Increasing the degree of control over physical qubits is a crucial component of quantum computing research. We report a superconducting qubit of fluxonium type with the Ramsey coherence
time reaching T∗2=1.48±0.13 ms, which exceeds the state of the art value by an order of magnitude. As a result, the average single-qubit gate fidelity grew above 0.9999, surpassing, to our knowledge, any other solid-state quantum system. Furthermore, by measuring energy relaxation of the parity-forbidden transition to second excited state, we exclude the effect of out-of-equilibrium quasiparticles on coherence in our circuit. Combined with recent demonstrations of two-qubit gates on fluxoniums, our result paves the way for the next generation of quantum processors.

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.

Proposal for entangling gates on fluxonium qubits via a two-photon transition

  1. Konstantin N. Nesterov,
  2. Quentin Ficheux,
  3. Vladimir E. Manucharyan,
  4. and Maxim G. Vavilov
We propose a family of microwave-activated entangling gates on two capacitively coupled fluxonium qubits. A microwave pulse applied to either qubit at a frequency near the half-frequency
of the |00⟩−|11⟩ transition induces two-photon Rabi oscillations with a negligible leakage outside the computational subspace, owing to the strong anharmonicity of fluxoniums. By adjusting the drive frequency, amplitude, and duration, we obtain the gate family that is locally equivalent to the fermionic-simulation gates such as SWAP−−−−−−√-like and controlled-phase gates. The gate error can be tuned below 10−4 for a pulse duration under 100 ns without excessive circuit parameter matching. Given that the fluxonium coherence time can exceed 1 ms, our gate scheme is promising for large-scale quantum processors.

Fast logic with slow qubits: microwave-activated controlled-Z gate on low-frequency fluxoniums

  1. Quentin Ficheux,
  2. Long B. Nguyen,
  3. Aaron Somoroff,
  4. Haonan Xiong,
  5. Konstantin N. Nesterov,
  6. Maxim G. Vavilov,
  7. and Vladimir E. Manucharyan
We demonstrate a controlled-Z gate between capacitively coupled fluxonium qubits with transition frequencies 72.3 MHz and 136.3 MHz. The gate is activated by a 61.6 ns long pulse at
the frequency between non-computational transitions |10⟩−|20⟩ and |11⟩−|21⟩, during which the qubits complete only 4 and 8 Larmor periods, respectively. The measured gate error of (8±1)×10−3 is limited by decoherence in the non-computational subspace, which will likely improve in the next generation devices. Although our qubits are about fifty times slower than transmons, the two-qubit gate is faster than microwave-activated gates on transmons, and the gate error is on par with the lowest reported. Architectural advantages of low-frequency fluxoniums include long qubit coherence time, weak hybridization in the computational subspace, suppressed residual ZZ-coupling rate (here 46 kHz), and absence of either excessive parameter matching or complex pulse shaping requirements.

Multiplexed photon number measurement

  1. Antoine Essig,
  2. Quentin Ficheux,
  3. Théau Peronnin,
  4. Nathanaël Cottet,
  5. Raphaël Lescanne,
  6. Alain Sarlette,
  7. Pierre Rouchon,
  8. Zaki Leghtas,
  9. and Benjamin Huard
The evolution of quantum systems under measurement is a central aspect of quantum mechanics. When a two level system — a qubit — is used as a probe of a larger system, it
naturally leads to answering a single yes-no question about the system state followed by its corresponding quantum collapse. Here, we report an experiment where a single superconducting qubit is counter-intuitively able to answer not a single but nine yes-no questions about the number of photons in a microwave resonator at the same time. The key ingredients are twofold. First, we exploit the fact that observing the color of a qubit carries additional information to the conventional readout of its state. The qubit-system interaction is hence designed so that the qubit color encodes the number of photons in the resonator. Secondly, we multiplex the qubit color observation by recording how the qubit reflects a frequency comb. Interestingly the amount of extracted information reaches a maximum at a finite drive amplitude of the comb. We evidence it by direct Wigner tomography of the quantum state of the resonator. Our experiment unleashes the full potential of quantum meters by bringing the measurement process in the frequency domain.

Sequential measurement of a superconducting qubit

  1. Théau Peronnin,
  2. Danijela Marković,
  3. Quentin Ficheux,
  4. and Benjamin Huard
We present a superconducting device that realizes the sequential measurement of a transmon qubit. The unitary evolution between system and probe is indeed separated in time and space
from the measurement of the probe itself. The device disables common limitations of dispersive readout such as Purcell effect or transients in the cavity mode by tuning the coupling to the measurement channel on demand. The probe is initially stored in a memory mode and coupled to the qubit until a microwave pump releases it into an output line in a characteristic time as low as 10 ns, which is 400 times shorter than the memory lifetime. The Wigner function of the memory allows us to characterize the non-Gaussian nature of the probe and its dynamics. A direct measurement of the released probe field quadratures demonstrates a readout fidelity of 97.5 % in a total measurement time of 220 ns.