High-sensitivity AC-charge detection with a MHz-frequency fluxonium qubit

  1. B.-L. Najera-Santos,
  2. R. Rousseau,
  3. K. Gerashchenko,
  4. H. Patange,
  5. A. Riva,
  6. M. Villiers,
  7. T. Briant,
  8. P.-F. Cohadon,
  9. A. Heidmann,
  10. J. Palomo,
  11. M. Rosticher,
  12. H. le Sueur,
  13. A. Sarlette,
  14. W. C. Smith,
  15. Z. Leghtas,
  16. E. Flurin,
  17. T. Jacqmin,
  18. and S. Deléglise
Owing to their strong dipole moment and long coherence times, superconducting qubits have demonstrated remarkable success in hybrid quantum circuits. However, most qubit architectures
are limited to the GHz frequency range, severely constraining the class of systems they can interact with. The fluxonium qubit, on the other hand, can be biased to very low frequency while being manipulated and read out with standard microwave techniques. Here, we design and operate a heavy fluxonium with an unprecedentedly low transition frequency of 1.8 MHz. We demonstrate resolved sideband cooling of the „hot“ qubit transition with a final ground state population of 97.7 %, corresponding to an effective temperature of 23 μK. We further demonstrate coherent manipulation with coherence times T1=34 μs, T∗2=39 μs, and single-shot readout of the qubit state. Importantly, by directly addressing the qubit transition with a capacitively coupled waveguide, we showcase its high sensitivity to a radio-frequency field. Through cyclic qubit preparation and interrogation, we transform this low-frequency fluxonium qubit into a frequency-resolved charge sensor. This method results in a charge sensitivity of 33 μe/Hz‾‾‾√, or an energy sensitivity (in joules per hertz) of 2.8 ℏ. This method rivals state-of-the-art transport-based devices, while maintaining inherent insensitivity to DC charge noise. The high charge sensitivity combined with large capacitive shunt unlocks new avenues for exploring quantum phenomena in the 1−10 MHz range, such as the strong-coupling regime with a resonant macroscopic mechanical resonator.

Dynamically enhancing qubit-oscillator interactions with anti-squeezing

  1. M. Villiers,
  2. W. C. Smith,
  3. A. Petrescu,
  4. A. Borgognoni,
  5. M. Delbecq,
  6. A. Sarlette,
  7. M. Mirrahimi,
  8. P. Campagne-Ibarcq,
  9. T. Kontos,
  10. and Z. Leghtas
The interaction strength of an oscillator to a qubit grows with the oscillator’s vacuum field fluctuations. The well known degenerate parametric oscillator has revived interest
in the regime of strongly detuned squeezing, where its eigenstates are squeezed Fock states. Owing to these amplified field fluctuations, it was recently proposed that squeezing this oscillator would dynamically boost its coupling to a qubit. In a superconducting circuit experiment, we observe a two-fold increase in the dispersive interaction between a qubit and an oscillator at 5.5 dB of squeezing, demonstrating in-situ dynamical control of qubit-oscillator interactions. This work initiates the experimental coupling of oscillators of squeezed photons to qubits, and cautiously motivates their dissemination in experimental platforms seeking enhanced interactions.

One hundred second bit-flip time in a two-photon dissipative oscillator

  1. C. Berdou,
  2. A. Murani,
  3. U. Reglade,
  4. W. C. Smith,
  5. M. Villiers,
  6. J. Palomo,
  7. M. Rosticher,
  8. A. Denis,
  9. P. Morfin,
  10. M. Delbecq,
  11. T. Kontos,
  12. N. Pankratova,
  13. F. Rautschke,
  14. T. Peronnin,
  15. L.-A. Sellem,
  16. P. Rouchon,
  17. A. Sarlette,
  18. M. Mirrahimi,
  19. P. Campagne-Ibarcq,
  20. S. Jezouin,
  21. R. Lescanne,
  22. and Z. Leghtas
Current implementations of quantum bits (qubits) continue to undergo too many errors to be scaled into useful quantum machines. An emerging strategy is to encode quantum information
in the two meta-stable pointer states of an oscillator exchanging pairs of photons with its environment, a mechanism shown to provide stability without inducing decoherence. Adding photons in these states increases their separation, and macroscopic bit-flip times are expected even for a handful of photons, a range suitable to implement a qubit. However, previous experimental realizations have saturated in the millisecond range. In this work, we aim for the maximum bit-flip time we could achieve in a two-photon dissipative oscillator. To this end, we design a Josephson circuit in a regime that circumvents all suspected dynamical instabilities, and employ a minimally invasive fluorescence detection tool, at the cost of a two-photon exchange rate dominated by single-photon loss. We attain bit-flip times of the order of 100 seconds for states pinned by two-photon dissipation and containing about 40 photons. This experiment lays a solid foundation from which the two-photon exchange rate can be gradually increased, thus gaining access to the preparation and measurement of quantum superposition states, and pursuing the route towards a logical qubit with built-in bit-flip protection.

Magnifying quantum phase fluctuations with Cooper-pair pairing

  1. W. C. Smith,
  2. M. Villiers,
  3. A. Marquet,
  4. J. Palomo,
  5. M. R. Delbecq,
  6. T. Kontos,
  7. P. Campagne-Ibarcq,
  8. B. Douçot,
  9. and Z. Leghtas
Remarkably, complex assemblies of superconducting wires, electrodes, and Josephson junctions are compactly described by a handful of collective phase degrees of freedom that behave
like quantum particles in a potential. The inductive wires contribute a parabolic confinement, while the tunnel junctions add a cosinusoidal corrugation. Usually, the ground state wavefunction is localized within a single potential well — that is, quantum phase fluctuations are small — although entering the regime of delocalization holds promise for metrology and qubit protection. A direct route is to loosen the inductive confinement and let the ground state phase spread over multiple Josephson periods, but this requires a circuit impedance vastly exceeding the resistance quantum and constitutes an ongoing experimental challenge. Here we take a complementary approach and fabricate a generalized Josephson element that can be tuned in situ between one- and two-Cooper-pair tunneling, doubling the frequency of the corrugation and thereby magnifying the number of wells probed by the ground state. We measure a tenfold suppression of flux sensitivity of the first transition energy, implying a twofold increase in the vacuum phase fluctuations.

Superconducting circuit protected by two-Cooper-pair tunneling

  1. W. C. Smith,
  2. A. Kou,
  3. X. Xiao,
  4. U. Vool,
  5. and M. H. Devoret
We present a protected superconducting qubit based on an effective circuit element that only allows pairs of Cooper pairs to tunnel. These dynamics give rise to a nearly degenerate
ground state manifold indexed by the parity of tunneled Cooper pairs. We show that, when the circuit element is shunted by a large capacitance, this manifold can be used as a logical qubit that we expect to be insensitive to multiple relaxation and dephasing mechanisms.

Driving forbidden transitions in the fluxonium artificial atom

  1. U. Vool,
  2. A. Kou,
  3. W. C. Smith,
  4. N. E. Frattini,
  5. K. Serniak,
  6. P. Reinhold,
  7. I. M. Pop,
  8. S. Shankar,
  9. L. Frunzio,
  10. S. M. Girvin,
  11. and M. H. Devoret
Atomic systems display a rich variety of quantum dynamics due to the different possible symmetries obeyed by the atoms. These symmetries result in selection rules that have been essential
for the quantum control of atomic systems. Superconducting artificial atoms are mainly governed by parity symmetry. Its corresponding selection rule limits the types of quantum systems that can be built using electromagnetic circuits at their optimal coherence operation points („sweet spots“). Here, we use third-order nonlinear coupling between the artificial atom and its readout resonator to drive transitions forbidden by the parity selection rule for linear coupling to microwave radiation. A Lambda-type system emerges from these newly accessible transitions, implemented here in the fluxonium artificial atom coupled to its „antenna“ resonator. We demonstrate coherent manipulation of the fluxonium artificial atom at its sweet spot by stimulated Raman transitions. This type of transition enables the creation of new quantum operations, such as the control and readout of physically protected artificial atoms.

Simultaneous monitoring of fluxonium qubits in a waveguide

  1. A. Kou,
  2. W. C. Smith,
  3. U. Vool,
  4. I. M. Pop,
  5. K. M. Sliwa,
  6. M. H. Hatridge,
  7. L. Frunzio,
  8. and M. H. Devoret
Most quantum-error correcting codes assume that the decoherence of each physical qubit is independent of the decoherence of any other physical qubit. We can test the validity of this
assumption in an experimental setup where a microwave feedline couples to multiple qubits by examining correlations between the qubits. Here, we investigate the correlations between fluxonium qubits located in a single waveguide. Despite being in a wide-bandwidth electromagnetic environment, the qubits have measured relaxation times in excess of 100 us. We use cascaded Josephson parametric amplifiers to measure the quantum jumps of two fluxonium qubits simultaneously. No correlations are observed between the relaxation times of the two fluxonium qubits, which indicates that the sources of relaxation are local to each qubit. Our architecture can easily be scaled to monitor larger numbers of qubits.

A fluxonium-based artificial molecule with a tunable magnetic moment

  1. A. Kou,
  2. W. C. Smith,
  3. U. Vool,
  4. R. T. Brierley,
  5. H. Meier,
  6. L. Frunzio,
  7. S. M. Girvin,
  8. L. I. Glazman,
  9. and M. H. Devoret
Engineered quantum systems allow us to observe phenomena that are not easily accessible naturally. The LEGO-like nature of superconducting circuits makes them particularly suited for
building and coupling artificial atoms. Here, we introduce an artificial molecule, composed of two strongly coupled fluxonium atoms, which possesses a tunable magnetic moment. Using an applied external flux, one can tune the molecule between two regimes: one in which the ground-excited state manifold has a magnetic dipole moment and one in which the ground-excited state manifold has only a magnetic quadrupole moment. By varying the applied external flux, we find the coherence of the molecule to be limited by local flux noise. The ability to engineer and control artificial molecules paves the way for building more complex circuits for protected qubits and quantum simulation.

Fluxonium-resonator system in the nonperturbative regime

  1. W. C. Smith,
  2. A. Kou,
  3. U. Vool,
  4. I. M. Pop,
  5. L. Frunzio,
  6. R. J. Schoelkopf,
  7. and M. H. Devoret
We present a method for calculating the low-energy spectra of superconducting circuits with arbitrarily strong anharmonicity and coupling. As an example, we numerically diagonalize
the Hamiltonian of a fluxonium qubit inductively coupled to a readout resonator. Our method treats both the anharmonicity of the Hamiltonian and the coupling between qubit and readout modes exactly. Calculated spectra are compared to measured spectroscopy data for this fluxonium-resonator system. We observe excellent quantitative agreement between theory and experiment that is not possible with a purely perturbative approach.