Instantons, semi-classical trajectories of quantum tunneling in imaginary time, have long been used to study thermodynamic and transport properties in a myriad of condensed matter andhigh energy systems. A recent experiment in superconducting circuits [Phys. Rev. Lett. 126, 197701, (2021)] provided first evidence for direct dynamical signatures of instantons (phase slips), manifested by order-unity inelastic decay probabilities for photons with which they interact, motivating the development of a scattering theory of instantons [Phys. Rev. Lett. 126, 137701, (2021)]. While this framework successfully predicted the measured inelastic decay rates of the photons for several experimental devices, it is valid only if the tunneling time of the instantons is much shorter than the relaxation time of the environment in which they are embedded, and requires a closed analytical expression for the instanton trajectory. Here, we amend these issues by incorporating numerical methods that lift some of the previously applied approximations. Our results agree with the experimental measurements, also for devices with shorter relaxation times, without fitting parameters. This framework should be useful in many other quantum field theoretical contexts.
Fluxonium qubit is a promising building block for quantum information processing due to its long coherence time and strong anharmonicity. In this paper, we realize a 60 ns direct CNOT-gateon two inductively-coupled fluxonium qubits using selective darkening approach, resulting in a gate fidelity as high as 99.94%. The fidelity remains above 99.9% for 24 days without any recalibration between randomized benchmarking measurements. Compared with the 99.96% fidelity of a 60 ns identity gate, our data brings the investigation of the non-decoherence-related errors during gate operations down to 2×10−4. The present result adds a simple and robust two-qubit gate into the still relatively small family of „the beyond three nines“ demonstrations on superconducting qubits.
We report a detailed characterization of two inductively coupled superconducting fluxonium qubits for implementing high-fidelity cross-resonance gates. Our circuit stands out becauseit behaves very closely to the case of two transversely coupled spin-1/2 systems. In particular, the generally unwanted static ZZ-term due to the non-computational transitions is nearly absent despite a strong qubit-qubit hybridization. Spectroscopy of the non-computational transitions reveals a spurious LC-mode arising from the combination of the coupling inductance and the capacitive links between the terminals of the two qubit circuits. Such a mode has a minor effect on our specific device, but it must be carefully considered for optimizing future designs.
We describe a superconducting qubit derived from operating a properly designed fluxonium circuit in a zero magnetic field. The qubit has a frequency of about 4 GHz and the energy relaxationquality factor Q≈0.7×107, even though the dielectric loss quality factor of the circuit components is in the low 105 range. The Ramsey coherence time exceeds 100 us, and the average fidelity of Clifford gates is benchmarked to >0.999. These figures are likely to improve by an order of magnitude with optimized fabrication and measurement procedures. Our work establishes a ready-to-use „partially protected“ superconducting qubit with an error rate comparable to the best transmons.
The tunnelling of cooper pairs across a Josephson junction (JJ) allow for the nonlinear inductance necessary to construct superconducting qubits, amplifiers, and various other quantumcircuits. An alternative approach using hybrid superconductor-semiconductor JJs can enable a superconducting qubit architecture with full electric field control. Here we present continuous-wave and time-domain characterization of gatemon qubits based on an InAs 2DEG. We show that the qubit undergoes a vacuum Rabi splitting with a readout cavity and we drive coherent Rabi oscillations between the qubit ground and first excited states. We measure qubit coherence times to be T1= 100 ns over a 1.5 GHz tunable band. While various loss mechanisms are present in III-V gatemon circuits we detail future directions in enhancing the coherence times of qubit devices on this platform.
We describe the generation of entangling gates on superconductor-semiconductor hybrid qubits by ac voltage modulation of the Josephson energy. Our numerical simulations demonstratethat the unitary error can be below 10−5 in a variety of 75-ns-long two-qubit gates (CZ, iSWAP, and iSWAP‾‾‾‾‾‾‾√) implemented using parametric resonance. We analyze the conditional ZZ phase and demonstrate that the CZ gate needs no further phase correction steps, while the ZZ phase error in SWAP-type gates can be compensated by choosing pulse parameters. With decoherence considered, we estimate that qubit relaxation time needs to exceed 70μs to achieve the 99.9% fidelity threshold.
Although quantum mechanics applies to many macroscopic superconducting devices, one basic prediction remained controversial for decades. Namely, a Josephson junction connected to aresistor must undergo a dissipation-induced quantum phase transition from superconductor to insulator once the resistor’s value exceeds h/4e2≈6.5 kΩ (h is Planck’s constant, e is the electron charge). Here we finally demonstrate this transition by observing the resistor’s internal dynamics. Implementing our resistor as a long transmission line section, we find that a junction scatters electromagnetic excitations in the line as either inductance (superconductor) or capacitance (insulator), depending solely on the line’s wave impedance. At the phase boundary, the junction itself acts as ideal resistance: in addition to elastic scattering, incident photons can spontaneously down-convert with a frequency-independent probability, which provides a novel marker of quantum-critical behavior.
The strong anharmonicity and high coherence times inherent to fluxonium superconducting circuits are beneficial for implementing quantum information processors. In addition to requiringhigh-quality physical qubits, a quantum processor needs to be assembled in a manner that reduces crosstalk and decoherence. In this letter, we report work on fluxonium qubits packaged in a flip-chip architecture. Here, the fluxonium qubits are embedded in a multi-chip module (MCM), where a classical control and readout chip is bump-bonded to the quantum chip. The modular approach allows for improved connectivity between qubits and control/readout elements, and separate fabrication processes. We demonstrate that this configuration does not degrade the fluxonium qubit performance, and identify the main decoherence mechanisms to improve on the reported results.
We revisit the superstrong coupling regime of multi-mode cavity quantum electrodynamics (QED), defined to occur when the frequency of vacuum Rabi oscillations between the qubit andthe nearest cavity mode exceeds the cavity’s free spectral range. A novel prediction is made that the cavity’s linear spectrum, measured in the vanishing power limit, can acquire an intricate fine structure associated with the qubit-induced cascades of coherent single-photon down-conversion processes. This many-body effect is hard to capture by a brute-force numerics and it is sensitive to the light-matter coupling parameters both in the infra-red and the ultra-violet limits. We focused at the example case of a superconducting fluxonium qubit coupled to a long transmission line section. The conversion rate in such a circuit QED setup can readily exceed a few MHz, which is plenty to overcome the usual decoherence processes. Analytical calculations were made possible by an unconventional gauge choice, in which the qubit circuit interacts with radiation via the flux/charge variable in the low-/high-frequency limits, respectively. Our prediction of the fine spectral structure lays the foundation for the „strong down-conversion“ regime in quantum optics, in which a single photon excited in a non-linear medium spontaneously down-converts faster than it is absorbed.
It is customary to use arrays of superconducting quantum interference devices (SQUIDs) for implementing magnetic field-tunable inductors. Here, we demonstrate an equivalent tunabilityin a (SQUID-free) array of single Al/AlOx/Al Josephson tunnel junctions. With the proper choice of junction geometry, a perpendicularly applied magnetic field bends along the plane of the superconductor and focuses into the tunnel barrier region due to a demagnetization effect. Consequently, the Josephson inductance can be efficiently modulated by the Fraunhoffer-type supercurrent interference. The elimination of SQUIDs not only simplifies the device design and fabrication, but also facilitates a denser packing of junctions and, hence, a higher inductance per unit length. As an example, we demonstrate a transmission line, the wave impedance of which is field-tuned in the range of 4−8 kΩ, centered around the important value of the resistance quantum h/(2e)2≈6.5 kΩ.