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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 T_{1}=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/sqrt[Hz], or an energy sensitivity (in joules per hertz) of 2.8ℏ. 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.

Objective: To analyse the COVID-19-related lockdown impact on University workers, to identify groups based on this information, and to study the factors associated with each group. Study design: Cross-sectional study. Methods: A survey was conducted 3.5 weeks after COVID-19-related lockdown in University workers in Spain. Sociodemographic variables, housing, work, health conditions, levels of anxiety, stress and depression (DASS-21), and social support (MSPSS) were collected. A cluster analysis was performed to identify groups depending on the impact of the lockdown. Differences between groups were tested using Chi-square and Mann-Whitney-U tests, and associated factors with binary logistic regression. Results: We identified two groups of workers. “G1: Consequences in the daily life routine” was mainly composed of men, Research and Teaching Personnel (RTP) with more stable professional categories, higher income level, and bigger houses than people in G2. Participants in “G2: Concerns for the current and future well-being” presented worse intensity of pain than before the lockdown, more anxiety, depression, stress and less social support than people in G1. ASP (Administration and Services Personnel) had more risk of belonging to G2 than RTP (OR = 5.863). A higher number of people living at home decreased the risk of being in G2 (OR = 0.439). People with lower pain intensity had less risk of being in G2 (OR = 0.014), and this risk decreased as friends support increased (OR = 0.833). Conclusions: In G1, the consequences were immediately reflected in the stress resulting from changes in their daily work routine. In G2, the concerns were related to their professional future, with worse mental health, greater intensity of pain and less social support.

Bistable dynamical systems are widely employed to robustly encode classical bits of information. However, they owe their robustness to inherent losses, making them unsuitable to encode quantum information. Surprisingly, there exists a loss mechanism, known as two-photon dissipation, that provides stability without inducing decoherence. An oscillator exchanging pairs of photons with its environment is expected to reach macroscopic bit-flip times between dynamical states containing only a handful of photons. However, previous implementations have observed bit-flip times saturating in the millisecond range. In this experiment, we design a superconducting resonator endowed with two-photon dissipation, and free of all suspected sources of instabilities and inessential ancillary systems. We attain bit-flip times exceeding 100 s in between states containing about 40 photons. Although a full quantum model is necessary to explain our data, the preparation of coherent superposition states remains inaccessible. This experiment demonstrates that macroscopic bit-flip times are attainable with mesoscopic photon numbers in a two-photon dissipative oscillator.

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. Almost all these circuits operate in the regime where quantum phase fluctuations are small—the associated flux is smaller than the superconducting flux quantum—although entering the regime of large fluctuations would have profound implications for metrology and qubit protection. The difficulty arises from the apparent need for circuit impedances vastly exceeding the resistance quantum. Independently, exotic circuit elements that require Cooper pairs to form pairs in order to tunnel have been developed to encode and topologically protect quantum information. In this work, we demonstrate that pairing Cooper pairs magnifies the phase fluctuations of the circuit ground state. We measure a tenfold suppression of flux sensitivity of the first transition energy only, implying a twofold increase in the vacuum phase fluctuations and showing that the ground state is delocalized over several Josephson wells.