Although Josephson junction qubits show great promise for quantum computing, the origin of dominant decoherence mechanisms remains unknown. Improving the operation of a Josephson junction based phase qubit has revealed microscopic two-level systems or resonators within the tunnel barrier that cause decoherence. We report spectroscopic data that show a level splitting characteristic of coupling between a two-state qubit and a two-level system. Furthermore, we show Rabi oscillations whose "coherence amplitude" is significantly degraded by the presence of these spurious microwave resonators. The discovery of these resonators impacts the future of Josephson qubits as well as existing Josephson technologies.

We propose the use of superconducting nanowires as both target and sensor for direct detection of sub-GeV dark matter. With excellent sensitivity to small energy deposits on electrons, and demonstrated low dark counts, such devices could be used to probe electron recoils from dark matter scattering and absorption processes. We demonstrate the feasibility of this idea using measurements of an existing fabricated tungsten-silicide nanowire prototype with 0.8-eV energy threshold and 4.3 nanograms with 10 thousand seconds of exposure, which showed no dark counts. The results from this device already place meaningful bounds on dark matter-electron interactions, including the strongest terrestrial bounds on sub-eV dark photon absorption to date. Future expected fabrication on larger scales and with lower thresholds should enable probing new territory in the direct detection landscape, establishing the complementarity of this approach to other existing proposals.

Integration is currently the only feasible route toward scalable photonic quantum processing devices that are sufficiently complex to be genuinely useful in computing, metrology, and simulation. Embedded on-chip detection will be critical to such devices. We demonstrate an integrated photon-number-resolving detector, operating in the telecom band at 1550 nm, employing an evanescently coupled design that allows it to be placed at arbitrary locations within a planar circuit. Up to five photons are resolved in the guided optical mode via absorption from the evanescent field into a tungsten transition-edge sensor. The detection efficiency is 7.2 ± 0.5 %. The polarization sensitivity of the detector is also demonstrated. Detailed modeling of device designs shows a clear and feasible route to reaching high detection efficiencies.Photonics provides a promising path for building and using complex quantum systems for both exploring fundamental physics and delivering quantum-enhanced technologies in information processing, metrology, and communications. Currently, the only feasible route toward sufficient complexity is integration, due to the high density of optical modes that can be contained within a single device and the extraordinary level of control that can be exercised over them. Although much research has gone into developing integrated elements at telecom wavelengths for classical applications, their use in the quantum regime has been limited, in large part because of intrinsic inefficiencies in input coupling, detector coupling, and propagation. The effect of these inefficiencies is to reduce or remove any quantum advantage attainable with a given device [1][2][3][4][5][6][7].Current single-photon-sensitive detectors for telecom wavelengths include avalanche photodiodes (APDs) [8], superconducting nanowires [9], and transition-edge sensors (TESs) [10,11]. In x Ga 1-x As APDs, the only commercially available telecom-band, single-photon-sensitive detectors, suffer from high dark-count rates, whereas nanowire detectors have much lower dark-count rates, are extremely fast, and can have high quantum efficiencies comparable to those of In x Ga 1-x As APDs [12]. In order to achieve high efficiencies with these normal incidence detectors, care must be taken to impedance match the incident field to the detector in order to avoid reflections of the optical signal. Moreover, normal incidencedetection schemes are intrinsically limited to monitoring the modes that emerge from the end facet of the device. As a result, inferring information about a quantum state or circuit element inside a device will only become more problematic as circuits move toward the complexities required to study effects beyond the scope of classical computational power [7,13,14]. Developing high-efficiency detectors that are compatible with these complex, high-density systems is therefore a critical enabling step for quantum photonics.In this paper, we demonstrate the operation of a new concept for broadband, efficient, single-photon detection, evanesc...

Current-biased Josephson junctions are prime candidates for the realization of quantum bits; however, a present limitation is their coherence time. In this paper it is shown qualitatively that quasiparticles create decoherence. We can decrease the number of quasiparticles present in the junctions by two methods-reducing the creation rate with current shunts and increasing the depletion rate with normal-metal traps. Experimental data demonstrate that both methods are required to significantly reduce the number of quasiparticles and increase the system's coherence. We conclude that these methods are effective and that the design of Josephson-junction qubits must consider the role of quasiparticles.Index Terms-Andreev reflection, Josephson junction, quantum computation, quasiparticle, qubit, superconducting devices. T HE quantized energy levels of the current-biasedJosephson junction, first observed over fifteen years ago [1], form the basis of several more-recent proposals and experiments [2]-[4] for a Josephson-junction realization of a quantum bit (qubit) [5]. Josephson junctions are promising systems for qubits because of the low dissipation inherent to the superconducting state and the relative ease of scaling to multiple qubits through integrated-circuit fabrication technology [6]. Recent experiments have demonstrated that Josephson-junction qubits can in principle perform the single-qubit basic functions needed for quantum computation-initialization of the state, controlled evolution, and state measurement-with coherence times sufficient for this demonstration [4], [7]-[10]. However, because coherence times must be further increased to perform multiple logic operations in a practical quantum computer, an important area of research is understanding the mechanisms of decoherence.The purposes of this paper are to experimentally demonstrate that quasiparticles can be a significant source of decoherence in a Josephson qubit and to understand how to minimize their presence and effect 1 . At the low temperatures typically used in Josephson qubit experiments 10-50 mK , the equilibrium quasiparticle density is computed to be exponentially small. However, because the state measurement procedure produces Manuscript 1 Experiments on SET devices have already addressed this issue by incorporating quasiparticle traps in their design. See [9], for example.Fig. 1. Josephson-junction qubit operation schematics. (a) Schematic circuit for a current-biased Josephson-junction qubit. 'S' represents the shunt that is used to minimize the generation of quasiparticles. Typical values for the qubits of this paper are I 40 A, C 6 pF. (b) Cubic potential U as a function of phase across the junction derived from an analysis of the circuit of (a). For the qubits in this paper ! =2 7:5 GHz. (c) Schematic single-qubit quantum-computation cycle showing periods during which quasiparticles (qps) are created and destroyed.a voltage across the junction, a significant number of quasiparticles are produced and remain in the system even after the qu...

A number of current approaches to quantum and neuromorphic computing use superconductors as the basis of their platform or as a measurement component, and will need to operate at cryogenic temperatures. Semiconductor systems are typically proposed as a top-level control in these architectures, with low-temperature passive components and intermediary superconducting electronics acting as the direct interface to the lowest-temperature stages.The architectures, therefore, require a low-power superconductorsemiconductor interface, which is not currently available. Here we report a superconducting switch that is capable of translating low-voltage superconducting inputs directly into semiconductor-compatible (above 1,000 mV) outputs at kelvin-scale temperatures (1 K or 4 K). To illustrate the capabilities in interfacing superconductors and semiconductors, we use it to drive a light-emitting diode (LED) in a photonic integrated circuit, generating photons at 1 K from a low-voltage input and detecting them with an on-chip superconducting single-photon detector. We also characterize our devices timing response (less than 300 ps turn-on, 15 ns turn-off), output 1