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We report on the design and performance of an on-chip microwave circulator with a widely (GHz) tunable operation frequency. Nonreciprocity is created with a combination of frequency conversion and delay, and requires neither permanent magnets nor microwave bias tones, allowing on-chip integration with other superconducting circuits without the need for high-bandwidth control lines. Isolation in the device exceeds 20 dB over a bandwidth of tens of MHz, and its insertion loss is small, reaching as low as 0.9 dB at select operation frequencies. Furthermore, the device is linear with respect to input power for signal powers up to hundreds of fW (≈103 circulating photons), and the direction of circulation can be dynamically reconfigured. We demonstrate its operation at a selection of frequencies between 4 and 6 GHz.

Many experiments that interrogate fundamental theories require detectors whose sensitivities are limited by the laws of quantum mechanics. In cavity-based searches for axionic dark matter, vacuum fluctuations in the two quadratures of the cavity electromagnetic field limit the sensitivity to an axion-induced field. In an apparatus designed to partially mimic existing axion detectors, we demonstrate experimentally that such quantum limits can be overcome through the use of squeezed states. By preparing a microwave cavity in a squeezed state and measuring just the squeezed quadrature, we enhance the spectral scan rate by a factor of2.12±0.08. This enhancement is in excellent quantitative agreement with a theoretical model accounting for both imperfect squeezing and measurement.

A single microwave photon is prepared in a superposition of two states of different frequency. This is achieved by using a superconducting quantum interference device to mediate the coupling between two harmonics of a superconducting resonator. Fully controlled coherent coupling of arbitrary harmonic oscillators is an important tool for processing quantum information 1 . Coupling between quantum harmonic oscillators has previously been demonstrated in several physical systems using a two-level system as a mediating element 2 , 3 . Direct interaction at the quantum level has only recently been realized by means of resonant coupling between trapped ions 4 , 5 . Here we implement a tunable direct coupling between the microwave harmonics of a superconducting resonator by means of parametric frequency conversion 6 , 7 . We accomplish this by coupling the mode currents of two harmonics through a superconducting quantum interference device (SQUID) and modulating its flux at the difference (∼7 GHz) of the harmonic frequencies. We deterministically prepare a single-photon Fock state 8 and coherently manipulate it between multiple modes, effectively controlling it in a superposition of two different ’colours’. This parametric interaction can be described as a beamsplitter-like operation that couples different frequency modes. As such, it could be used to implement linear optical quantum computing protocols 9 , 10 on-chip 11 .

Time division SQUID multiplexing is being developed as the TES readout technology for the ATHENA X-ray integral field unit and CMB-S4. Close packing of TDM and dc-biased SQUID components is motivated by chip area constraints but has resulted in significant physical neighbor crosstalk in previous generation chips. We present techniques to reduce physical neighbor crosstalk in both linear and two dimensional (2D) TDM chips as well as measurements of crosstalk in these chips.

Several current and proposed cosmic microwave background experiments use transition edge sensor bolometer focal planes coupled to the digital frequency-domain multiplexing (DfMux) electronics. This readout architecture sums bolometer signals in a SQUID array amplifier (SAA). In this study, we investigate the properties of two SAA designs, the SA4b, which is currently used in the DfMux system, and the SA13a. The SA13a design is gradiometric, making it less sensitive to stray magnetic field pickup. It has lower input inductance and is laid out on the chip as a re-configurable array with 6 banks of 64 series SQUIDs that can be arranged in any series and parallel configurations to optimize array noise, peak-to-peak modulation depth, and dynamic output resistance. The SA13a design reported on here is configured with 3 banks in series × 2 banks in parallel. The SA4b is a series array of 100 SQUIDs in series, each with an 8-turn input coil.

The microwave SQUID multiplexer ( μ MUX) is a high channel-count multiplexer that, when coupled to low-temperature detectors such as Transition Edge Sensor (TES) Bolometers, has applications across astronomy and physics. Our primary application is for the Simons Observatory, an array of CMB polarimeters utilizing over 70,000 μ MUX readout channels, located in the Atacama Desert. To facilitate the delivery of high-quality multiplexers to the project, we have developed a high-throughput microwave SQUID screening measurement system, capable of measuring microwave devices operating over the frequency range of 4–8 GHz. Here, we present the hardware design comprised of a cryogen-free 100 mK 2-stage Adiabatic Demagnetization Refrigerator (ADR) cryostat, microwave packages which hold μ MUX chips, and the microwave readout chain necessary to do these measurements. In addition, we describe the screening protocols and show example results.

The manipulation of quantum states of light 1 holds the potential to enhance searches for fundamental physics. Only recently has the maturation of quantum squeezing technology coincided with the emergence of fundamental physics searches that are limited by quantum uncertainty 2 , 3 . In particular, the quantum chromodynamics axion provides a possible solution to two of the greatest outstanding problems in fundamental physics: the strong-CP (charge–parity) problem of quantum chromodynamics 4 and the unknown nature of dark matter 5 – 7 . In dark matter axion searches, quantum uncertainty manifests as a fundamental noise source, limiting the measurement of the quadrature observables used for detection. Few dark matter searches have approached this limit 3 , 8 , and until now none has exceeded it. Here we use vacuum squeezing to circumvent the quantum limit in a search for dark matter. By preparing a microwave-frequency electromagnetic field in a squeezed state and near-noiselessly reading out only the squeezed quadrature 9 , we double the search rate for axions over a mass range favoured by some recent theoretical projections 10 , 11 . We find no evidence of dark matter within the axion rest energy windows of 16.96–17.12 and 17.14–17.28 microelectronvolts. Breaking through the quantum limit invites an era of fundamental physics searches in which noise reduction techniques yield unbounded benefit compared with the diminishing returns of approaching the quantum limit. A quantum enhanced search for dark matter that uses vacuum squeezing to overcome the quantum noise limit finds no evidence of dark matter axions in a well motivated mass range.

We present a method of creating high-density superconducting flexible wiring on flexible thin silicon substrates. The flexible wiring, called SOI flex , is created by depositing superconducting wiring on a silicon-on-insulator (SOI) wafer, selectively etching away the thicker silicon section handle layer, and bending the thinner silicon device layer. We show measurements of superconducting transition temperature and critical current for Mo, Nb, and Al on SOI flex. We discuss the expected advantages of SOI flex for low-temperature detector applications, as well as the role of stress and strain in bent silicon and niobium.