Low-noise cryogenic microwave amplifier characterization with a calibrated noise source.

Parametric amplifiers have become a workhorse in superconducting quantum computing; however, research and development of these devices has been hampered by inconsistent and, sometimes, misleading noise performance characterization methodologies. The concepts behind noise characterization are deceptively simple, and there are many places where one can make mistakes, either in measurement or in interpretation and analysis. In this article, we cover the basics of noise performance characterization and the special problems it presents in parametric amplifiers with limited power handling capability. We illustrate the issues with three specific examples: a high-electron mobility transistor amplifier, a Josephson traveling-wave parametric amplifier, and a Josephson parametric amplifier. We emphasize the use of a 50-Ω shot noise tunnel junction (SNTJ) as a broadband noise source, demonstrating its utility for cryogenic amplifier amplifications. These practical examples highlight the role of loss as well as the additional parametric amplifier "idler" input mode.

Performance of a Kinetic Inductance Traveling-Wave Parametric Amplifier at 4 Kelvin: Toward an Alternative to Semiconductor Amplifiers.

Most microwave readout architectures in quantum computing or sensing rely on a semiconductor amplifier at 4 K, typically a high-electron mobility transistor (HEMT). Despite its remarkable noise performance, a conventional HEMT dissipates several milliwatts of power, posing a practical challenge to scale up the number of qubits or sensors addressed in these architectures. As an alternative, we present an amplification chain consisting of a kinetic inductance traveling-wave parametric amplifier (KITWPA) placed at 4 K, followed by a HEMT placed at 70 K, and demonstrate a chain-added noise TΣ=6.3±0.5K between 3.5 and 5.5 GHz. While, in principle, any parametric amplifier can be quantum limited even at 4 K, in practice we find the performance of the KITWPA to be limited by the temperature of its inputs and by an excess of noise Tex=1.9K. The dissipation of the rf pump of the KITWPA constitutes the main power load at 4 K and is about 1% that of a HEMT. These combined noise and power dissipation values pave the way for the use of the KITWPA as a replacement for semiconductor amplifiers.

A quantum enhanced search for dark matter axions.

The manipulation of quantum states of light1 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 uncertainty2,3. 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 chromodynamics4 and the unknown nature of dark matter5-7. 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 limit3,8, 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 quadrature9, we double the search rate for axions over a mass range favoured by some recent theoretical projections10,11. 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.

First Results from a Microwave Cavity Axion Search at 24 µeV.

We report on the first results from a new microwave cavity search for dark matter axions with masses above 20 µeV. We exclude axion models with two-photon coupling g_{aγγ}≳2×10^{-14} GeV^{-1} over the range 23.55

Freezing and thawing of artificial ice by thermal switching of geometric frustration in magnetic flux lattices.

The problem of an ensemble of repulsive particles on a potential-energy landscape is common to many physical systems and has been studied in multiple artificial playgrounds. However, the latter usually involve fixed energy landscapes, thereby impeding in situ investigations of the particles' collective response to controlled changes in the landscape geometry. Here, we experimentally realize a system in which the geometry of the potential-energy landscape can be switched using temperature as the control knob. This realization is based on a high-temperature superconductor in which we engineer a nanoscale spatial modulation of the superconducting condensate. Depending on the temperature, the flux quanta induced by an applied magnetic field see either a geometrically frustrated energy landscape that favours an ice-like flux ordering, or an unfrustrated landscape that yields a periodic flux distribution. This effect is reflected in a dramatic change in the superconductor's magneto-transport. The thermal switching of the energy landscape geometry opens new opportunities for the study of ordering and reorganization in repulsive particle manifolds.