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Applied Physics Letters

Squeezing electronic noise with two tones

C. Paradis, Pierre Février, Edouard Pinsolle, Bertrand Reulet

Published Jun 26, 2023• 2 citations
Publisher

Abstract

We have calculated and measured the effect of two ac excitations on the quantum shot noise of a tunnel junction. Using a phase-sensitive detection, we demonstrate that the junction generates squeezed microwave, in agreement with theory. While being weak, this squeezing should be extremely broadband.
DOI
10.1063/5.0147473

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We report the observation of photon pairs in the photoassisted shot noise of a tunnel junction in the quantum regime at very high frequency and very low temperature. We have measured the fluctuations of the noise power generated by the junction at two different frequencies, f(1) = 4.4 and f(2) = 7.2 GHz, while driving the junction with a microwave excitation of frequency f(0) = f(1) + f(2). We observe clear correlations between the fluctuations of the two noise powers even when the mean photon number per measurement is much smaller than one. This is strong evidence for photons being emitted in pairs. We also demonstrate that the electromagnetic field generated by the junction exhibits two-mode amplitude squeezing, a proof of its nonclassicality. The data agree very well with predictions based on the fourth cumulant of the current fluctuations generated by the junction.
Bright sources of quantum microwave light are an important building block for various quantum technological applications. Josephson junctions coupled to microwave cavities are a particularly versatile and simple source for microwaves with quantum characteristics, such as different types of squeezing. Due to the inherent nonlinearity of the system, a pure dc-voltage bias can lead to the emission of correlated pairs of photons into a stripline resonator. However, a drawback of this method is that it suffers from bias voltage noise, which disturbs the phase of the junction and consequently destroys the coherence of the photons, severely limiting its applications. Here we describe how adding a small ac reference signal either to the dc bias or directly into the cavity can stabilize the system and counteract the sensitivity to noise. We first consider the injection locking of a single-mode device before turning to the more technologically relevant locking of two-mode squeezed states, where phase locking preserves the entanglement between photons. Finally, we describe locking by directly injecting a microwave into the cavity, which breaks the symmetry of the squeezing ellipse. In all cases, locking can mitigate the effects of voltage noise and enable the use of squeezed states in quantum technological applications.
In the quest to realize a quantum computer, physicists are dealing with signals that can be as weak as a single microwave photon. To detect such signals with sufficient quantum efficiency to implement e.g.e.g.e.g. quantum error correction, they must be amplified within the cryostat, and the amplifier itself must be a quantum device. To this end, the authors present a broadband, nearly quantum-limited parametric amplifier based on the tunneling of quasiparticles in a superconducting junction. This development may allow the generation of microwave continuous-variable cluster states, and is expected to have great impact on quantum information science.
We consider the electromagnetic field generated by a coherent conductor in which electron transport is described quantum mechanically. We obtain an input-output relation linking the quantum current in the conductor to the measured electromagnetic field. This allows us to compute the outcome of measurements on the field in terms of the statistical properties of the current. We moreover show how under ac bias the conductor acts as a tunable medium for the field, allowing for the generation of single- and two-mode squeezing through fermionic reservoir engineering. These results explain the recently observed squeezing using normal tunnel junctions [G. Gasse et al., Phys. Rev. Lett. 111, 136601 (2013); J.-C. Forgues et al., Phys. Rev. Lett. 114, 130403 (2015)].