Beyond the standard quantum limit for parametric amplification of broadband signals

Abstract

The low-noise amplification of weak microwave signals is crucial for countless protocols in quantum information processing. Quantum mechanics sets an ultimate lower limit of half a photon to the added input noise for phase-preserving amplification of narrowband signals, also known as the standard quantum limit (SQL). This limit, which is equivalent to a maximum quantum efficiency of 0.5, can be overcome by employing nondegenerate parametric amplification of broadband signals. We show that, in principle, a maximum quantum efficiency of unity can be reached. Experimentally, we find a quantum efficiency of 0.69 ± 0.02, well beyond the SQL, by employing a flux-driven Josephson parametric amplifier and broadband thermal signals. We expect that our results allow for fundamental improvements in the detection of ultraweak microwave signals.

Perspectives of microwave quantum key distribution in open-air

“…In experiments, JPAs operating in the GHz regime were shown to reach noise levels on the order of 0.1 added noise photons in the phase-sensitive regime [17]. Presently, the noise performance of JPAs is limited by fabrication imperfections, pump-induced noise [30,31], or higher-order nonlinearities [32]. The displacement operation required by our CV-QKD protocol can be experimentally realized by applying strong coherent drive tones to cryogenic directional couplers [26].…”

“…The quantum efficiency is defined as the ratio between vacuum fluctuations and fluctuations in output signals resulting from additional noise photons n amp due to amplification, where n amp is referred to the input of the detection chain. Therefore, we can express the quantum efficiency as [30]…”

Bimodal Approach for Noise Figures of Merit Evaluation in Quantum-Limited Josephson Traveling Wave Parametric Amplifiers

“…The amplification is associated with the creation of a second mode at frequency ω ′ = ω p − ω (the so-called idler mode of a three-wave mixing parametric amplification [15]) that is commonly considered as an internal mode of the amplifier that causes the onset of noise at the output port. Here, we extend and give a different perspective of this description considering the case in which an uncorrelated idler mode is already present at the input port (i.e., considering a bimodal input field), analyzing the effect of the interaction between these modes inside the amplifier in terms of typical noise estimators.This operative condition may arise in real measurement setups where the amplifier is exploited, for instance, for the multiplexed readout of broadband signals [16] or for the joint detection and amplification of probing signals in a microwave quantum illumination experiment [8]. The theoretical framework presented in this manuscript is supported with numerical simulations of the noise estimators for a realistic implementation of a quantum-limited amplifier [13], [14] such as the rf-SQUID based Josephson Travelling Wave Parametric Amplifier (JTWPA) [17]- [21], which represents a promising realization of a microwave amplifier with high gain, large bandwidth and quantum-limited added noise [22].…”

“…To evaluate the performance of an ideal phase-preserving linear amplifier it is possible to define its quantum efficiency spectrum as the ratio between the vacuum fluctuations in the input field and the fluctuations of the output field, this latter quantity normalized on the bimodal gain of the amplifier [16]…”

Propagating Quantum Microwaves: Towards Applications in Communication and Sensing

“…However, for squeezing beyond 3 dB, induced noise results in a significant thermal contribution and, hence, a significantly reduced purity of the squeezed state [37]. Recent experiments on the amplification properties of JPAs suggest pump-induced noise as a limiting factor for quantum efficiencies [43]. Figure 2 shows an exemplary plot for the pump power dependence of reconstructed singlemode squeezing (a) and purity (b) for a single-SQUID niobium JPA, fabricated by VTT.…”