Near-quantum-limited amplification from inelastic Cooper-pair tunnelling

Abstract

Nature sets fundamental limits regarding how accurate the amplification of analog signals may be. For instance, a linear amplifier unavoidably adds some noise which amounts to half a photon at best. While for most applications much higher noise levels are acceptable, the readout of microwave quantum systems, such as spin or superconducting qubits, requires noise as close as possible to this ultimate limit. To date, it is approached only by parametric amplifiers exploiting non-linearities in superconducting circuits and driven by a strong microwave pump tone. However, this microwave drive makes them much more difficult to implement and operate than conventional DC powered amplifiers, which so far suffer from much higher noise. Here we present the first experimental proof that a simple DC-powered setup allows for amplification close to the quantum limit. Our amplification scheme is based on the stimulated microwave photon emission accompanying inelastic Cooper pair tunneling through a DC-biased Josephson junction, with the key to low noise lying in a well defined auxiliary idler mode, in analogy to parametric amplifiers.The quantum limit on the noise of a linear amplifier can be derived from first principles [1]. This derivation shows that in order to be amplified irrespectively of its phase, the signal necessarily has to be coupled to at least one complementary mode, called idler, and that the photon noise of this mode is added to the signal. In the ideal case, where the idler mode is in its quantum ground state and the gain is large, the added input noise is half a photon. This limit is reached by Josephson parametric amplifiers (JPAs) [2][3][4][5][6][7][8], where the nonlinear inductance of Josephson junctions is used to couple a microwave pump tone to the signal mode. They have a perfectly well defined idler mode at the frequency f i , the difference of (a multiple of) the pump frequency and the signal frequency. This frequency can be matched to a dedicated mode in the circuit which can then be put in its ground state by strongly coupling it to a dedicated cold dissipator with temperature T hf i /k B .In DC-powered amplifiers, on the other hand, this idler mode is usually not well identified and difficult to engineer. For example, in high electron mobility transistor VNA RF DC 20dB 30dB 50 Ω 10 MΩ 50 Ω

Theory of quantum-circuit refrigeration by photon-assisted electron tunneling

“…Although the early work on photon-assisted tunneling at NIS junctions focused on the effect of the electromagnetic circuit on the tunnel current [19][20][21]32,33], recent studies also demonstrate the effect of the tunneling events on the state of the circuit [34][35][36][37][38][39][40][41][42][43]. Importantly, a quantum-circuit refrigerator and cryogenic microwave source based on photonassisted tunneling of electrons through NIS junctions were demonstrated in Refs.…”

Bright On-Demand Source of Antibunched Microwave Photons Based on Inelastic Cooper Pair Tunneling

“…Josephson photonics has recently emerged as a way to directly generate and manipulate microwave frequency signals at milliKelvin temperatures without the need for complicated microwave control drives [9][10][11][12][13][14][15][16] . It relies on the phenomenon of inelastic Cooper pair tunneling where a DC voltage biased Josephson tunnel junction at zero temperature can admit a finite direct current, even if the applied bias voltage is smaller than its gap voltage 17 .…”

“…While early experiments focused on the rate Γ and the associated direct current 18,[21][22][23] , recent advances in low-noise high-frequency measurements have made it possible to investigate the photonic side of this energy transfer [14][15][16]19,24 . In all previous implementations, the tunneling events were happening either independently 18,19 or through stimulated emission [14][15][16] , in both cases leading to classical Poisson statistics of the emitted photons. In the present work, we demonstrate a device where the electromagnetic environment of the junction is engineered to create anti-bunching in the photon emission, thus showing quantum statistics generated through inelastic Cooper pair tunneling.…”

Optimal work extraction from quantum states by photo-assisted Cooper pair tunneling

“…This scenario is relevant when the engine is used to extract work from a non-passive state that is continuously being generated by another machine. The continuous mode of operation allows us to consider situations that can be implemented with current-day technology [70][71][72]. In particular, we discuss how our engine can be used to convert the power from a laser into an electrical current that flows against a voltage bias.…”