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arXiv:1810.06217 · Physical Review Letters

Antibunched Photons Emitted by a dc-Biased Josephson Junction

Chloé Rolland, A. Peugeot, Simon Dambach, M. Westig, Björn Kubala, Yury Mukharsky, Carles Altimiras, H. le Sueur, P. Joyez, D. Vion, P. Roche, D. Esteve, Joachim Ankerhold, F. Portier

Published May 10, 2019• 61 citations
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Abstract

We show experimentally that a dc biased Josephson junction in series with a high-enough-impedance microwave resonator emits antibunched photons. Our resonator is made of a simple microfabricated spiral coil that resonates at 4.4 GHz and reaches a 1.97 kΩ characteristic impedance. The second order correlation function of the power leaking out of the resonator drops down to 0.3 at zero delay, which demonstrates the antibunching of the photons emitted by the circuit at a rate of 6×10^{7} photons per second. Results are found in quantitative agreement with our theoretical predictions. This simple scheme could offer an efficient and bright single-photon source in the microwave domain.

DOI
10.1103/physrevlett.122.186804

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Single-photon generation is an important proof of the underlying quantum nature of a physical process and a ubiquitous tool for scientific exploration, with applications ranging from spectroscopy and metrology to quantum computing.In the microwave regime, emission of antibunched radiation has so far relied on coherent control of Josephson qubits requiring precisely calibrated microwave pulses.In this work, we experimentally demonstrate the operation of a bright on-demand source of quantum microwave radiation driven by a simple dc voltage bias across a Josephson junction.Our source is based on photon emission into a microwave resonator through inelastic Cooper pair tunneling regulated by a high-impedance RC circuit preventing simultaneous tunnel events.It is characterized by its normalized second-order correlation function of g ð2Þ ð0Þ ≈ 0.43 corresponding to antibunching in the single-photon regime.Our device can be triggered, and its in situ tunable emission rate exceeds those obtained with current microwave singlephoton sources by more than 1 order of magnitude.
We propose an experimentally feasible scheme for generating antibunched NNN-photon bundles by a dc voltage-biased Josephson junction in series with a superconducting microwave resonator and a charge qubit. Each resonant tunneling Cooper pair leads to the excitation of the charge qubit and resonator with NNN photons simultaneously via the ac Josephson effect. Meanwhile, the charge qubit with strong anharmonicity is utilized to regulate the tunneling behavior of the Cooper pairs; that is, the presence of NNN photons in the resonator prevents the next tunnel event. So the resonator contains only NNN photons or none, and all other possibilities are greatly suppressed. Combined with the system's dissipation, the resonator can emit its energy in antibunched bundles of NNN strongly correlated photons with high purity and an in situ tunable emission rate. Such a nonclassical source could be useful in applications in the field of quantum information science.
Abstract We propose to generate photon-pair blockade in a Josephson-photonics circuit that consists of a dc voltage-biased Josephson junction in series with a superconducting charge qubit and two nondegenerate microwave resonators. The two-level charge qubit is utilized to create anticorrelations of the charge transport; that is, the simultaneous Cooper pair tunneling events are inhibited. When the Josephson frequency matches the sum of resonance frequencies of the charge qubit and the two resonators, we demonstrate that the two resonators can release their energies in the form of antibunched pairs of two strongly correlated photons. The present work provides a practical way for producing a bright microwave source of antibunched photon pairs, with potential applications ranging from spectroscopy and metrology to quantum information processing.
We show experimentally that a dc-biased Josephson junction in series with two microwave resonators emits entangled beams of microwaves leaking out of the resonators.In the absence of a stationary phase reference for characterizing the entanglement of the outgoing beams, we measure second-order coherence functions to prove the entanglement.The experimental results are found in quantitative agreement with theory, proving that the low-frequency noise of the dc bias is the main limitation for the coherence time of the entangled beams.This agreement allows us to evaluate the entropy of entanglement of the resonators, estimate the entanglement flux at their output, and to identify the improvements that could bring this device closer to a useful bright source of entangled microwaves for quantum-technological applications.
We present a novel scheme to detect itinerant microwave radiation at the single photon level. Using existing Josephson-photonics devices, where two microwave cavities are coupled by a dc-voltage biased superconducting junction, we theoretically show how to implement a stroboscopically repeated, near-projective measurement of a photon impinging on one of the cavities. Optimizing rate, duration, and strength of the measurement by flux control of the junction and developing a threshold protocol to detect the photon from a homodyne measurement of the radiation output of the other cavity, we achieve highly efficient detection with low dark counts. By cascading the detector with a preamplifier, where a similar two-cavity Josephson-photonics device acts as a photon multiplier, we can further improve the device to reach a detection efficiency of 88.5%88.5 \%88.5% with a dark count rate of ∼10−4γa\sim10^{-4} γ_a∼10−4γa​, set by the resonance width γaγ_aγa​ of the absorbing cavity. These results for a multiplication factor of two suggest that near-unity efficiencies may be reached for higher multiplication factors.
We demonstrate theoretically that charge transport across a Josephson junction, voltage-biased through a resistive environment, produces antibunched photons. We develop a continuous-mode description of the emitted radiation field in a semi-infinite transmission line terminated by the Josephson junction. Within a perturbative treatment in powers of the tunneling coupling across the Josephson junction, we capture effects originating in charging dynamics of consecutively tunneling Cooper pairs. We find that within a feasible experimental setup the Coulomb blockade provided by high zero-frequency impedance can be used to create antibunched photons at a very high rate and in a very versatile frequency window ranging from a few GHz to a THz.