Quantum non-demolition detection of an itinerant microwave photon

Abstract

Photon detectors are an elementary tool to measure electromagnetic waves at the quantum limit 1,2 and are heavily demanded in the emerging quantum technologies such as communication 3 , sensing 4 , and computing 5 . Of particular interest is a quantum non-demolition (QND) type detector, which projects the quantum state of a photonic mode onto the photon-number basis without affecting the temporal or spatial properties 6-9 . This is in stark contrast to conventional photon detectors 2 which absorb a photon to trigger a 'click' and thus inevitably destroy the photon. The long-sought QND detection of a flying photon was recently demonstrated in the optical domain using a single atom in a cavity 10,11 . However, the counterpart for microwaves has been elusive despite the recent progress in microwave quantum optics using superconducting circuits 12-18 . Here, we implement a deterministic entangling gate between a superconducting qubit and a propagating microwave pulse mode reflected by a cavity containing the qubit. Using the entanglement and the highfidelity qubit readout, we demonstrate a QND detection of a single photon with the quantum efficiency of 0.84, the photon survival probability of 0.87, and the dark-count probability of 0.0147. Our scheme can be a building block for quantum networks connecting distant qubit modules as well as a microwave photon counting device for multiple-photon signals.Microwave quantum optics in superconducting circuits enables us to investigate unprecedented regimes of quantum optics. The strong nonlinearity brought by Josephson junctions together with the strong coupling of the qubits with resonators/waveguides reveals rich physics not seen in the optical domain before. It has also been applied in demonstrations of the generation and characterization of non-classical states in cavity modes 12-14 and propagating modes 15,16 as well as the remote entanglement of localized superconducting qubits 17,18 . However, single-photon detection in the microwave domain is still a challenging task because of the photon energy four to five orders of magnitude smaller than in optics. The sensitivities of conventional incoherent detectors such as avalanche photodiodes, bolometers, and superconducting nanowires are not sufficient for single microwave photons 2 . Therefore, resonant absorption of a microwave photon with a superconducting qubit was ex-ploited for single-photon detection recently 19 . Note also that QND measurements of cavity-confined microwave photons have been realized by using a Rydberg atom or a superconducting qubit as a probe 20,21 .For a QND detection of an itinerant photon, we use a circuit quantum-electrodynamics (QED) architecture with a transmon qubit in a largely detuned 3D cavity 22 . An input pulse mode through a 1D transmission line to the cavity is entangled with the qubit upon the reflection and is projected to a number state by the subsequent qubit readout without destroying the photon (Fig. 1).In our setup, the qubit-cavity interaction is described with the Hamiltonian...

Single-Shot Quantum Nondemolition Detection of Individual Itinerant Microwave Photons

“…Later, systems involving absorption into artificial atoms (and thus destruction) of traveling photons [25,26] were implemented. Very recently, a quantum nondemolition (QND) detection scheme based on a photon-qubit entangling gate, similar in spirit to this work, was implemented using a strong dispersive shift in a 3D cavity [27]. Projective measurements of coherent input states into single-photon Fock states were realized in that work [27].…”

“…Very recently, a quantum nondemolition (QND) detection scheme based on a photon-qubit entangling gate, similar in spirit to this work, was implemented using a strong dispersive shift in a 3D cavity [27]. Projective measurements of coherent input states into single-photon Fock states were realized in that work [27].Here, we demonstrate single-shot QND detection of itinerant single photons in the microwave domain, based on a cavity-assisted controlled phase gate operated between an artificial atom and a propagating photon [28]. We show the unconditional detection of an itinerant wave packet containing a Fock state at the single-photon level.…”

“…However, protocols such as entanglement heralding require the intrinsic nonlinearity of a single-photon detector in order to yield high-purity states despite losses between the source and the detector. Such a component has therefore raised interest in the community, leading to a variety of theoretical proposals [10][11][12][13][14][15][16][17][18][19][20], as well as initial experimental demonstrations, in the last decade [21][22][23][24][25][26][27].The first microwave photodetection experiment with superconducting circuits that did not require photons to be stored in high-quality factor cavities [21-23] was based on current biased Josephson junctions [24], but it was destructive and involved a long dead time. Later, systems involving absorption into artificial atoms (and thus destruction) of traveling photons [25,26] were implemented.…”

A unidirectional on-chip photonic interface for superconducting circuits

“…The photon can then be detected by measuring the qubit state after applying a Ramsey π/2-pulse, which realizes a quantum non-demolition measurement of the itinerant photon, in analogy to the cavity-QED experiments in Refs. [60][61][62][63][64]. The resonance frequency ω 0 of this detector can be tuned, while the detection bandwidth is given by γ r (see details in Supplementary Section E).…”

On-demand generation and characterization of a microwave time-bin qubit

“…Our scheme allows us to generate and shape the single-photon wave packet as well as to generate any superposition state of the time-bin qubit with variable spacing between the temporal modes. We perform Wigner tomography of microwave signal in two temporal modes by measuring the quadrature distributions with a flux-driven Josephson parametric amplifier [22] and a single heterodyne detector [23]. With the JPA, we can rapidly change the measurement quadrature for each temporal mode independently in a single shot.…”