The recent development of hybrid systems based on superconducting circuits provides the possibility of engineering quantum sensors that exploit different degrees of freedom. Quantum magnonics, which aims to control and read out quanta of collective spin excitations in magnetically ordered systems, provides opportunities for advances in both the study of magnetism and the development of quantum technologies. Using a superconducting qubit as a quantum sensor, we report the detection of a single magnon in a millimeter-sized ferrimagnetic crystal with a quantum efficiency of up to 0.71. The detection is based on the entanglement between a magnetostatic mode and the qubit, followed by a single-shot measurement of the qubit state. This proof-of-principle experiment establishes the single-photon detector counterpart for magnonics.

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...

Information thermodynamics bridges information theory and statistical physics by connecting information content and entropy production through measurement and feedback control. Maxwell’s demon is a hypothetical character that uses information about a system to reduce its entropy. Here we realize a Maxwell’s demon acting on a superconducting quantum circuit. We implement quantum non-demolition projective measurement and feedback operation of a qubit and verify the generalized integral fluctuation theorem. We also evaluate the conversion efficiency from information gain to work in the feedback protocol. Our experiment constitutes a step toward experimental studies of quantum information thermodynamics in artificially made quantum machines.

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