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Quantum non-demolition detection of an itinerant microwave photon


Photon detectors are an elementary tool to measure electromagnetic waves at the quantum limit1,2 and are heavily demanded in the emerging quantum technologies such as communication3, sensing4 and computing5. Of particular interest is a quantum non-demolition (QND)-type detector, which projects an electromagnetic wave onto the photon-number basis6,7,8,9,10. This is in stark contrast to conventional photon detectors2 that absorb a photon to trigger a ‘click’. The long-sought QND detection of a flying photon was recently demonstrated in the optical domain using a single atom in a cavity11,12. However, the counterpart for microwaves has been elusive despite the recent progress in microwave quantum optics using superconducting circuits13,14,15,16,17,18,19. Here, we implement a deterministic entangling gate between a superconducting qubit and an itinerant microwave photon reflected by a cavity containing the qubit. Using the entanglement and the high-fidelity qubit readout, we demonstrate a QND detection of a single photon with the quantum efficiency of 0.84 and the photon survival probability of 0.87. Our scheme can serve as a building block for quantum networks connecting distant qubit modules as well as a microwave-photon-counting device for multiple-photon signals.

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Fig. 1: Circuit quantum electrodynamics set-up for the QND detection of an itinerant microwave photon.
Fig. 2: QND detection of an itinerant microwave photon.
Fig. 3: Quantum state tomography of the reflected pulse mode.
Fig. 4: Qubit–photon entanglement.


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We acknowledge the fruitful discussions with T. Serikawa, T. Sugiyama, Y. Shikano, R. Yamazaki and K. Usami. This work was supported in part by the Advanced Leading Graduate Course for Photon Science (ALPS), the University of Tokyo, the Japan Society for the Promotion of Science (JSPS) Grants-in-Aid for Scientific Research (KAKENHI) (no. 16K05497 and no. 26220601), and the Japan Science and Technology Agency (JST) Exploratory Research for Advanced Technology (ERATO) (grant no. JPMJER1601).

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S.K. performed the experiments. K.K. provided the theoretical support. S.K., Y.T. and A.N. participated in discussions on the analysis. S.K., K.K. and Y.N. wrote the manuscript with feedback from all authors. Y.N. supervised the project.

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Correspondence to S. Kono or Y. Nakamura.

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The authors declare no competing interests.

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Kono, S., Koshino, K., Tabuchi, Y. et al. Quantum non-demolition detection of an itinerant microwave photon. Nature Phys 14, 546–549 (2018).

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