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Quantum non-demolition measurement of an electron spin qubit


Measurements of quantum systems inevitably involve disturbance in various forms. Within the limits imposed by quantum mechanics, there exists an ideal projective measurement that does not introduce a back action on the measured observable, known as a quantum non-demolition (QND) measurement1,2. Here we demonstrate an all-electrical QND measurement of a single electron spin in a gate-defined quantum dot. We entangle the single spin with a two-electron, singlet–triplet ancilla qubit via the exchange interaction3,4 and then read out the ancilla in a single shot. This procedure realizes a disturbance-free projective measurement of the single spin at a rate two orders of magnitude faster than its relaxation. The QND nature of the measurement protocol5,6 enables enhancement of the overall measurement fidelity by repeating the protocol. We demonstrate a monotonic increase of the fidelity over 100 repetitions against arbitrary input states. Our analysis based on statistical inference is tolerant to the presence of the relaxation and dephasing. We further exemplify the QND character of the measurement by observing spontaneous flips (quantum jumps)7 of a single electron spin. Combined with the high-fidelity control of spin qubits8,9,10,11,12,13, these results will allow for various measurement-based quantum state manipulations including quantum error correction protocols14.

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Fig. 1: QND readout of a single electron spin via an ancillary qubit.
Fig. 2: Demonstration of QND measurement and fidelity analysis.
Fig. 3: Fidelity boost in repetitive QND readouts.
Fig. 4: Quantum jumps of a single electron spin in a quantum dot.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.


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The authors thank N. Imoto for fruitful discussions and A. Gutierrez-Rubio and Y. Kojima for careful reading of the manuscript. The authors also thank the RIKEN CEMS Emergent Matter Science Research Support Team and the Microwave Research Group at Caltech for technical assistance. Part of this work was financially supported by CREST, JST (JPMJCR15N2, JPMJCR1675), the ImPACT Program of the Council for Science, Technology and Innovation (Cabinet Office, Government of Japan), JSPS KAKENHI grants nos. 26220710, JP16H02204 and 18H01819, RIKEN Incentive Research Projects and Q-LEAP project initiated by MEXT, Japan. T.O. acknowledges support from JSPS KAKENHI grants nos. 16H00817 and 17H05187, PRESTO (JPMJPR16N3), JST, a Yazaki Memorial Foundation for Science and Technology Research Grant, Advanced Technology Institute Research Grant, a Murata Science Foundation Research Grant, an Izumi Science and Technology Foundation Research Grant, a TEPCO Memorial Foundation Research Grant, The Thermal & Electric Energy Technology Foundation Research Grant, The Telecommunications Advancement Foundation Research Grant, a Futaba Electronics Memorial Foundation Research Grant and an MST Foundation Research Grant. A.D.W. and A.L. acknowledge support from BMBF – Q.Link.X 16KIS0867, TRR160 and DFH/UFA CDFA-05-06.

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Authors and Affiliations



T.N., M.R.D. and S.T. conceived and designed the experiments. A.L. and A.D.W. grew the heterostructure. T.N. and A.N. fabricated the device. T.N. and A.N. conducted the experiments with the assistance of K.K. T.N. and A.N. analysed the data and wrote the manuscript with input from J.Y. and P.S. All authors discussed the results and commented on the manuscript. The project was supervised by S.T.

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Correspondence to Takashi Nakajima or Seigo Tarucha.

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The authors declare no competing interests.

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Journal peer review information: Nature Nanotechnology thanks John Morton and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Nakajima, T., Noiri, A., Yoneda, J. et al. Quantum non-demolition measurement of an electron spin qubit. Nat. Nanotechnol. 14, 555–560 (2019).

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