Optical deterministic measurement of a nitrogen
nuclear spin in diamond
Riyo Enyo1, Takaaki Nakamura1, Yuhei Sekiguchi1, and Hideo Kosaka1*
1

Yokohama National University, 79-5 Tokiwadai, Hodogaya, Yokohama 240-8501, Japan*Corresponding author: kosaka-hideo-yp@ynu.ac.jp

Abstract: We demonstrate the deterministic measurement of the quantum state of anitrogen nuclear spin in a nitrogen vacancy (NV) center in diamond under a zero field.
We succeeded to measure the nuclear spin state of a target 14N impurity atom byrepeatedly measuring the spin state of an entangled ancillary electron. It indicates that wecan perform deterministic measurement of a nuclear spin with one measurement asfaithful as 97% fidelity.
1. Introduction
Quantum repeaters, which enables long-distance quantum communications, require complete Bellmeasurement with extremely high fidelity for the entanglement swapping or the quantum teleportation [1,2].
However, it is impossible to measure a quantum state to any arbitrary accuracy by itself. We thus need todevelop the technique called quantum-non-demolition measurement to perform a single-shot measurement,
which allows single preparation and repeated measurement.

2. Experimental setups
We used a native NV center in a high-purity type-IIa chemical-vapor-deposition grown bulk diamond witha <001> crystal orientation (electronic grade from Element Six) without any electron-beam dose orannealing. A negatively-charged NV center located at about 30 m below the surface was found using a
confocal laser microscope. A 25-m copper wire mechanically attached to the surface of the diamond wasused to apply a microwave. An external magnetic field was applied to carefully compensate thegeomagnetic field of about 0.045 mT using a permanent magnet. The NV center used in the experimentshowed hyperfine splittings caused by 14N nuclear spin at 2.2 MHz. All experiments were performed at 5 K
to reduce the optical linewidth of the Ex and A1 transition to as narrow as 35 MHz. All the light beams werefocused onto the sample using a 0.8 NA 100x objective inside the vacuum.
The experimental setup was the same as in Kosaka and Niikura [3]. A green laser (532nm, 100 W) anda red light resonant to the A1 state were first used to initialize the electron spin states to the ground ms=0state (bright state), and then a microwave (2.878 GHz) was used to excited the nitrogen nuclear spin statemI=±1 states leaving only the mI=0 state. Finally, a red light resonant to the Ex state was used to read out
the mI=0 state for 10 s. Then the A1 initialization and the Ex readout sequence was repeated for N times(Fig. 1).
3. Experimental results
We demonstrate the single-shot measurement of the quantum state of a nuclear spin in a nitrogenvacancy (NV) center in diamond under a zero field [4,5]. We were able to measure the nuclear spin state ofa target 14N impurity atom by repeatedly measuring the spin state of an entangled ancillary electron. After
preparing the nuclear spin into the mI=0 state, we observed 3.7 photons in average with 44-time repeated
optical readouts of the electron spin in the mS=0 state (Fig. 2). From the time decay of the photon counts,
we estimated the signal includes 0.03 photon from the ms=±1 state (dark state). This indicates that we can
optically perform the single-shot measurement to determine the nuclear spin state whether mI=0 or ±1 withthe fidelity of 95% in average even under a zero field.
4. Acknowledgements
We thank Yuichiro Matsuzaki, Kae Nemoto, William Munro, Norikazu Mizuochi, Fedor Jelezko, andJoerg Wrachtrup for their discussions and experimental help. This work was supported by National Instituteof Information and Communications Technology (NICT) Quantum Repeater Project, and by Japan Society
for the Promotion of Science (JSPS) Grant-in-Aid for Scientific Research (24244044, 16H06326,16H01052) and Ministry of Education, Culture, Sports, Science and Technology (MEXT) as “Exploratory
Challenge on Post-K computer” (Frontiers of Basic Science: Challenging the Limits).

5. References[1] S. Yang et al., Nat. Photon. 10, 507-511 (2016).
[2] L. Robledo et al., Nature 477, 574-578 (2011).
[3] H. Kosaka and N. Niikura, Phys. Rev. Lett. 114, 053603 (2015).
[4] Y. Sekiguchi, Y. Komura, S. Mishima, T. Tanaka, N. Niikura, and H. Kosaka, Nat. Commun. 7, 11668 (2016).
[5] Y. Sekiguchi, N. Niikura, R. Kuroiwa, H. Kano, and H. Kosaka, Nat. Photon. in press (2017).

Fig.1. Quantum circuit for the conventional (upper) and single-shot (lower) measurements.

Fig.2 Accumulated photon counts for the bright and dark states and the corresponding measurement fidelity
in average.