Entanglement between a Diamond Spin Qubit and a Photonic Time-Bin Qubit at Telecom Wavelength

Anna Tchebotareva, Sophie L. N. Hermans, Peter C. Humphreys, Dirk Voigt, Peter J. Harmsma, Lun K. Cheng, Ad L. Verlaan, Niels Dijkhuizen, Wim de Jong, Anaïs Dréau, and Ronald Hanson

Phys. Rev. Lett. 123, 063601 – Published 5 August 2019

Abstract

We report on the realization and verification of quantum entanglement between a nitrogen-vacancy electron spin qubit and a telecom-band photonic qubit. First we generate entanglement between the spin qubit and a 637 nm photonic time-bin qubit, followed by photonic quantum frequency conversion that transfers the entanglement to a 1588 nm photon. We characterize the resulting state by correlation measurements in different bases and find a lower bound to the Bell state fidelity of $\geq 0.77 \pm 0.03$ . This result presents an important step towards extending quantum networks via optical fiber infrastructure.

Received 21 May 2019

DOI:https://doi.org/10.1103/PhysRevLett.123.063601

Physics Subject Headings (PhySH)

Color centers Light-matter interaction Nonlinear optics Quantum entanglement Quantum optics

Atomic, Molecular & Optical

Authors & Affiliations

Anna Tchebotareva^1,2,*, Sophie L. N. Hermans^1,3,*, Peter C. Humphreys^1,3, Dirk Voigt^1,2, Peter J. Harmsma^1,2, Lun K. Cheng^1,2, Ad L. Verlaan^1,2, Niels Dijkhuizen^1,2, Wim de Jong^1,2, Anaïs Dréau^1,3,4, and Ronald Hanson^1,3,†

¹QuTech, Delft University of Technology, P.O. Box 5046, 2600 GA Delft, Netherlands
²Netherlands Organisation for Applied Scientific Research (TNO), P.O. Box 155, 2600 AD Delft, Netherlands
³Kavli Institute of Nanoscience, Delft University of Technology, P.O. Box 5046, 2600 GA Delft, Netherlands
⁴Laboratoire Charles Coulomb, Université de Montpellier and CNRS, 34095 Montpellier, France

^*A. T. and S. L. N. H. contributed equally to this work.
^†R.Hanson@tudelft.nl

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Vol. 123, Iss. 6 — 9 August 2019

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Images

Figure 1
(a) In a long-distance quantum network, heralded entanglement between the nodes is generated by interference on a beam splitter and subsequent measurement of telecom photonic qubits. (b) Concept of the experiment. A NV spin qubit–photon qubit entangled state is generated. The photonic qubit is converted to a telecom wavelength by the difference frequency generation (DFG) process [28]. For measuring the photonic qubit in the $X$ and $Y$ bases, an imbalanced interferometer is used. Inset: Simplified level scheme of the NV center. (c) DFG process. A 637 nm photon is converted to a wavelength of 1588 nm using a nonlinear PPLN crystal. Left inset: Electron microscope image of the diamond device. Right inset: Image of the PPLN crystal with ridge waveguides (NTT/NEL).
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Figure 2
(a) Experimental setup for the spin-telecom photon entangled state generation. Emitted 637 nm photons are combined with the pump laser (1064 nm) in the difference frequency generation setup (DFG1). The two lasers are frequency locked to an external reference cavity. Tomography in the $Z$ basis: the frequency-converted photons are detected using a superconducting nanowire detector ( $D 2$ ) discriminating the early and late time bins. (b) Experimental protocol for generating and detecting spin-telecom photon entangled states (see main text). (c) Results for correlations measured in the $Z$ basis for both the red and the frequency-converted photons at telecom wavelength.
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Figure 3
(a) Polarization-maintaining fiber-based imbalanced interferometer used for the photon state readout in $X$ and $Y$ bases. The frequency-converted single photons are directed into the interferometer. One output port of the interferometer is connected to a superconducting nanowire detector (detector $D 3$ ). Every second the phase of the interferometer is stabilized. Classical frequency-converted light created by a second DFG setup (DFG2) is sent into the interferometer via a shutter $S$ and a circulator. Light intensities measured by photodiodes PD2 and PD3 are used to generate a feedback signal to the fiber piezostretcher (FPS) to maintain the target phase $Δ ϕ$ . (b) Bloch sphere with the selected photon qubit readout bases indicated on it, and the corresponding phase setpoints of the imbalanced interferometer.
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Figure 4
(a) Experimental protocol for measurements in the photon $X$ and $Y$ bases. (b) Measured phase difference $Δ ϕ$ just before stabilization (orange, with 900 ms free evolution time) and directly after stabilization (blue) for the two setpoints $Δ ϕ_{X} = π / 4$ and $Δ ϕ_{Y} = 3 π / 4$ . From the standard deviations in these data, we estimate a residual phase drift of 0.05 and $0.01 rad / s$ for the $X$ and $Y$ photon qubit readout bases, respectively. (c) Results for the correlations in the $X$ and $Y$ bases in purple. The top panel shows ideal correlations. In total we have measured 1595 photon detection events.
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