Quantum entanglement between spatially separated nodes is a key ingredient for future technologies, such as secure communication, distributed quantum computing, and quantum-enhanced sensing. Photon-mediated entanglement motivates development of quantum nodes with an optical interface, and the tin-vacancy center is a compelling building block. It is a spin-photon interface with coherent and indistinguishable emission, compatible with integration in nanostructures. Additionally, it has shown long spin coherence times at temperatures above 1 K. For photonic qubits, time-bin encoding is robust against photon loss and phase noise in fibers, making it especially suitable for long-range entanglement mediation.
An important challenge is thus, to demonstrate entanglement between the spin of the tin-vacancy center and a photonic time bin qubit. Quantum state tomography of the entangled state requires measurements of the photonic qubit in various bases. To this end, we have constructed a Time Delay Interferometer (TDI), in the form of an Unbalanced Mach-Zehnder Interferometer. A probe signal facilitates feedback to set (‘lock’) the phase of the TDI to any angle, allowing controlled quantum state measurements in a continuum of bases. We can lock the phase successfully in only 30 ms as shown by a subsequent probability distribution with a Root Mean Square of 0.046 𝜋, centered around the target phase. During locking, no quantum state measurements on single photons are possible, which necessitates passive phase stability. Laser frequency noise limited verification of passive phase stability on time scales below 200 ms. During the time between 200 ms and 8 s, the RMS increased with only 0.05 𝜋. This promises quantum state tomography measurements with little overhead.
Next, we measured the single photon purity of emission when attempting excitation of a tin-vacancy center. A second order autocorrelation 𝑔(2)(0) = 0.07(7) clearly demonstrates antibunching. In addition, we probed the indistinguishability of two-photon interference in the TDI. We measured a raw Hong-Ou-Mandel visibility 𝑉raw = 0.85(14) and a corrected indistinguishability of 0.87(14) (upper bound at 1). Together with measured imperfections in the interferometer, this data determines an upper bound on the fidelity of single shot quantum state measurements of 0.72(4).
This work lays the foundation to demonstrate spin-photon entanglement between a tin-vacancy spin qubit and an emitted time bin photonic qubit. This is an important step in realizing long-range quantum entanglement via emission based protocols ... Read more