Optical spin tomography in a telecom C-band quantum dot

Abstract

A central challenge for scalable quantum networks is the realization of coherent interfaces between stationary qubits and telecom-band photonic qubits for long-distance entanglement distribution. Semiconductor quantum dots emitting at telecom wavelengths present a promising spin-photon platform, and a precise understanding of the properties of the confined spin is crucial for optimizing its interplay with the photonic qubit. Here, we simultaneously benchmark the electron and hole g-factors and coherence properties of a droplet epitaxy QD, solely from time and polarization resolved photon correlations. These measurements identify the hole as the preferable qubit for spin-photon entanglement in quantum network nodes. We then perform full state tomography of the confined hole ground state to reveal subtle anisotropies in the spin precession, providing essential diagnostics for minimizing phase errors critical for deterministic multiphoton entanglement generation.

Figures (4)

• Figure 1: Device architecture and hole-trion transition under phonon-assisted excitation. (a) Schematics of the QD-microcavity device. Droplet epitaxy InAs QDs (purple) are embedded within a distributed Bragg reflector structure composed of InP/AlInGaAs, and capped with a solid immersion lens (SIL) to enhance both pulsed laser excitation and photon collection efficiency. The QD growth axis defines the $z$ direction, with a weak magnetic field applied along the $y$ axis (Voigt geometry). (b) Polarization selection rules governing the hole-trion optical transitions under coherent phonon-assisted excitation. (c) Bloch sphere depiction of spin qubits in the ground and excited states. The spin-up and spin-down eigenstates are aligned along the $z$ axis, while coherent superposition states reside in the $x-y$ plane. Blue arrows illustrate the spin precession of the trion and hole states within the $x-z$ plane under the magnetic field.
• Figure 2: Spin precession dynamics in ground and excited states. (a) Experimental scheme illustrating the polarized optical excitation pulse sequence (purple arrows) and detection configuration to measure emitted photons in the circular basis, using electronic polarization controllers and polarizing beamsplitters. (b) Time-resolved, two-photon correlation map for revealing spin dynamics for both ground and excited states at $B$ = 1.2 T. The two-dimensional histogram displays correlated photon arrival times, measured in orthogonal circular polarization channels (R-L). Horizontal axis shows $t_{e}$, the time spent in the excited state after the second excitation pulse. $\Delta$$t$ on the y-axis denotes the time spent in the excited state after the first excitation pulse, complementary to the ground-state precession time $t_{g}$ (i.e. $T$ = $\Delta$$t$ + $t_{g}$). (c) Temporal cross-section along a 32-ps-high horizontal slice (dashed line in (b)), isolating the slow excited-state electron spin precession, with theoretical fit shown as a solid line. (d) Corresponding 32-ps-width vertical slice of (b), revealing the fast ground-state hole spin precession.The binning size in (c) and (d) is 16 ps.
• Figure 3: Ground state spin evolution and DCP analysis. (a) Schematic of the excitation pulse sequence and detection setup. The QD is driven by linearly polarized excitation pulses in either H or D polarization, while detection is performed in the circular basis. Ground state precession time is controlled by selecting different temporal slices along the decay of the first excitation (red dashed lines). (b-e) Time-resolved photon correlation measurements for varying ground state precession intervals under H-polarized (b, c) and D-polarized (d, e) excitation, with in-plane magnetic field $B$=1.2 T. In all cases, detection of the first photon is conditioned on R polarization, and the second photon’s detection in R or L polarization allows measurement of the spin’s oscillatory dynamics in the excited state. Panels (b) and (d) correspond to correlations taken at $\Delta$$t$ = $T$$t_{g}$ = 0 ps, while (c) and (e) represent measurements at 64 ps delay. Red and blue data points denote R-R and R-L coincidence counts, respectively. Lower panels show the extracted DCP from the polarization-dependent correlations, fitted with a damped oscillator model. (f) Standard deviation $\sigma$ of DCP as a function of the ground state precession time under H- and D-polarized excitation.
• Figure 4: Tomographic reconstruction of ground state hole spin. (a) Time evolution of the ground-state spin polarization components $S_x$ (blue), $S_y$ (yellow), $S_z$ (green), and the total spin magnitude $|\boldsymbol{S}|$ (red), as a function of the residence time in the ground state $t_{g}$, fitted with damped oscillator model. (b, c) Bloch sphere visualization of the reconstructed spin trajectories during coherent precession. (b) shows projection of the spin evolution onto the $y$–$z$ plane, while (c) reveals an inclination of the precession plane relative to the $y$–$z$ plane. Color gradient encodes elapsed $t_{g}$.