A Spin-Photon Interface in the Telecom C-Band with Long Hole Spin Dephasing Time

TL;DR

The work addresses the challenge of building a spin–photon interface in the telecom C-band by integrating InAs/InAlGaAs quantum dots into a deterministically placed circular Bragg grating. It characterizes the system with polarization-resolved spectroscopy to extract electron and hole g-factors, and demonstrates long-lived ground-state hole-spin coherence via both continuous-wave and pulsed two-photon correlation measurements, yielding $T_{2}^{*}$ around 16 ns. The combination of a telecom-emitting quantum dot, cavity-enhanced emission, and heralded spin readout achieves the longest reported pure dephasing time for telecom QD spins to date, advancing prospects for scalable spin–photon networks and cluster-state generation. The results lay groundwork for more complex spin-control protocols and integration with silicon photonics in quantum communication architectures.

Abstract

Matter qubits that maintain coherence over extended timescales are essential for many pursued applications in quantum communication and quantum computing. Significant progress has already been made on extending coherence times of spins in semiconductor quantum dots while interfacing them with photons in the near-infrared wavelength range. However, similar results for quantum dots emitting at the telecom range, crucial for many applications, have so far lagged behind. Here, we report on InAs/InAlGaAs quantum dots integrated in a deterministically placed circular Bragg grating emitting at $1.55\,μ\mathrm{m}$. We quantify the g-factors of electrons and holes from polarization-resolved measurements of a positive trion in an in-plane magnetic field and study the dynamics of the ground-state hole spin qubit. We then herald the hole spin in a pulsed two-photon correlation measurement and determine its inhomogeneous dephasing time to $T_{2}^{*}=(15.9 \pm 1.7)$ ns.

Figures (10)

• Figure 1: Splittings and g-factors from polarizations resolved measurements in magnetic field.(a) Spectral line filtered in H- and V-polarization. The lines split for increasing magnetic field, with a stronger splitting observed for the H-component. (b) Sketch of the trion energy level scheme. When the splitting is sufficiently large, the optical transitions are governed by linearly (H/V) polarized photon emission. (c) Energy splitting between H (brown)- and V (green) polarized emission lines. From the splitting, the g-factors of excited and ground level spin can be calculated. (d) Time- and polarization-resolved trace of the $T^{+}$ emission at zero magnetic field and for small fields $B_x\leq$ 0.15T. For 0T, a polarization memory of $0.865 \pm 0.001$ can be extracted. For $B_x>$ 0T, the exponential decays of the trion lifetime traces are modulated by oscillations that stem from spin precessions. The frequency is proportional to the induced energy splitting of the excited state, as illustrated in (e). (f) Linear fit to the observed energy splitting. From this, a g-factor of $2.09 \pm 0.03$ can be calculated.
• Figure 2: Polarization resolved autocorrelation measurement. The trion was excited continuously at different magnetic fields and with varying laser powers. The polarizations were set to either R/R (excitation/detection) or R/L, enabling the observation of ground state spin precessions. (a) R/R (top, red) and R/L (bottom, blue) time traces for 0.5µ W and $B=$ 37.5mT. Calculating $(RR-RL)/(RR+RL)$ yields the DOCP displayed in (b). The decaying envelope used to modulate the fitted cosine function yields a coherence time of $\tau=(16.51\pm0.06)$ ns. (c) Oscillation frequency / energy splitting extracted at various magnetic fields. From the energy splittings, the g-factor can be determined to $0.35 \pm 0.01$. Additionally, one can observe a decrease in $\tau$ for increasing magnetic fields. (d) Laser-power dependent dephasing time of the hole spin. Similar to increasing magnetic fields, $\tau$ follows an exponential decay for increasing power.
• Figure 3: Pulsed coherence time measurement. The hole spin is probed utilizing a two-photon-correlation experiment. (a) Schematic illustration of the measurement sequence. The experiment is synchronized by an electronic trigger signal from the laser which starts the measurement clock. The emitted laser pulse is split in two pulses and the second pulse arrives delayed due to an optical path of controllable length. The polarization for both pulses can be controlled individually. The same applies for the projection of the resulting photons, whose arrival in two separate channels CH1 and CH2 stops the clock. Detecting the first photon in $R$ (after $R$-excitation) heralds the spin in $\ket{{\Downarrow}}$. After undisturbed precession with the Larmor frequency around an in-plane magnetic field of 150m T, the hole spin is probed with an $H$-polarized laser pulse and projected to both $R$ and $L$. (b) Resulting two-dimensional time trace of photon 2 for a pulse delay of 1.6n s. The data of both circular polarizations is superimposed in a colormap. The time traces are analyzed along the horizontal direction with regard to a fixed arrival time of the CH1 photon at $t_{X,CH1}=$ 370p s (cf. dashed black line). The resulting one-dimensional time traces are displayed in (c). Top panel: Correlated $\langle RR \rangle$ (red) and $\langle RL \rangle$ (blue) signals. A phase shift of $\pi$ can be observed between the oscillating time traces. Bottom panel: Fit of the DOCP resulting from the datasets in the top panel. A vertical dashed line marks the point of readout, which is varied over a window of 220p s (shaded area). (d) DOCP at $t_{2, read}(\Delta t)$. The resulting oscillation reflects the ground-state spin-precession of the hole. From a fit, a precession frequency of $763 \pm 1$ M Hz and spin dephasing time of $17.0 \pm 2.2$ ns are extracted.
• Figure S1: $\mu$-PL spectrum and cavity enhanced radiative lifetime measurements. Black: Spectrum under above-bandgap laser illumination. Two prominent emission lines appear below 1550nm. Red: Cavity emission under strong laser pumping. The cavity is red-shifted around 10nm from the targeted center wavelength. Blue: Measurements of the radiative lifetime at the position of prominent PL-features. The lifetime significantly decreases for lines that couple to the cavity. Green: Purcell factor calculated from the decreased radiative decay times. The Purcell enhancement follows the cavity's spectral shape.
• Figure S2: Analysis of peak position and intensity under varying excitation powers and detection polarizations. (a) QD spectrum with the exciton and trion line highlighted. (b) Colormap constructed from all spectra of a polarization series, with the HWP angle in the detection tuned from 0180°. (c-d) Peak position of $X^{0}$ and $X^{+}$ from polarization series. The exciton shows a fine-structure-splitting, whereas the trion wavelength remains polarization-independent. (e-f) Respective power dependent peak intensities. Both fitted lines showcase a power-law coefficient of $\sim 1.2$.