Hole spin coherence in InAs/InAlGaAs self-assembled quantum dots emitting at telecom wavelengths

TL;DR

This work measures hole-spin coherence in self-assembled InAs/InAlGaAs quantum dots emitting in the telecom band, addressing a critical gap for quantum repeater applications. Using spin inertia and spin mode-locking in a heavily inhomogeneous QD ensemble, the authors extract a longitudinal relaxation time of $T_1 = 0.5$ μs and a transverse coherence time range of $T_2 = 0.02$–0.40 μs, with $|g_e|=1.88$ and $|g_h|=0.60$ and substantial g-factor dispersion. The study reveals that holes exhibit longer coherence than electrons under these conditions and identifies a non-oscillating molecular-state signal that warrants further investigation. The results imply potential pathways to longer coherence, possibly approaching millisecond scales via nuclear-spin coupling, relevant for scalable telecom-based quantum information processing.

Abstract

We report measurements of the longitudinal and transverse spin relaxation times of holes in an ensemble of self-assembled InAs/InAlGaAs quantum dots (QDs), emitting in the telecom spectral range. The spin coherence of a single carrier is determined using spin mode-locking in the inhomogeneous ensemble of QDs. Modeling the signal allows us to extract the hole spin coherence time to be in the range of T$_2 = 0.02-0.4$ $μ$s. The longitudinal spin relaxation time T$_1 = 0.5$ $μ$s is measured using the spin inertia method.

Figures (5)

• Figure 1: (a) Schematic of the potential profile and the sample structure. (b) Photoluminescence spectrum measured at $T = 6$ K. (c) Dynamics of differential transmission ($\Delta T/T$) excited and detected at the energy of $E_\text{p} = 0.784$ eV (blue). The red line is fit by a biexponential decay with $\tau_\text{X} = 0.5$ ns and $\tau_\text{MX} = 2$ ns. Pump and probe powers are 17 mW and 1 mW, respectively.
• Figure 2: (a) Faraday rotation signal for $B_\text{V} = 0.2$ T, $T = 6$ K, and $E_\text{p} = 0.785$ eV shown by blue trace. The pump and probe powers are 15 mW and 1 mW, respectively. The black and red traces are the electron and the hole spin contributions, respectively. The green trace is the signal of the molecular states. (b) The Larmor precession frequencies of electron (black) and hole (red) versus the transverse magnetic field ($B_\text{V}$). Linear fits give $|g_\text{e}| = 1.88$ and $|g_\text{h}| = 0.60$. (c) Dephasing times ($T_2^*$) of electron (black) and hole (red) as functions of $B_\text{V}$. The data are fitted using Eq. (\ref{['T2']}).
• Figure 3: (a) Red dots present the dependence of $|g_\text{h}|$ on the central laser energy $E_\text{p}$. The red line is a linear fit to the data. The brown dots show the spectral dependence of the normalized hole spin amplitude contribution to the FR signal. The solid line is a guide for the eye. (b) Black dots present the dependence of $|g_\text{e}|$ on $E_\text{p}$. The black solid line is a linear fit. The green dots show the spectral dependence of the normalized electron contribution. The solid line is a guide for the eye.
• Figure 4: (a) Faraday rotation amplitude (dots) at the pump-probe delay of $-50$ ps as a function of the pump modulation frequency $f_\text{m}$ measured in a longitudinal magnetic field of $B_\text{F} = 4.6$ mT for different pump powers. $E_\text{p} = 0.784$ eV. The lines are fita according to Eq. (\ref{['eq_SIN']}) describing the spin inertia effect. (b) Power dependence of inverse spin lifetimes $1/T_\text{s}$. A linear extrapolation to zero power gives the intrinsic spin relaxation time $T_1 = 0.5$$\mu$s. (c) $T_1$ dependence on temperature.
• Figure 5: (a) Faraday rotation signal measured at $B_\text{V} = 0.2$ T, $T = 6$ K and $E_\text{p} = 0.785$ eV. The pump and probe powers are 15 mW and 1 mW. The inset shows the zoomed-in signal at negative time delays. (b) The blue trace is the spin mode-locking signal of the holes, and the red solid lines shows its modeling. (c) Spectral distribution of the precession modes calculated for $T_2 = 400$ ns and $\Theta=0.6\pi$.