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Physical Review Letters

Optical Amplification Using Raman Transitions between Spin-Singlet and Spin-Triplet States of a Pair of Coupled In-GaAs Quantum Dots

J. M. Elzerman, K. M. Weiss, Javier Miguel-Sanchez, Ataç İmamoğlu

Published Jun 30, 2011• 28 citations
Publisher

Abstract

We report the observation of steady-state optical amplification in Raman transitions between the lowest-energy spin states of a single quantum-dot molecule. Absorption and resonance fluorescence experiments demonstrate that the entangled two-electron singlet and triplet states have electric-dipole coupling to a common optically excited state. Fast spin relaxation ensures optical gain on the triplet transition when the singlet transition is driven resonantly. By embedding the quantum-dot molecule in a cavity of modest quality factor, a solid-state single-emitter laser can be realized.

DOI
10.1103/physrevlett.107.017401

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We report the observation of steady-state optical amplification in Raman transitions between the lowest-energy spin states of a single quantum-dot molecule. Absorption and resonance fluorescence experiments demonstrate that the entangled two-electron singlet and triplet states have electric-dipole coupling to a common optically excited state. Fast spin relaxation ensures optical gain on the triplet transition when the singlet transition is driven resonantly. By embedding the quantum-dot molecule in a cavity of modest quality factor, a solid-state single-emitter laser can be realized.
We report the observation of Raman amplification in a three-level lambda scheme realized in a single coupled quantum dot (CQD) molecule. When both QDs contain a single electron, the lowest energy levels correspond to spin singlet (S) or triplet (T) states, which share a common optically excited state (X). Electron tunneling between the two dots gives rise to an exchange splitting between the S and T states, which allows us to selectively address them optically. We take advantage of fast (-10 ns) relaxation from T to S to generate population inversion and laser amplification on the T-to-X transition by pumping the S transition.
We show that spin-orbit coupling in a quantum dot molecule allows for coherent manipulation of two-electron spin states using Raman transitions. Such two-electron spin states defined by the singlet and triplet states of two exchange-coupled quantum dots can have favorable coherence properties. In addition, two of the four metastable ground states in this system can be used as auxiliary states that could facilitate implementation of tasks such as mapping of spin states to that of a single propagating photon. We find that even weak spin-orbit effects---manifesting themselves as slightly different ggg factors for the electron and the hole---would allow for the coherent Raman coupling of the singlet-triplet states. We also discuss the possibilities for implementing quantum optical techniques for spin preparation and manipulation.
Two-electron charged self-assembled quantum dot molecules exhibit a decoherence-avoiding singlet-triplet qubit subspace and an efficient spin-photon interface. Here, we demonstrate that the cycling transitions originating from auxiliary ground states in the same system allow for an efficient optical read-out of a singlet-triplet qubit. By implementing a spin-selective state transfer to the auxiliary state using a resonant laser field, we observe an improvement approaching two orders of magnitude in fidelity as compared to spin measurement by light scattering directly from the qubit states. Embedding the quantum dot molecule inside a low quality-factor micro-cavity structure should enable single-shot qubit read-out.
The integration of InAs/GaAs quantum dots into nanophotonic cavities has led to impressive demonstrations of cavity quantum electrodynamics. However, these demonstrations are primarily based on two-level excitonic systems. Efforts to couple long-lived quantum dot electron spin states with a cavity are only now succeeding. Here we report a two-spin-cavity system, achieved by embedding an InAs quantum dot molecule within a photonic crystal cavity. With this system we obtain a spin singlet-triplet Λ-system where the ground-state spin splitting exceeds the cavity linewidth by an order of magnitude. This allows us to observe cavity-stimulated Raman emission that is highly spin-selective. Moreover, we demonstrate the first cases of cavity-enhanced optical nonlinearities in a solid-state Λ-system. This provides an all-optical, local method to control the spin exchange splitting. Incorporation of a highly engineerable quantum dot molecule into the photonic crystal architecture advances prospects for a quantum network.
Tunnel-coupled optically active quantum dot molecules (QDMs) have the potential to operate as spin-photon interfaces with coupled spins that interact with two different photon frequencies at the same time. A prerequisite is to deterministically prepare two (electron or hole) spins in the QDM and be able to electrically tune the orbital state couplings. Here, we demonstrate the sequential optical charging of a single QDM with two-electron spins while simultaneously maintaining the ability to widely tune orbital couplings using static electric fields and optically drive the system for quantum light generation. We optically prepare one- and two-spin states, initialize via optical pumping and explore orbital and spin relaxation dynamics for one- and two-spin states as a function of the energy detuning and hybridization of orbital states. For two-spin states, remarkably long singlet-triplet relaxation times are observed, extending beyond ∼ 100 μ s with strong dependence on the relative energy of ground and excited two-spin states. Qualitative agreement is observed with k · p calculations of phonon-mediated spin relaxation. Our results provide new quantitative understanding of the dynamics of one- and two-spin states and address a number of key prerequisites for using QDMs to create multidimensional photonic cluster states.
A pair of tunnel-coupled quantum dots has advantages over single dots in tunability and spin coherence, but far less work has been done to measure and understand the quantum optics of this system. In particular, the two-electron singlet-triplet system with one electron in each dot generates an eight-level system in which two-level and four-level double-\ensuremath{\Lambda} systems are coupled only through the hyperfine interaction and other spin relaxation processes. We first measure the emission spectrum under resonant driving with a continuous-wave laser and then perform time-correlated Hanbury Brown-Twiss (HBT) and Hong-Ou-Mandel (HOM) interferometric measurements to examine nonclassical photon properties of spin-flip Raman emission and resonance fluorescence. The shapes of second-order correlation functions $[{g}^{(2)}(\ensuremath{\tau})]$ from HBT are strikingly different between resonance fluorescence and spin-flip Raman emission, reflecting the spin dynamics of the singlet-triplet system. In addition, our two-photon HOM measurements demonstrate a high raw visibility of 0.96, with a coherence time exceeding the radiative lifetime.