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Physical review. B./Physical review. B

Spin-dependent quantum optics in a quantum dot molecule

Bumsu Lee, Brennan C. Pursley, Sam Carter, Sophia E. Economou, Michael K. Yakes, Joel Q. Grim, Allan S. Bracker, D. Gammon

Published Sep 30, 2019• 3 citations
Publisher

Abstract

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.

DOI
10.1103/physrevb.100.125438

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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)(\ensuremathτ)][{g}^{(2)}(\ensuremath{\tau})][g(2)(\ensuremathτ)] 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.
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.
The work presented in this dissertation contains the first observation of spin-resolved resonance fluorescence from a single quantum dot and its application of direct measurement of electron spin dynamics. The Mollow triplet and the Mollow quintuplet, which are the hallmarks of resonance fluorescence, are presented as the non-spin-resolved and spin-resolved resonance fluorescence spectrum, respectively. The negligible laser background contribution, the near pure radiative broadened spectrum and the anti-bunching photon statistics imply the sideband photons are background-free and near transform-limited single photons. This demonstration is a promising step towards the heralded single photon generation and electron spin readout. Instead of resolving spectrum, an alternative spin-readout scheme by counting resonance fluorescence photons under moderate laser power is demonstrated. The measurements of n-shot time-resolved resonance fluorescence readout are carried out to reveal electron spin dynamics of the measurement induced back action and the spin relaxation. Hyperfine interaction and heavy- light hole mixing are identified as the relevant mechanisms for the back action and phonon-assistant spin-orbit interaction dominates the spin relaxation. After a detailed discussion on charge-spin configurations in coupled quantum dots system, the single-shot readout on electron spin are proposed.
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.
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.
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 S-T relaxation times are observed extending beyond ∼100μs\sim 100μs∼100μs with strong dependence on the relative energy of ground and excited two-spin states. Qualitative agreement is observed with k⋅p\mathbf{k \cdot p}k⋅p calculations of phonon-mediated spin-relaxation. Our results provide new quantitative understanding of the dynamics of one and two-spin states and confirm their suitability of QDMs for creating multidimensional photonic cluster states by exploiting tunable spin-spin exchange couplings at zero magnetic fields combined with optical driving.
A single spin confined to a semiconductor quantum dot is a system of significant interest for quantum information science, as a potential optically-addressable qubit. In many respects, a quantum dot behaves like a single atom with high quality single photon emission. By controlling the light-matter coupling in such a system, it is possible to generate highly non-classical states of light and coherently control a single spin confined to the quantum dot. A departure from the ideal atomic picture appears once we consider the mesoscopic environment with which the quantum dot interacts. Charge fluctuations in the surroundings of the quantum dot affect the photon emission frequency leading to inhomogeneous broadening. Further broadening of the emission is caused by coupling to phonon modes of the host semiconductor material. Finally, coupling between the spin of a confined electron and a large bath of nuclear spins residing in the quantum dot leads to fast dephasing of the electron spin. All of these effects are typically considered detrimental to the potential use of quantum dots for quantum technologies. In this thesis, we develop the environmental coupling of a negatively charged quantum dot as a resource for quantum optics and spin control. First, the phonon-assisted fluorescence is shown to be a useful independent channel for feedback stabilisation of the quantum dot emission frequency, without requiring a measurement of the indistinguishable zero-phonon line. With stabilisation, the corresponding frequency broadening is drastically improved, and the sub-Hz frequency fluctuations are no longer resolved. Next, we show low-power resonance fluorescence emission spectra of the negatively charged trion transition. In the low power regime of resonance fluorescence, the excited state is not populated and most of the emission is coherent. In addition to elastic Rayleigh scattering, we observe coherent Raman sidebands, linked to an effective magnetic field created by the hyperfine interaction, the Overhauser field. This fluctuating effective field lifts the electron spin degeneracy in the absence of a magnetic field, and dictates the optical selection rules of the trion system. These spectra therefore allow for a measurement of the time-averaged distributions of in-plane and out-of-plane Overhauser field components. In the final part of the thesis, we use this hyperfine-generated Λ -scheme to optically create electron spin superpositions through two-colour excitation and coherent population trapping. We then show that rapid shifts in the relative phase of the lasers lead to initialisation of the electron spin into a rotated dark state.