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arXiv:1105.3737 · Physical Review Letters

Exciton-Spin Memory with a Semiconductor Quantum Dot Molecule

A. Boyer de la Giroday, Niklas Sköld, R. M. Stevenson, Ian A. Farrer, D. A. Ritchie, Andrew J. Shields

Published May 24, 2011• 56 citations
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Abstract

We report on a single photon and spin storage device based on a semiconductor quantum dot molecule. Optically excited single electron-hole pairs are trapped within the molecule, and their recombination rate is electrically controlled over 3 orders of magnitude. Single photons are stored up to 1 μs and read out on a subnanosecond time scale. By using resonant excitation, the circular polarization of individual photons is transferred into the spin state of electron-hole pairs with a fidelity above 80%, which does not degrade for storage times up to the 12.5 ns repetition period of the experiment.

DOI
10.1103/physrevlett.106.216802

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We report on a single photon and spin storage device based on a semiconductor quantum dot molecule. Optically excited single electron-hole pairs are trapped within the molecule, and their recombination rate is electrically controlled over 3 orders of magnitude. Single photons are stored up to 1 μs and read out on a subnanosecond time scale. By using resonant excitation, the circular polarization of individual photons is transferred into the spin state of electron-hole pairs with a fidelity above 80%, which does not degrade for storage times up to the 12.5 ns repetition period of the experiment.
We present an exciton-spin memory scheme using a single QDM consisting of two InAs QDs separated by a thin GaAs tunnelling barrier. Electrical control of the electron-hole spatial separation allowed the exciton lifetime to be tuned in the 1-1000 ns range. Storage and read-out operations were implemented using sub nanosecond modulation of the lifetime and photon storage up to 1μs was demonstrated. Using resonant excitation the circular polarisation of photons was transferred and stored in the exciton Zeeman states of a QDM under a magnetic field. The delayed emission was clearly co-polarised with the excitation light and a combined fidelity of 80% for the write and read operations was achieved.
New technologies based on the properties of quantum mechanics promise to revolutionise the way information is processed by outperforming what is possible using classical devices. Examples include massively parallel processing using quantum computers, verifiably secure communication using quantum cryptography, and measurement with sensitivity beyond classical limitation with quantum metrology. Realising the full potential of these technologies necessitates the ability to communicate quantum information over large distances, a key requirement for future quantum networks. However, developing practical implementations of long-distance quantum communication is challenging as it necessitates three major ingredients: light-matter interfaces, elementary quantum operations, and quantum memories. This thesis describes work that has been undertaken to address these requirements using semiconductor nanotechnology. We have first demonstrated that single InAs quantum dots embedded inside conventional diode structures constitute high-fidelity controllable interfaces between optical qubits and solid-state qubits. Indeed, the polarisation state of a photon was transferred into the spin state of an electron-hole pair and eventually restored through radiative recombination of the electron and the hole with a fidelity up to 95%. Moreover, spins were manipulated using subnanosecond modulation of a vertical electric field applied to the quantum dots. By controlling this electrical modulation, we demonstrated elementary phase-shift and spin-flip gate operations with near-unity fidelities. An electron-hole pair confi ned in a single quantum dot has a short radiative lifetime limiting therefore its use as an excitonic quantum memory. The solution we proposed was to use a quantum dot molecule to control the spatial separation of the electron and the hole and therefore prevent their recombination. Comprehensive studies of electric field eff ects upon the photoluminescence of quantum dot molecules lead to a clear understanding and a good control over their physical properties. Single photons were stored in individual quantum dot molecules up to 1μs and read out on a subnanosecond time scale. Moreover, the circular polarisation of individual photons was transferred into the spin state of electron-hole pairs with a fidelity above 90%, which does not degrade for storage times up to the 12.5 ns repetition period of the experiment. Our work on single quantum dots could be extended in the near future to allow for two-qubits quantum operations by con fining a second electron-hole pair to be electrically manipulated. Storage of a superposition of spin states in a quantum dot molecule should also be possible if the spin states are made degenerate, which is feasible using the electric fi eld dependence of the energy splitting between the spin states discussed in this thesis. We believe that combining both approaches will lead to the development of a controllable multi-qubit quantum memory for polarised light, a building block for long distance quantum communication based on semiconductor nanotechnology.
We show storage of the circular polarization of an optical field, transferring it to the spin-state of an individual electron confined in a single semiconductor quantum dot. The state is subsequently read out through the electronically-triggered emission of a single photon. The emitted photon shares the same polarization as the initial pulse but has a different energy, making the transfer of quantum information between different physical systems possible. With an applied magnetic field of 2 T, spin memory is preserved for at least 1000 times more than the exciton's radiative lifetime.
Storage and retrieval of excitons were demonstrated with semiconductor self-assembled quantum dots (QDs). The optically generated excitons were dissociated and stored as separated electron-hole pairs in coupled QD pairs. A bias voltage restored the excitons, which recombined radiatively to provide a readout optical signal. The localization of the spatially separated electron-hole pair in QDs was responsible for the ultralong storage times, which were on the order of several seconds. The present limits of this optical storage medium are discussed.