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Register a new userDemonstration of quantum entanglement between a single electron spin confined to an InAs quantum dot and a photon
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The electron spin state of a singly charged semiconductor quantum dot has been shown to form a suitable single qubit for quantum computing architectures with fast gate times. A key challenge in realizing a useful quantum dot quantum computing architecture lies in demonstrating the ability to scale the system to many qubits. In this letter, we report an all optical experimental demonstration of quantum entanglement between a single electron spin confined to single charged semiconductor quantum dot and the polarization state of a photon spontaneously emitted from the quantum dots excited state. We obtain a lower bound on the fidelity of entanglement of 0.59, which is 84% of the maximum achievable given the timing resolution of available single photon detectors. In future applications, such as measurement based spin-spin entanglement which does not require sub-nanosecond timing resolution, we estimate that this system would enable near ideal performance. The inferred (usable) entanglement generation rate is 3 x 10^3 s^-1. This spin-photon entanglement is the first step to a scalable quantum dot quantum computing architecture relying on photon (flying) qubits to mediate entanglement between distant nodes of a quantum dot network.
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Strong interactions between single spins and photons are essential for quantum networks and distributed quantum computation. They provide the necessary interface for entanglement distribution, non-destructive quantum measurements, and strong photon-photon interactions. Achieving spin-photon interactions in a solid-state device could enable compact chip-integrated quantum circuits operating at gigahertz bandwidths. Many theoretical works have suggested using spins embedded in nanophotonic structures to attain this high-speed interface. These proposals exploit strong light-matter interactions to implement a quantum switch, where the spin flips the state of the photon and a photon flips the spin-state. However, such a switch has not yet been realized using a solid-state spin system. Here, we report an experimental realization of a spin-photon quantum switch using a single solid-state spin embedded in a nanophotonic cavity. We show that the spin-state strongly modulates the cavity reflection coefficient, which conditionally flips the polarization state of a reflected photon on picosecond timescales. We also demonstrate the complementary effect where a single photon reflected from the cavity coherently rotates the spin. These strong spin-photon interactions open up a promising direction for solid-state implementations of high-speed quantum networks and on-chip quantum information processors using nanophotonic devices.
We propose a scheme to efficiently couple a single quantum dot electron spin to an optical nano-cavity, which enables us to simultaneously benefit from a cavity as an efficient photonic interface, as well as to perform high fidelity (nearly 100%) spin initialization and manipulation achievable in bulk semiconductors. Moreover, the presence of the cavity speeds up the spin initialization process beyond GHz.
Semiconductor quantum dots (QDs) have potential applications in quantum information processing due to the fact that they are potential on-demand sources of single and entangled photons. Generation of polarization-entangled photon pairs was demonstrated using the biexciton-exciton radiative cascade. One obvious way to increase the number of quantum correlated photons that the QDs emit is to use higher-order multiexcitons, in particular the triexciton. Towards achieving this goal, we first demonstrate deterministic generation of the QD-confined triexciton in a well-definedcoherent state and then spectrally identify and directly measure a three-photon radiative cascade resulting from the sequential triexciton-biexciton-exciton radiative recombination.
Epitaxially grown quantum dots (QDs) are promising sources of non-classical states of light such as single photons and entangled photons. However, in order for them to be used as a resource for long-distance quantum communication, distributed quantum computation, or linear optics quantum computing, these photons must be coupled efficiently to long-lived quantum memories as part of a quantum repeater network. Here, we theoretically examine the prospects for efficient storage and retrieval of a QD-generated single photon with a 1 ns lifetime in a multi-level atomic system. We calculate using an experimentally demonstrated optical depth of 150 that the storage (total) efficiency can exceed 46% (28%) in a dense, ultracold ensemble of $^{87}$Rb atoms. Furthermore, we find that the optimal control pulse required for storage and retrieval can be obtained using a diode laser and an electro-optic modulator rather than a mode-locked, pulsed laser source. Increasing the optical depth, for example by using Bose-condensed ensembles or an optical cavity, can increase the efficiencies to near unity. Aside from enabling a high-speed quantum network based on QDs, such an efficient optical interface between an atomic ensemble and a QD can also lead to entanglement between collective spin-wave excitations of atoms and the spin of an electron or hole confined in the QD.
We present a scheme for efficient state teleportation and entanglement swapping using a single quantum-dot spin in an optical microcavity based on giant circular birefringence. State teleportation or entanglement swapping is heralded by the sequential detection of two photons, and is finished after the spin measurement. The spin-cavity unit works as a complete Bell-state analyzer with a built-in spin memory allowing loss-resistant repeater operation. This device can work in both the weak coupling and the strong coupling regime, but high efficiencies and high fidelities are only achievable when the side leakage and cavity loss is low. We assess the feasibility of this device, and show it can be implemented with current technology. We also propose a spin manipulation method using single photons, which could be used to preserve the spin coherence via spin echo techniques.
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