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Physica E Low-dimensional Systems and Nanostructures

First-order photon interference of a single photon from a single quantum dot

S. Ekuni, H. Nakajima, H. Sasakura, Ikuo Suemune, H. Kumano

Published Nov 21, 2009• 1 citation
Publisher
DOI
10.1016/j.physe.2009.11.029

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A well controlled, triggered source of entangled photons is desirable for many applications in quantum information processing. The two-photon cascade from a biexciton state, two electrons and holes, confined in a quantum dot can be such a source of polarisation-entangled photons provided the two decay paths from the biexciton carry no “which-path” information. In recent years many techniques have been employed to make the two optical decay paths from the biexciton state indistinguishable. A number of these techniques will be discussed and entangled-photon emission from a single quantum dot with a high fidelity in the expected Bell-state from the cascade will be demonstrated. This source of entangled photons allows optical interferometry beyond the limits imposed by the photon wavelength. Interference fringes of the entangled biphoton state reveals a periodicity half of that obtained with the single photon, and much less than that of the pump laser. High fringe visibility indicates that biphoton interference is less sensitive to decoherence than interference of two sequential single photons. The effect of the exciton fine-structure splitting on our entangled photon source will be shown. Surprisingly the entanglement is found to persist despite relatively large separations between the intermediate energy levels of up to 4µeV. Measurements demonstrate that entanglement of the photon pair is robust to the dephasing of the intermediate exciton state responsible for the first order coherence time of either single photon. We distinguish between the first-order coherence time, and a parameter defined as the cross-coherence time, this is illustrated in the figure.
Single photons with ultralong coherence time and high indistinguishability are key resources for interference-based photonic quantum technologies. While laser light exhibits excellent coherence, it cannot be attenuated to single photons through linear optics. Here, we demonstrate high-quality single photons coherently reflected from a single quantum dot (QD) coupled to a double-sided optical microcavity and driven by a continuous-wave single-frequency laser in the low-driving regime. Measuring the reflected light without cross-polarization filtering to suppress laser scattering, we observe a second-order correlation of g (2) (0)=0.030±0.002 and a two-photon interference visibility of V (0)=94.3%±0.2%, which remains robust for photon separations up to 8 km. Under weak driving, these photons exhibit an ultralong coherence time of 258±2µs—six orders of magnitude longer than the coherence time (115 ps) of spontaneously emitted photons—directly inherited from the driving laser. The observed anti-bunching arises from single-photon switching governed by the QD’s saturation nonlinearity and is further explained by quantum interference between the reflected driving field and incoherent scattering. By inheriting the laser’s ultralong first-order coherence and robust photon indistinguishability, coherently scattered single photons hold promise as valuable resources for quantum information science and photonic quantum technologies.