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arXiv:0907.0437 · Physical Review B

Entangled photons on demand: Erasing which-path information with sidebands

W. A. Coish, Jay Gambetta

Published Dec 8, 2009• 12 citations
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Abstract

The biexciton cascade in a quantum dot can be used to generate entangled-photon pairs rapidly and deterministically (on demand). However, due to a large fine-structure splitting between intermediate exciton energy levels, which-path information encoded in the frequencies of emitted photon pairs leads to a small degree of entanglement. Here we show that this information can be efficiently erased by modulating the exciton and biexciton energy levels, giving rise to additional decay paths through sidebands. The resulting degree of entanglement is substantial and can be made maximal through spectral filtering, with only a nominal reduction in collection efficiency.

DOI
10.1103/physrevb.80.241303

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The biexciton cascade in a quantum dot can be used to generate entangled-photon pairs rapidly and deterministically (on demand). However, due to a large fine-structure splitting between intermediate exciton energy levels, which-path information encoded in the frequencies of emitted photon pairs leads to a small degree of entanglement. Here we show that this information can be efficiently erased by modulating the exciton and biexciton energy levels, giving rise to additional decay paths through sidebands. The resulting degree of entanglement is substantial and can be made maximal through spectral filtering, with only a nominal reduction in collection efficiency.
Fine-structure splitting of the exciton energy levels in a quantum dot introduces an energy difference between the two biexciton decay paths. This energy difference greatly reduces the entanglement of the photon pairs generated via biexciton recombination. Here, we propose a feasible method to eliminate the ``which-path'' information by using a fast frequency down-conversion technique, which substantially improves the entanglement of the photon pairs. Moreover, the wavelengths of both of the entangled photons can be transferred to the telecommunication band, which makes this method further appropriate for quantum communication.
We theoretically investigate the production of polarization-entangled photons through the biexciton cascade decay in a single semiconductor quantum dot. To accomplish a high degree of entanglement, despite the exciton finestructure splitting, one must either energetically align the two exciton states by means of external fields or erase the which-path information by post-selecting photons within the correct frequency range. Here we show that in the latter scheme, as well as in related proposals, the photon state becomes deteriorated through dephasing processes in the solid, and the degree of entanglement remains low despite filtering.
Tomographic analysis demonstrates that the polarization state of pairs of photons emitted from a biexciton decay cascade becomes entangled when spectral filtering is applied. The measured density matrix of the photon pair satisfies the Peres criterion for entanglement by more than 3 standard deviations of the experimental uncertainty and violates Bell's inequality. We show that the spectral filtering erases the "which path" information contained in the photons' color and that the remanent information in the quantum dot degrees of freedom is negligible.
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. We will demonstrate the effect of the exciton fine-structure splitting on a single-dot based entangled photon source. 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 of the single photons. We distinguish between the first-order coherence time (2*), and a parameter defined as the cross-coherence time (HV) which is illustrated in the figure. The cross-coherence time is found to be at least an order of magnitude greater than the first-order coherence time. We will also report on recent progress towards an ideal source of triggered polarisation-entangled photons with a single quantum dot.
We theoretically investigate the production of polarization-entangled photons through the biexciton cascade decay in a single semiconductor quantum dot. To accomplish a high degree of entanglement, despite the exciton fine-structure splitting, one must either energetically align the two exciton states by means of external fields or erase the which-path information by postselection or time reordering of photons. Here we show that in the latter schemes the photon state becomes deteriorated through dephasing processes in the solid, and the degree of entanglement remains low.