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Applied Physics Letters

On-demand entanglement source with polarization-dependent frequency shift

Xiang‐Bin Wang, Chengxi Yang, Yanbing Liu

Published May 17, 2010• 16 citations
Publisher

Abstract

The polarization entanglement photon pairs generated from the biexciton cascade decay in a single semiconductor quantum dot is distorted due to the fine structure splitting. We show that, frequency of light can be shifted when it passes through an electro-optic modulator under voltage ramping. Based on this, we can realize polarization-dependent frequency shift and the hidden entanglement due to fine structure splitting can be revealed.

DOI
10.1063/1.3427485

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Acta Physica Sinica•Jan 1, 2014•0 citations
The polarization entanglement photon pairs generated from the biexciton cascade decay in a single semiconductor quantum dot is distorted due to the fine structure splitting. We show that, frequency of light can be shifted when it passes through an electro-optic modulator under voltage ramping. Based on this, we can realize polarization-dependent frequency shift and the hidden entanglement due to fine structure splitting can be revealed.
The polarization entanglement photon pairs generated from the biexciton cascade decay in a single semiconductor quantum dot is corrupted by the position-dependent (time-dependent) phase difference of the two polarization mode due to the fine structure splitting. We show that, by taking voltage ramping to an electro-optic modulator, such phase-difference can be removed. In our first proposed set-up, two photons are sent to two separate Pockels cell under reverse voltage ramping, as a result, the position-dependent phase difference between the two polarization mode is removed in the outcome state. In our second proposed set-up, the polarization of the first photon is flipped and then both photons fly into the same Pokels cell. Since we only need to separate the two photons rather than separate the two polarization modes, our schemes are robust with respect to fluctuations of the optical paths.
We propose a polarization-dependent spatial phase modulation method to purify the two-photon polarization entanglement generated by the biexciton cascade decay in a single semiconductor quantum dot. In principle, our method can completely compensate the random phase acquired from the decay of the non-degenerate exciton states in time domain. In frequency domain, our method is equivalent to shifting photon frequency according to its polarization. The method can be applied immediately by existing experimental set-ups.
Entangled photons are an important resource for quantum optics. Quantum dots are a source of on-demand and highly entangled photon pairs at a high repetition rate. However, fine structure splitting (FSS) in the biexciton-exciton cascade causes the photons to be emitted in a time-dependant state instead of an ideal Bell state. Current techniques to remove the FSS include applying a strain, electric, or magnetic field and require post-processing of the quantum dots which reduces device yield. We use a novel all-optical approach implemented by emulating a fast-rotating half-wave plate in a Lithium Niobate waveguide using a electro-optic modulator. This method allows us to frequency shift single photons and produce a time-independant entangled photon.
We propose to use a time-delayed quantum-coherent feedback mechanism to increase and control the entanglement of photon pairs emitted by a quantum dot biexciton cascade. The quantum dot biexciton cascade is a well-known source of entangled photons on demand, however excitonic fine-structure splitting decreases the achievable polarization entanglement. We demonstrate that feedback can change the spectrum of the emitted photons in a way that the entanglement is either strongly increased or decreased, depending on the feedback time and phase. We analyze the dependence on parameters such as the delay time and the robustness of the proposed mechanism.
Semiconductor quantum dots are potential sources for generating polarization-entangled photons efficiently. The main prerequisite for such generation based on biexciton–exciton cascaded emission is to control the exciton fine-structure splitting. Among various techniques investigated for this purpose, an electric field is a promising means to facilitate the integration into optoelectronic devices. Here we demonstrate the generation of polarization-entangled photons from single GaAs quantum dots by an electric field. In contrast to previous studies, which were limited to In(Ga)As quantum dots, GaAs island quantum dots formed by a thickness fluctuation were used because they exhibit a larger oscillator strength and emit light with a shorter wavelength. A forward voltage was applied to a Schottky diode to control the fine-structure splitting. We observed a decrease and suppression in the fine-structure splitting of the studied single quantum dot with the field, which enabled us to generate polarization-entangled photons with a high fidelity of 0.72±0.05. As quantum information and communication experiments grow in sophistication, the need for efficient sources of entangled photons escalates. Using exciton and biexciton emission in GaAs island quantum dots, Ghaliet al. demonstrate the electric field-induced generation of entangled photons with high fidelity.
Through the electro-optic modulation on photons with different polarization modes from quantum dot entanglement sources, the way to eliminate the photon frequency difference is analyzed systematically. We design an electro-optic modulation scheme with Pockels cell, to which a rising or falling voltage of 8 V·ns-1 is applied, thereby realizing a photon frequency shift with a bandwidth of 18 MHz. Simply increasing the modulation voltage slope would realize over 1 GHz frequency shift effectively, which provides a practical basis for realizing the future quantum dot entanglement sources with a perfect certainty.