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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

A light emitting diode for entangled photons

R. M. Stevenson, C. L. Salter, Ian A. Farrer, Christine A. Nicoll, D. A. Ritchie, Andrew J. Shields

Published Feb 10, 2011• 0 citations
Publisher

Abstract

Existing sources of entangled photons require a laser excitation, imposing a practical limit on their potential for large-scale quantum information applications. For the widely used parametric down-conversion sources, zero or multiple photon-pairs are usually emitted due to the probabilistic nature of the non-linear process. This presents an additional fundamental limitation in the form of efficiency and errors. Here we demonstrate the first electrically driven entangled light source, based on a layer of InAs quantum dots embedded in a p-i-n light emitting diode structure, with potential to operate 'on-demand'.

DOI
10.1117/12.879765

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Existing sources of entangled photons require a laser excitation, imposing a practical limit on their potential for large-scale quantum information applications. For the widely used parametric down-conversion sources, zero or multiple photon-pairs are usually emitted due to the probabilistic nature of the non-linear process. This presents an additional fundamental limitation in the form of efficiency and errors. Here we demonstrate the first electrically driven entangled light source, based on a layer of InAs quantum dots embedded in a p-i-n light emitting diode structure, with potential to operate 'on-demand'.
Current sources for entangled photon pairs rely mostly on spontaneous parametric down-conversion in nonlinear optical crystals. Such sources have the drawback of producing pairs randomly and require powerful optical pump lasers. C. L. Salter et al. have now developed a new source that generates entangled photon pairs on demand under electrical excitation. The device is similar to a classical light-emitting diode (see picture). Its robust and integrated design makes it suitable for applications in quantum information processing.
Introduction Quantum information technology promises to offer incredible advantages over current digital systems, allowing intractable problems in science and engineering to be tackled almost instantaneously through quantum computing, and unconditionally secure communication over long distances using quantum key distribution. Many schemes have been developed to implement quantum computing, including using linear optics [28]. The linear optical approach has proved popular due to the limited decoherence of photons with the environment, and accessibility of the components required for simple experiments. At the heart of an optical quantum computer, or extended range quantum key distribution using quantum relays or repeaters [14, 8, 24], lie entangled photons. The characteristics of the sources that create entangled photons, and their properties, are therefore central to realizing the full potential of such applications. Quantum dots are one technology with which entangled light sources can be built [6]. Although first realised only relatively recently [49], they in principle offer key fundamental and practical advantages over other entangled photon sources. In the fundamental sense, quantum dots can be triggered, so that no more than one entangled photon pair is emitted at a time. This is in stark contrast to Poissonian entangled light sources [47, 27, 13], including the most widely used parametric down-conversion, where zero or multiple photon-pairs are usually emitted due to their probabilistic nature. Furthermore quantum dots have the potential to operate with high efficiency, with current experiments reporting up to 72% collection efficiency for the first and second photon [9, 12].
The generation of entangled photon pairs is a fundamental cornerstone of quantum optics, and an essential technology in the high profile field of quantum communication and computing. Currently the most widely used techniques for generating entangled photon pairs are nonlinear optical processes, such as parametric down conversion, which produce a probabilistic number of pairs per excitation cycle. Such a source is of limited use in quantum information/processing applications where a regular stream of single entangled photon pairs is usually required. We have produced such a triggered source from a semiconductor device for the first time, using a two-photon cascade in a single quantum dots, demonstrating efficiencies up to 70%, close to the level required for useful applications. Single quantum dots could prove to be the first robust and compact triggered source of entangled photons.
Sources of entangled pairs of photons can be used for encoding signals in quantum-encrypted communications, allowing a sender, Alice, and a receiver, Bob, to exchange keys without the possibility of eavesdropping. In fact, any quantum information system would require single and entangled photons to serve as qubits. For this purpose, semiconductor quantum dots (QD) have been extensively studied for their ability to produce entangled light and function as single photon sources. The quality of such sources is evaluated based on three criteria: high efficiency, small multi-photon probability, and quantum indistinguishability. In this work, a simple quantum dot-based LED (E-LED) was used as a quantum light source for on-demand emission, indicating the potential for use as quantum information devices. Limitations of the device include the fine-structure splitting of the quantum dot excitons, their coherence lengths and charge carrier interactions in the structure. The quantum dot-based light emitting diode was initially shown to operate in pulsed mode under AC bias frequencies of up to several hundreds of MHz, without compromising the quality of emission. In a Hong-ou-Mandel interference type experiment, the quantum dot photons were shown to interfere with dissimilar photons from a laser, achieving high two-photon interference (TPI) visibilities. Quantum entanglement from a QD photon pair was also measured in pulsed mode, where the QD-based entangled-LED (E-LED) was electrically injected at a frequency of 203 MHz. After verifying indistinguishability and good entanglement properties from the QD photons under the above conditions, a quantum relay over 1km of fibre was demonstrated, using input qubits from a laser source. The average relay fidelity was high enough to allow for error correction for this BB84-type scheme. To improve the properties of the QD emission, an E-LED was developed based on droplet epitaxy (D-E) QDs, using a different QD growth technique. The relevant chapter outlines the process of QD growth and finally demonstration of quantum entanglement from an electrically injected diode, yielding improvements compared to previous E-LED devices. For the same reason, an alternative method of E-LED operation based on resonant two-photon excitation of the QD was explored. Analysis of Rabi oscillations in a quantum dot with a bound exciton state demonstrated coupling of the ground state and the biexciton state by the external oscillating field of a laser, therefore allowing the transition between the two states. The results include a considerable improvement in the coherence length of the QD emission, which is crucial for future quantum network applications. We believe that extending this research can find application in quantum cryptography and in realising the interface of a quantum network, based on semiconductor nanotechnology.