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Experiment looks to increase security in quantum communication

Friday, September 4, 2015

Researchers develop first source of on-demand time-bin entangled photon pairs using quantum dot

An international team of researchers, including Institute for Quantum Computing (IQC) faculty member Michael Reimer, has developed the first source of on-demand single time-bin entangled photon pairs with no possibility of producing extra unwanted pairs. Ensuring that only a single pair is produced may lead to increased security in quantum communication, improved transfer of quantum information and advances in the development of compact and scalable quantum information devices.

Quantum communication networks often rely on polarization entangled photons, strongly correlated quantum particles to transmit quantum information through an optical fibre. The polarization-entangled photons are vulnerable to thermal and mechanical disturbances in the fibre – this causes the photons to lose entanglement over long distances and disrupt the network. Unlike polarization entangled photons, time-bin entangled photons are not affected by these disturbances in the fibre, which makes time-bin entanglement ideal for robust transfer of quantum information over long distances.

Reimer, a faculty member with the Department of Electrical and Computer Engineering at the University of Waterloo and collaborators at the Technical University of Delft created a source of on-demand single time-bin entangled photon pairs. The team used a quantum dot – a nano-sized object containing thousands of semiconductor atoms – to generate single pairs of polarization entangled photons on-demand, emitted only when triggered by a laser pulse. Next the photon pairs were sent through an interferometer to convert to time-bin entanglement.

Time-bin entangled photons can be in either the early time-bin, meaning they take the short path, or the late time-bin if they take the long path through the polarization-time-bin interface (see Figure 1). The conversion process is reversible, allowing for the time-bin entanglement to be distributed over long distances of optical fibre and then converted back to polarization entanglement for processing quantum information.

Set-up based on a quantum dot and a polarization-time-bin interface.

Figure 1: Set-up based on a quantum dot and a polarization-time-bin interface. Single polarization entangled photon pairs from the quantum dot are converted into single time-bin entangled photon pairs.

This new optical approach to generate time-bin entanglement developed by Reimer and collaborators opens up other possibilities for future research, including spin-photon entanglement distribution, entanglement purification and more. Currently, the conversion process requires a bulky laser and large optical set-up. “It’s possible to embed the quantum dot into a semiconductor device and operate it electrically – then only a compact, nano-sized device would generate the time-bin entangled photons,” Reimer notes.

The experiment was conducted at the Delft University of Technology, Netherlands, in the lab of Professor Val Zwiller where Reimer was a research associate at the time. The team included colleagues from Delft, University of Vienna, Austrian Academy of Sciences and University College Cork. Their paper Single pairs of time-bin entangled photons appeared in Physical Review A on September 4, 2015.

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