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arXiv:1507.01876 · Physical Review A

Single pairs of time-bin-entangled photons

Marijn A. M. Versteegh, Michael E. Reimer, Aafke A. van den Berg, G. Juška, V. Dimastrodonato, Agnieszka M. Gocalinska, E. Pelucchi, Val Zwiller

Published Sep 2, 2015• 36 citations
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Abstract

Time-bin-entangled photons are ideal for long-distance quantum communication via optical fibers. Here we present a source where, even at high creation rates, each excitation pulse generates, at most, one time-bin-entangled pair. This is important for the accuracy and security of quantum communication. Our site-controlled quantum dot generates single polarization-entangled photon pairs, which are then converted, without loss of entanglement strength, into single time-bin-entangled photon pairs.

DOI
10.1103/physreva.92.033802

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Time-bin-entangled photons are ideal for long-distance quantum communication via optical fibers. Here we present a source where, even at high creation rates, each excitation pulse generates, at most, one time-bin-entangled pair. This is important for the accuracy and security of quantum communication. Our site-controlled quantum dot generates single polarization-entangled photon pairs, which are then converted, without loss of entanglement strength, into single time-bin-entangled photon pairs.
Temporal encodings of quantum information are prevalent in applications because of their suitability for long-distance quantum communication and their compatibility with optical fiber communication networks. Perhaps the simplest temporal encoding is time-bin encoding, i. e. in superpositions of two (or more) temporally separated optical pulses. Early attempts at generating time-bin entanglement from single quantum emitters was not able to avoid the problem of re-excitation or was converted probabilistically from polarization entangled photon pairs from a quantum dot. Direct generation requires a metastable level to carry the coherence and avoid double pair emission into the desired time bins. In order to use dark exciton states as metastable states we have worked on their efficient creation and coherent control in the presence of in-plane magnetic fields. Much of this is based on our recent work on advanced excitation schemes using chirped pulses. With chirped pulses we are now able to deterministically populate a dark exciton state and to transfer this population to the biexciton, which can then emit a photon pair.
Single self-assembled quantum dots are established emitters of single photons and entangled photon pairs. The entanglement of photons generated by quantum dots has been shown in polarization [1] , in time bin [2] , and also as hyperentanglement [3] . However, the achievable degree of entanglement and readiness of the source for use in quantum communication protocols, depend on several additional functionalities such as high collection efficiency, coherence of the emitted photon pairs, and fabrication scalability.
We create pairs of nondegenerate time-bin entangled photons at telecom wavelengths with ultrashort pump pulses. Entanglement is shown by performing Bell kind tests of the Franson type with visibilities of up to 91%. As time-bin entanglement can easily be protected from decoherence as encountered in optical fibers, this experiment opens the road for complex quantum communication protocols over long distances. We also investigate the creation of more than one photon pair in a laser pulse and present a simple tool to quantify the probability of such events to happen.
We present two types of photon sources designed for secure quantum communication, e.g. for quantum cryptography. Both types are based on the creation of photon pairs by spontaneous parametric downconversion in nonlinear crystals. The first is a heralded single photon source and the second is a source of polarization-entangled photon pairs. For the heralded single photon source the detection of one of the photons of a downconversion pair is used as a trigger to announce the presence of the other: the single photon. The source is characterized by a highly sub-Poisson photon number statistics making it very suitable for use in quantum cryptography protocols using single photonic qubits to create correlated information between a sender and a receiver. The entanglement source instead uses the inherent non-classical correlations between entangled qubits. We also present a hybrid-encoding where the sender uses polarization to encode information while the receiver uses time-bins. Both sources create photons with highly non-degenerate wavelengths of 810 nm and 1550 nm, taking advantage of the efficient detectors at near-infrared and the low transmission loss of optical fibers at telecommunication wavelengths.
High Resolution Image Download MS PowerPoint Slide Entangled photon pairs at telecom wavelengths are essential for quantum communication, distributed computing, and quantum-enhanced sensing. The telecom O-band offers low chromatic dispersion and fiber loss, which is ideal for long-distance networks. Site-controlled nanowire quantum dots have emerged as a promising platform for generating single and entangled photons, offering high extraction efficiency and scalability. However, their operation has largely been restricted to the visible and first near-infrared (NIR-I) windows. Here, we demonstrate a bright source of entangled photon pairs in the telecom O-band based on site-controlled nanowire quantum dots. We measure a fine-structure splitting of 4.6 μeV, confirming suitability for high-fidelity polarization entanglement. Quantum-state tomography of the biexciton–exciton cascade reveals a maximum fidelity of 85.8 ± 1.1% to the Φ + Bell state and a maximum concurrence of 75.1 ± 2.1%. This work establishes nanowire quantum dots as viable entangled photon sources at telecom, advancing scalable quantum technologies for fiber-based networks.
On-demand sources of entangled photons for the transmission of quantum information in the telecom C-band are required to realize fiber-based quantum networks. So far, non-deterministic sources of quantum states of light were used for long distance entanglement distribution in this lowest loss wavelength range. However, they are fundamentally limited in either efficiency or security due to their Poissonian emission statistics. Here, we show on-demand generation of entangled photon pairs in the telecom C-band by an InAs/GaAs semiconductor quantum dot. Using a robust phonon-assisted excitation scheme we measure a concurrence of 91.4 %91.4\,\%91.4% and a fidelity of 95.2 %95.2\,\%95.2% to Φ+Φ^+Φ+. On-demand generation of polarization entangled photons will enable secure quantum communication in fiber-based networks. Furthermore, applying this excitation scheme to several remote quantum dots tuned into resonance will enable first on-demand entanglement distribution over large distances for scalable real-life quantum applications.