Differential-phase-shift quantum key distribution using heralded narrow-band single photons

Universal relation between the conditional auto-correlation function and the cross-correlation function of biphotons

We systematically studied the relation between the conditional auto-correlation function (CACF) and cross-correlation function (CCF) of biphotons or pairs of single photons. The biphotons were generated from a heated atomic vapor via the spontaneous four-wave mixing (SFWM) process. In practical usage, one single photon of a pair is utilized as the heralding photon, and another is employed as the heralded photon. Motivated by the data of CACF of the heralded photons versus CCF, we proposed a universal formula to predict the CACF. The derived formula was based on general theory and is also valid for the biphoton generation process of spontaneous parametric down-conversion (SPDC). With the formula, we utilized the experimentally determined parameters to predict CACFs, which can well agree with the measured CACFs. The proposed formula enables one to quantitatively know the CACF of heralded single photons without the measurement of Hanbury-Brown-Twiss-type three-fold coincidence count. This study provides a better understanding of biphoton generation using the SFWM or SPDC process. Our work demonstrates a valuable tool for analyzing a vital property of how the heralded photons are close to Fock-state single photons.

Field test of quantum key distribution with high key creation efficiency

Quantum key distribution (QKD) promises unconditional security for communication. However, the random choices of the measurement basis in QKD usually result in low key creation efficiency. This drawback is overcome in the differential-phase-shift QKD, provided that each photon can be prepared in a large number of time slots with a proper waveform. In this work we develop a miniature room-temperature 1550-nm single-photon source to generate narrowband single photon in 50 time slots with a nearly optimal waveform for achieving unity key creation efficiency. By utilizing these single photons in the field test, we demonstrate the differential-phase-shift QKD with a key creation efficiency of 97%. Our work shows that the practical QKD can benefit from the narrowband single photons with controllable waveforms.

Uniaxial strain tuning of organic molecule single photon sources

Organic fluorophores are excellent single photon sources, combining high brightness, lifetime-limited linewidths and useful emission wavelengths. A key factor in their performance as photon emitters is their dynamic frequency tunability, which can be used to render the emission from multiple molecules indistinguishable. In this work we demonstrate dynamic tuning of dibenzoterrylene molecules embedded in anthracene crystals through the application of uniaxial strain fields. By bending a piezoelectric strip in two opposite directions in linear steps, we impose an escalating compressive or tensile strain on the molecular crystals, resulting in two opposite dynamic detunings of the dopant dibenzoterrylene emission wavelength. To validate that the tuning mechanism is strain, we performed a similar measurement using an identical strip that was depolarised by annealing in which the tuning was absent. Finally, we simulated the effect of strain on the dopant dibenzoterrylene emission wavelength by combining molecular dynamics and density functional theory techniques to determine the strain tuning rate which matched well with that found experimentally.

Experimental test of the no-go theorem for continuous ψ-epistemic models

Quantum states are the key mathematical objects in quantum theory; however, there is still much debate concerning what a quantum state truly represents. One such century-old debate is whether a quantum state is ontic or epistemic. Recently, a no-go theorem was proposed, stating that the continuous ψ-epistemic models cannot reproduce the measurement statistic of quantum states. Here we experimentally test this theorem with high-dimensional single photon quantum states without additional assumptions except for the fair-sampling assumption. Our experimental results reproduce the prediction of quantum theory and support the no-go theorem.

A high-speed multi-protocol quantum key distribution transmitter based on a dual-drive modulator

We propose a novel source based on a dual-drive modulator that is adaptable and allows Alice to choose between various practical quantum key distribution (QKD) protocols depending on what receiver she is communicating with. Experimental results show that the proposed transmitter is suitable for implementation of the Bennett and Brassard 1984 (BB84), coherent one-way (COW) and differential phase shift (DPS) protocols with stable and low quantum bit error rate. This could become a useful component in network QKD, where multi-protocol capability is highly desirable.