Rui-Bo Jin

Widely tunable single photon source with high purity at telecom wavelength

We theoretically and experimentally investigate the spectral tunability and purity of photon pairs generated from spontaneous parametric down conversion in periodically poled KTiOPO(4) crystal with group-velocity matching condition. The numerical simulation predicts that the spectral purity can be kept higher than 0.81 when the wavelength is tuned from 1460 nm to 1675 nm, which covers the S-, C-, L-, and U-band in telecommunication wavelengths. We also experimentally measured the joint spectral intensity at 1565 nm, 1584 nm and 1565 nm, yielding Schmidt numbers of 1.01, 1.02 and 1.04, respectively. Such a photon source is useful for quantum information and communication systems.

Pulsed Sagnac polarization-entangled photon source with a PPKTP crystal at telecom wavelength.

We demonstrate pulsed polarization-entangled photons generated from a periodically poled KTiOPO₄ (PPKTP) crystal in a Sagnac interferometer configuration at telecom wavelength. We achieved fidelities of 0.981 ± 0.0002 for |ψ^-〉 and 0.980 ± 0.001 for |ψ⁺〉 respectively.

Nonclassical interference between independent intrinsically pure single photons at telecommunication wavelength

We demonstrate a Hong-Ou-Mandel interference between two independent, intrinsically pure, heralded single photons from spontaneous parametric down-conversion (SPDC) at telecommunication wavelength. A visibility of

Efficient generation of twin photons at telecom wavelengths with 2.5 GHz repetition-rate-tunable comb laser

85.5\ifmmode\pm\else\textpm\fi{}8.3%

Highly efficient entanglement swapping and teleportation at telecom wavelength.

was achieved without using any bandpass filter. Thanks to the group-velocity-matched SPDC and superconducting nanowire single-photon detectors (SNSPDs), the fourfold coincidence counts are one order higher than that in the previous experiments. The combination of bright single-photon sources and SNSPDs is a crucial step for future practical quantum infocommunication systems at telecommunication wavelength.

High-visibility nonclassical interference between pure heralded single photons and weak coherent photons

We demonstrate a down-conversion-based twin photon source pumped by a 10 GHz repetition-rate-tunable comb laser at 1553 nm wavelength. We show high Hong-Ou-Mandel interference visibilities, which are free from the pump-power induced degradation.

Full analysis of multi-photon pair effects in spontaneous parametric down conversion based photonic quantum information processing

Entanglement swapping at telecom wavelengths is at the heart of quantum networking in optical fiber infrastructures. Although entanglement swapping has been demonstrated experimentally so far using various types of entangled photon sources both in near-infrared and telecom wavelength regions, the rate of swapping operation has been too low to be applied to practical quantum protocols, due to limited efficiency of entangled photon sources and photon detectors. Here we demonstrate drastic improvement of the efficiency at telecom wavelength by using two ultra-bright entangled photon sources and four highly efficient superconducting nanowire single photon detectors. We have attained a four-fold coincidence count rate of 108 counts per second, which is three orders higher than the previous experiments at telecom wavelengths. A raw (net) visibility in a Hong-Ou-Mandel interference between the two independent entangled sources was 73.3 ± 1.0% (85.1 ± 0.8%). We performed the teleportation and entanglement swapping, and obtained a fidelity of 76.3% in the swapping test. Our results on the coincidence count rates are comparable with the ones ever recorded in teleportation/swapping and multi-photon entanglement generation experiments at around 800 nm wavelengths. Our setup opens the way to practical implementation of device-independent quantum key distribution and its distance extension by the entanglement swapping as well as multi-photon entangled state generation in telecom band infrastructures with both space and fiber links.

Quantum Photonic Network: Concept, Basic Tools, and Future Issues

We present an experiment of nonclassical interference between an intrinsically pure heralded single-photon state and a weak coherent state. Our experiment demonstrates that, without the use of bandpass filters, spectrally pure single photons can have high-visibility (89.4{+-}0.5%) interference with photons from a weak coherent field. Our scheme lays the groundwork for future experiments requiring quantum interference between photons in nonclassical states and those in coherent states.

Simple method of generating and distributing frequency-entangled qudits

In spontaneous parametric down conversion (SPDC) based quantum information processing (QIP) experiments, there is a tradeoff between the coincidence count rates (i.e. the pumping power of the SPDC), which limits the rate of the protocol, and the visibility of the quantum interference, which limits the quality of the protocol. This tradeoff is mainly caused by the multi-photon pair emissions from the SPDCs. In theory, the problem is how to model the experiments without truncating these multi-photon emissions while including practical imperfections.In this paper, we establish a method to theoretically simulate SPDC-based QIPs which fully incorporates the effect of multi-photon emissions and various practical imperfections. The key ingredient in our method is the application of the characteristic function formalism which has been used in continuous variable QIPs. We apply our method to three examples, the Hong–Ou–Mandel interference and the Einstein–Podolsky–Rosen interference experiments, and the concatenated entanglement swapping protocol. For the first two examples, we show that our theoretical results quantitatively agree with the recent experimental results. Also we provide the closed expressions for these interference visibilities with the full multi-photon components and various imperfections. For the last example, we provide the general theoretical form of the concatenated entanglement swapping protocol in our method and show the numerical results up to five concatenations. Our method requires only a small computational resource (a few minutes by a commercially available computer), which was not possible in the previous theoretical approach. Our method will have applications in a wide range of SPDC-based QIP protocols with high accuracy and a reasonable computational resource.

Spectrally resolved Hong-Ou-Mandel interference between independent photon sources

We present practical GHz-clocked QKD systems, next generation entanglement QKD technologies, and QKD platform to manage the secure keys and to support a variety of applications. We then show the intrinsic limit of QKD, i.e., a key rate bound, and discuss how to realize the provable (information theoretic) security with a larger secrecy capacity over longer distances. In particular, we present a basic theory of physical layer cryptography, which characterizes the secrecy capacity, and engineers the tradeoff between the efficiency of reliable transmission and secrecy of communication. We introduce a concept to unify these schemes in photonic network, referred to as quantum photonic network. Future issues for realizing this new network paradigm are discussed.