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Frontiers in Optics

Quantum Cryptography with Optical Entanglement at 1.5 µm

Alexander V. Sergienko, Martin A. Jaspan, Bahaa E. A. Saleh, Malvin Carl Teich

Published Jan 1, 2006• 0 citations
Publisher

Abstract

We report on the engineering, preparation, and utilization of polarization entangled-photon states in the telecommunications window of 1.5 µm for secure quantum key distribution. Traditionally, such sources have been based on the use of the process of spontaneous parametric down conversion (SPDC) when nonlinear crystal is pumped by a continuous wave (cw) or a pulsed laser. The pump photon disintegrates spontaneously giving birth to two twin photons that are strongly correlated in frequency, polarization and wave vector (direction of propagation). Photon entanglement results from the fact that the possible values of correlated parameters of twin photons are made indistinguishable from each other either at the generation or at the single-photon detection stage. This leads to non-classical interference effects.

DOI
10.1364/ls.2006.lwa4

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We report on the engineering, preparation, and utilization of polarization entangled-photon states in the telecommunications window of 1.5 µm for secure quantum key distribution. Traditionally, such sources have been based on the use of the process of spontaneous parametric down conversion (SPDC) when nonlinear crystal is pumped by a continuous wave (cw) or a pulsed laser. The pump photon disintegrates spontaneously giving birth to two twin photons that are strongly correlated in frequency, polarization and wave vector (direction of propagation). Photon entanglement results from the fact that the possible values of correlated parameters of twin photons are made indistinguishable from each other either at the generation or at the single-photon detection stage. This leads to non-classical interference effects.
We report on the construction of polarization-entangled photon sources for quantum key distribution (QKD) testbed using nonlinear optical process of spontaneous parametric down-conversion (SPDC) pumped by a 266-nm and 351-nm continuous wave (cw) and a 415-nm femtosecond-pulsed laser sources. The efficient coupling of down-converted photons at 702 nm and 830 nm into optical single-mode fiber has enabled us to increase the rate of entangled-photon pairs available for transmission over communication channels with a high degree of polarization entanglement. The detection and characterization of the entangled-photon-state properties has been performed using commercially available Silicon avalanche photodiodes (APD) as well as using a novel photon-number-resolving cryogenic photodetector, which has been developed by our colleagues at the Boulder Division of NIST and brought to BU for tests with elements of the future DARPA Quantum Network.
Quantum computation and quantum communication require sources of entangled photons, especially from low-cost diode lasers. This study considered a polarization entangled photons pulse source produced via a spontaneous parametric down-conversion process in the type-I phase-matching of two nonlinear crystals, incorporating a continuous wave laser diode operating in pulse mode as a pump. The generated photons were measured based on visibility as a function of the pump frequency and collection apertures. A polarization correlation between photon pairs of approximately 93% and a Bell parameter S value of 2.56 ± 0.05 were obtained using the source described here.
A new scheme for quantum cryptography, based on a distributed polarization quantum intensity interferometer, is presented. Two-photon entangled states generated via the optical nonlinear process of type-II phase-matched spontaneous parametric down-conversion are used to securely distribute secret cryptographic keys. The high contrast and stability of the quantum interference pattern obtained by using this design renders it superior to the best existing single-photon polarization technique. In addition, the use of high-repetition-rate femtosecond pump pulses for down-conversion significantly enhances the production rate of entangled photon pairs for key distribution.
This thesis deals with the production of entangled photons using spontaneous parametric down-conversion (SPDC). We start with a short overview of some important theoretical concepts. First we provide a brief reminder of the theory of entanglement. We then discuss how the state of quantum systems can be determined using quantum state tomography. We also explain SPDC, the physical process which we use to produce entangled photons. Finally, we give an overview of the methods which have been used to produce entangled photons in the past, both for two- and three-photon entanglement. \n \nThe first experiment is the design of an efficient source of entangled photon pairs based on a polarizing Sagnac interferometer configuration. With this configuration, we can use quasi-phasematched materials which allow for higher efficiencies than standard bulk nonlinear materials. The source is pumped by a low-power continuous-wave laser diode, and produces degenerate photon pairs at 809nm. It has a spectral brightness of 87,500 pairs/(s mW nm), and the fidelity of the produced quantum states with a Bell state is 98.9%. The source is used for experiments in quantum key distribution, cluster state quantum computing, remote state preparation, state discrimination, and entanglement-enhanced classical communication. \n \nThe second experiment discussed in this thesis is the generation of photon triplets using cascaded SPDC. In this experiment, a primary SPDC source is pumped with a low-power, continuous-wave laser diode, producing photon pairs. Single photons from these pairs serve as the pump for a second down-conversion, resulting in photon triplets. This is the first demonstration of the direct production of photon triplets, and the first observation of SPDC at the single photon level. This method could potentially be used to produce entangled photon triplets without post-selection, and as a source of triggered Bell pairs.
Quantum-enhanced optical systems operating within the 2-2.5 µm spectral region have the potential to revolutionize emerging applications in communications, sensing and metrology. However, until now, sources of entangled photons have been realized mainly in the near-infrared 700-1550 nm spectral window. Above 2 µm lies an atmospheric transparency window with nearly one-third of the solar blackbody radiation as what is typical at telecom wavelengths. There is, therefore, a growing interest to deliver sources and detectors operating in this wavelength range for free-space optical communications and in emerging fields of high-sensitivity metrology, specifically, gravitational wave detection [1,2]. Here, using a 130 fs pulsed laser at wavelength 1.05 μm to pump custom-designed non-linear crystals for spontaneous parametric down-conversion (SPDC) process and tailored superconducting nanowire single-photon detectors (SNSPD), we demonstrate for the first-time two-photon quantum interference and polarization entanglement in the mid-infrared, at ≃ 2.1 μm.
It is well-known that multi-photon signals in quantum key distribution (QKD) with weak laser pulses represent a security threat resulting in a reduced rate for secret-key production. Similarly, multi-pair contributions from spontaneous-parametric-down-conversion (SPDC) sources may cause information leak. In particular, in the case of polarization coding (i.e., when the polarization of photons is used to encode the logical qubits) a security risk occurs when the polarizations show some degree of correlation. In such a case an eavesdropper can perform a photon number splitting (PNS) attack by getting hold of, and analyzing, one photon out of the multi-photon state without causing any disturbance to the polarization of the other ones. It is therefore important to estimate the relative amount of polarization- correlated multi-pairs in SPDC signals. We present and discuss the results of a recent investigation on this subject. In our study, we consider a CW-SPDC-source scheme of Kwiat type, fully based on polarization-entanglement. The down-converted field is produced continuously, and a suitably chosen coincidence window replaces the concept of pulses.