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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Hyper-entangled states and free-space quantum cryptography

Alexander V. Sergienko, Mete Atatüre, Giovanni Di Giuseppe, Gregg S. Jaeger, Bahaa E. A. Saleh, Malvin Carl Teich

Published Dec 1, 2002• 0 citations
Publisher

Abstract

We describe the development of a quantum key distribution (QKD) scheme based on ultrafast laser pumped sources of entangled photon pairs and the engineering of their entanglement properties. Though quantum entanglement has been shown to be a useful resource for quantum key distribution, little work has been carried out in making use of the full range of joint entanglement behavior present in hyper-entangled photon pairs. We consider the principal advantages of our QKD scheme in connection with the way it makes use of ultrafast laser pumped spontaneous parametric down-conversion and hyper-entanglement. In particular, we consider how polarization quantum interference may be modified by manipulating the spatial features of the down-converted light.

DOI
10.1117/12.450941

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We describe the development of a quantum key distribution (QKD) scheme based on ultrafast laser pumped sources of entangled photon pairs and the engineering of their entanglement properties. Though quantum entanglement has been shown to be a useful resource for quantum key distribution, little work has been carried out in making use of the full range of joint entanglement behavior present in hyper-entangled photon pairs. We consider the principal advantages of our QKD scheme in connection with the way it makes use of ultrafast laser pumped spontaneous parametric down-conversion and hyper-entanglement. In particular, we consider how polarization quantum interference may be modified by manipulating the spatial features of the down-converted light.
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.
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.
We present an experimental setup for generation of entangled-photon pairs via spontaneous parametric down-conversion, based on the femtosecond-pulsed laser. Our entangled-photon source utilizes a 76-MHz-repetition-rate, 100-fs-pulsewidth, mode-locked, ultrafast femtosecond laser, which can produce, on average, more photon pairs than a cw laser of an equal pump power. The output infrared pump photons (λ = 810 nm) are first up-converted to blue light (λ = 405 nm) and, subsequently, down-converted in a 1.5-mm-thick, type-II BBO crystal via spontaneous down-conversion. The resulting entangled pairs are counted by a pair of high-quantum-efficiency, single-photon, silicon avalanche photodiodes. The total down-conversion efficiency of our system, corresponding criterion of the pump power for real entangled coincident events, has been calculated to be 0.86 × 10-9. Our apparatus is intended as an efficient source/receiver system for the quantum communications and quantum cryptography applications.
Quantum cryptography with entangled photon pairs can be more powerful than protocols based onsingle photons or weak coherent pulses: they can tolerate higher losses and thus allow thedistribution of quantum secret keys (QKD) over longer distances [1], they also provide a waytowards device-independent quantum cryptography [2]. However, in order to enable a wide use ofentangled photon pairs in future quantum telecommunication systems, further developments areneeded to demonstrate performant sources that can be easily fabricated and integrated into Telecomfiber networks.Here we present a source consisting of an aluminium gallium arsenide waveguide generatingphoton pairs in the Telecom band by type II spontaneous parametric down-conversion [3]. Such adevice has already been proven to work under electrical pumping [4]. Thanks to the very smallbirefringence of the guided modes, the pairs are directly generated in a polarization-entangled Bellstate, without the need for any post-compensation. Moreover, as the photons are emitted over alarge bandwidth (about 100 nm) with a joint spectrum that exhibits frequency anticorrelation, thesame source can be used to simultaneously distribute keys among multiple pairs of users by usingstandard Telecom wavelength demultiplexers [5]. Here, we experimentally show the distribution ofquantum secret keys with the BBM92 QKD protocol [6] between four different pairs of users with acommercial 100 GHz demultiplexer (0.8 nm channel width and spacing). Under CW pumpingconditions, using free-running InGaAs single-photon detectors, we achieve a secret key rate of 0.21bits/s and a qubit error rate (QBER) of 6.9% over 50 km of standard optical fiber.Our results, obtained with a robust and simple experimental set-up, open the way towards theimplementation of practical device-independent quantum communication protocols.
Entanglement-based quantum key distribution (EB-QKD) has several advantages over prepare-and-measure protocols, such as eliminating the need for decoy states and active modulators. At present, entanglement sources based on the spontaneous parametric down-conversion process have been widely used in EB-QKD experiments. Most of the existing models regard the photon-number distribution of pulse-pumped entanglement sources as thermal. However, doing so ignores any effects of spectral modes, which will lead to the overestimation of the secure key rate. In this paper, we analyze the correlation between the photon-number distribution and the joint spectral distribution and rewrite the quantum state of the photon pairs. Then we apply the results to EB-QKD and present a general model which can be used to calculate the secure key rate not only for any pulse-pumped systems but even for cw-pumped systems.
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.