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Quantum Electronics and Laser Science Conference

Free space quantum key distribution in daylight

Richard Hughes, J. E. Nordholt, George L. Morgan, Charles G. Peterson

Published Jun 25, 2003• 3 citations
Publisher

Abstract

Summary form only given. The Los Alamos free-space quantum key distribution (QKD) system operates at a 1-MHz clock rate. On each clock cycle a /spl sim/1-ns "bright timing pulse" is emitted from an array of /spl sim/1550-nm lasers at the transmitter ("Alice"). After a /spl sim/100 ns delay one of four temperature-controlled "data" diode lasers emits a /spl sim/1-ns, 772-nm optical pulse that is attenuated to the single-photon level. We present results from operations over multi-kilometer atmospheric ranges. The spectral, spatial and temporal filtering allow practical secret bit rates even in full daylight. We also present a study of a satellite-to-ground QKD system, and an analysis of the expected secret bit rate.

DOI
10.1109/qels.2002.1031406

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Summary form only given. The Los Alamos free-space quantum key distribution (QKD) system operates at a 1-MHz clock rate. On each clock cycle a /spl sim/1-ns "bright timing pulse" is emitted from an array of /spl sim/1550-nm lasers at the transmitter ("Alice"). After a /spl sim/100 ns delay one of four temperature-controlled "data" diode lasers emits a /spl sim/1-ns, 772-nm optical pulse that is attenuated to the single-photon level. We present results from operations over multi-kilometer atmospheric ranges. The spectral, spatial and temporal filtering allow practical secret bit rates even in full daylight. We also present a study of a satellite-to-ground QKD system, and an analysis of the expected secret bit rate.
In quantum key distribution (QKD) single-photon transmissions generate the shared, secret random number sequences, known as cryptographic keys that are used to encrypt and decrypt secret communications. Because the security of QKD is based on principles of quantum physics an adversary can neither successfully tap the key transmissions, nor evade detection (eavesdropping raises the key error rate above a threshold value). We have developed an experimental QKD system that uses the four-state BB84 protocol with non-orthogonal photon polarization states and lowest-order adaptive optics to generate shared key material over multi-kilometer atmospheric, line-of-sight paths. I will present results of a daylight demonstration of this system. Key material is built up using the transmission of a photon-pulse per bit of an initial secret random sequence. I will describe the design and operation of the system, present an analysis of the system's security, efficiency, error rate, and secret key rate, as well as cryptographic testing of the key bits produced. I will describe the prospects for longer-distance applications of free-space QKD, particularly for satellite communications.
Summary form only given.We demonstrated quantum cryptography, or quantum key distribution (QKD), in free space over an outdoor optical path of /spl sim/950 m under nighttime conditions. The success of free space QKD against a high background depends on the transmission and detection of single photons through an optically thick and turbulent medium. It has been shown that a combination of sub-nanosecond timing, narrow filters, spatial filtering, and adaptive optics can render the difficult detection problem tractable. Further, the nonbirefringent nature of the atmosphere at optical wavelengths allows the transmission of the single photon polarization states used in the free-space QKD protocol.
Summary form only given. The QKD transmitter, Alice, consists of three diode lasers (a timing laser and two data lasers: one data laser transmits vertical polarized photons, while the other transmits 45 degree polarized photons, two matching interference filters (IF), two optical attenuators, two linear polarizers, two non-polarizing beamsplitters, a 27/spl times/ beam expander, and a single-photon detector (SPD). The two data-laser (dimpulse) wavelengths are temperature controlled and constrained by the IF to 773/spl plusmn/0.5 nm, while the timing-laser (bright-pulse) transmitted wavelength is temperature controlled to /spl sim/768 nm. The data-lasers are configured to emit dim-light pulses of /spl sim/1 ns duration, and the SPD measures the average photon number per transmitted dim-pulse. This system uses the B92 QKD protocol, but could be easily adapted to the BB84 QKD protocol. The transmitter incorporates no active polarization switching.
Free-space quantum key distribution (QKD), more popularly know as quantum cryptography, uses single-photon free-space optical communications to distribute the secret keys required for secure communications. At Los Alamos National Laboratory we have demonstrated a fully automated system that is capable of operations at any time of day over a horizontal range of several kilometers. This has proven the technology is capable of operation from a spacecraft to the ground, opening up the possibility of QKD between any group of users anywhere on Earth. This system, the prototyping of a new system for use on a spacecraft, and the techniques required for world-wide quantum key distribution will be described. The operational parameters and performance of a system designed to operate between low earth orbit (LEO) and the ground will also be discussed.