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

Quantum cryptography in free space with coherent-state light

Geraldo A. Barbosa, Eric Corndorf, Prem Kumar, Horace P. Yuen

Published Dec 1, 2002• 5 citations
Publisher

Abstract

In this paper, we present a proof-of-concept experimental demonstration of the secret key quantum cryptographic scheme. A tabletop communication link was set up in the free-space channel using ordinary lasers as transmitters, which emit coherent states of light, and quantum-limited direct detection was employed in the receivers. In the secret key scheme, one needs a supply of M possible quantum states that are uniformly distributed over some random variable. In the free-space case, we used polarization angle as the variable determining the state. In the proof-of-concept emonstration, we aimed towards sending data messages encrypted with a short secret key from the transmitter to the receiver. The messages could be successfully deciphered by the receiver by its knowledge of the secret key. However, when the secret key was taken away, in order to mimic an eavesdropper, the messages could not be deciphered.

DOI
10.1117/12.450942

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In this paper, we present a proof-of-concept experimental demonstration of the secret key quantum cryptographic scheme. A tabletop communication link was set up in the free-space channel using ordinary lasers as transmitters, which emit coherent states of light, and quantum-limited direct detection was employed in the receivers. In the secret key scheme, one needs a supply of M possible quantum states that are uniformly distributed over some random variable. In the free-space case, we used polarization angle as the variable determining the state. In the proof-of-concept emonstration, we aimed towards sending data messages encrypted with a short secret key from the transmitter to the receiver. The messages could be successfully deciphered by the receiver by its knowledge of the secret key. However, when the secret key was taken away, in order to mimic an eavesdropper, the messages could not be deciphered.
Abstract Quantum key distribution, that transmits single-photons or strongly attenuated coherent light, has been studied for secret-key cryptography. This paper experimentally demonstrates a novel key distribution scheme, where a transmitter sends slightly intensity-modulated light with a power level used in conventional fiber communication systems and a receiver directly detects it with a conventional photodiode followed by threshold processing. Its security relies on that the slight intensity modulation is masked by quantum noise. Experiments are presented, which demonstrate that such slight modulation is feasible in practice and a secret key is actually created. The present key distribution scheme employs a setup similar to that in conventional optical communication, featuring practicality.
The secure distribution of the secret random bit sequences known as 'key' material, is an essential precursor to their use for the encryption and decryption of confidential communications. Quantum cryptography is a new technique for secure key distribution with single-photon transmissions: Heisenberg's uncertainty principle ensures that 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 experimental quantum cryptography systems based on the transmission of non- orthogonal photon polarization states to generate shared key material over line-of-sight optical links. Key material is built up using the transmission of a single-photon per bit of an initial secret random sequence. A quantum-mechanically random subset of this sequence is identified, becoming the key material after a data reconciliation stage with the sender. We have developed and tested a free-space quantum key distribution (QKD) system over an outdoor optical path of approximately 1 km at Los Alamos National Laboratory under nighttime conditions. Results show that free-space QKD can provide secure real-time key distribution between parties who have a need to communicate secretly. Finally, we examine the feasibility of surface to satellite QKD.
This paper proposes a quantum communication scheme for sending a definite binary sequence while confirming the security of the transmission. The scheme is very suitable for sending a ciphertext in a secret-key cryptosystem so that we can detect any eavesdropper who attempts to decipher the key. Thus we can continue to use a secret key unless we detect eavesdropping and the security of a key that is used repeatedly can be enhanced to the level of one-time-pad cryptography. In our scheme, a pair of entangled photon twins is employed as a bit carrier which is encoded in a two-term superposition of four Bell states. Different bases are employed for encoding the binary sequence of a ciphertext and a random test bit. The photon twins are measured with a Bell state analyzer and any bit can be decoded from the resultant Bell state when the receiver is later notified of the coding basis through a classical channel. By opening the positions and the values of test bits, ciphertext can be read and eavesdropping is simultaneously detected.
Digital signatures are frequently used in data transfer to prevent impersonation, repudiation and message tampering. Currently used classical digital signature schemes rely on public key encryption techniques, where the complexity of so-called ‘one-way’ mathematical functions is used to provide security over sufficiently long timescales. No mathematical proofs are known for the long-term security of such techniques. Quantum digital signatures offer a means of sending a message, which cannot be forged or repudiated, with security verified by information-theoretical limits and quantum mechanics. Here we demonstrate an experimental system, which distributes quantum signatures from one sender to two receivers and enables message sending ensured against forging and repudiation. Additionally, we analyse the security of the system in some typical scenarios. Our system is based on the interference of phase-encoded coherent states of light and our implementation utilizes polarization-maintaining optical fibre and photons with a wavelength of 850 nm. Quantum digital signatures exploit quantum mechanics to provide verification of messages at the limits of information theory. Clarkeet al.demonstrate a photonic system that provides quantum digital signatures for messages sent to two receivers and is secure against forgery and repudiation.