Quantum Cryptography with Entangled Photons

Abstract

By realizing a quantum cryptography system based on polarization entangled photon pairs we establish highly secure keys, because a single photon source is approximated and the inherent randomness of quantum measurements is exploited. We implement a novel key distribution scheme using Wigner's inequality to test the security of the quantum channel, and, alternatively, realize a variant of the BB84 protocol. Our system has two completely independent users separated by 360 m, and generates raw keys at rates of 400 -800 bits/second with bit error rates arround 3%.The primary task of cryptography is to enable two parties (commonly called Alice and Bob) to mask confidential messages such, that the transmitted data are illegible to any unauthorized third party (called Eve). Usually this is done using shared secret keys. However, in principle it is always possible to intercept classical key distribution unnoticedly. The recent development of quantum key distribution 1 can cover this major loophole of classical cryptography. It allows Alice and Bob to establish two completely secure keys by transmitting single quanta (qubits) along a quantum channel. The underlying principle of quantum key distribution is that nature prohibits to gain information on the state of a quantum system without disturbing it. Therefore, in appropriately designed schemes, no tapping of the qubits is possible without showing up to Alice and Bob. These secure keys can be used in a One-Time-Pad protocol 2 , which makes the entire communication absolutely secure.Two well known concepts for quantum key distribution are the BB84 scheme and the Ekert scheme. The BB84 scheme 1 uses single photons transmitted from Alice to Bob, which are prepared at random in four partly orthogonal polarization states: 0 • , 45 • , 90 • , 135 • . If Eve tries to extract information about the polarization of the photons she will inevitably introduce errors, which Alice and Bob can detect by comparing a random subset of the generated keys.The Ekert scheme 3 is based on entangled pairs and uses Bell's inequality 4 to establish security. Both Alice and Bob receive one particle out of an entangled pair. They perform measurements along at least three different directions on each side, where measurements along parallel axes are used for key generation and oblique angles are used for testing the inequality. In 3 , Ekert pointed out that eavesdropping inevitably affects the entanglement between the two constituents of a pair and therefore reduces the degree of violation of Bell's inequality. While we are not aware of a general proof that the violation of a Bell inequality implies the security of the system, this has been shown 5 for the BB84 protocol adapted to entangled pairs and the CHSH inequality 6 .In any real cryptography system, the raw key generated by Alice and Bob contains errors, which have to be corrected by classical error correction 7 over a public channel. Furthermore it has been shown that whenever Alice and Bob share a sufficiently secure key, they can enhance its...

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