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arXiv:quant-ph/9912117 · Physical Review Letters

Quantum Cryptography with Entangled Photons

Thomas D. Jennewein, Christoph Simon, Gregor Weihs, Harald Weinfurter, Anton Zeilinger

Published May 15, 2000• 937 citations
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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/s with bit error rates around 3%.

DOI
10.1103/physrevlett.84.4729

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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/s with bit error rates around 3%.
We describe a four-dimensional (D=4\mathcal{D} = 4D=4) single-photon quantum cryptography protocol with up to twenty (D×(22+1)\mathcal{D} \times (2^2 + 1)D×(22+1)) possible states generated by a polarization-, phase- and time-encoding transmitter. This protocol can be experimentally realized with existing technology, drawing from time- and polarization-encoded systems. The protocol is error tolerant and has a maximum raw bit rate of two raw bits per detection, which when combined with state detection efficiency yields a qubit rate of up to one per transmission under ideal assumptions, or up to twice the raw bit rate of two-dimensional protocols such as the well-known BB84 protocol.
Summary form only given. Quantum cryptography uses single photons to allow secure distribution of secret key material to sender and receiver, without the possibility of an undetected eavesdropper. In the protocol suggested by Ekert, each photon of a quantum-mechanically entangled pair is sent to the sender and receiver, who randomly measure the polarization in various bases. If the bases are the same, then sender and receiver will have completely correlated measurement results, which become the shared secret key. Other combinations of measurements are used to test Bell's inequalities, which limit the possible correlations achievable with any local realistic theory. The presence of an intermediate eavesdropper can be readily detected by an inability to violate Bell's inequality. Using the polarization-entangled photon pairs from a novel spontaneous parametric down-conversion source, we have experimentally implemented. Ekert's proposal.
Quantum cryptography allows the establishing of a highly confidential sequence of random bits between two parties who share no secret information in advance. Its security is guaranteed by the basic laws of quantum mechanics rather than mathematical difficulty. An experimental system based on a novel protocol for quantum cryptography using only two nonorthogonal photon polarization states is implemented. The photon sources are two semiconductor laser diodes operated at 680 nm, their light pulses which are heavily attenuated to far below one photon per pulse are employed to encode the key. Gated single photon detection technique is used to minimize the dark count effect of the photon multiplier tube. Key establishing rate of 40 b/s and error rate of 2.1% are obtained. Compared with other systems, our system has the simplest experimental setup.
We have implemented the 'six-state' quantum cryptography protocol using polarization-entangled photon pairs, in which the polarization of each photon of a pair is measured in one of three randomly chosen bases. For a given amount of eavesdropping, this protocol results in a larger error rate than in four- or two-state protocols, but reduces the number of key-producing events. We have experimentally investigated several incoherent eavesdropping strategies, and verified the predicted enhancement in error rate. However, we demonstrate that for low error rates, the efficiency for secret key generation is higher when using the four-state protocol.
Semiconductor quantum dots are capable of emitting polarization entangled photon pairs with ultralow multipair emission probability even at maximum brightness. Using a quantum dot source with a fidelity as high as 0.987(8), we implement here quantum key distribution with an average quantum bit error rate as low as 1.9% over a time span of 13 hours. For a proof of principle, the key generation is performed with the BBM92 protocol between two buildings, connected by a 350-m-long fiber, resulting in an average raw (secure) key rate of 135 bits/s (86 bits/s) for a pumping rate of 80 MHz, without resorting to time- or frequency-filtering techniques. Our work demonstrates the viability of quantum dots as light sources for entanglement-based quantum key distribution and quantum networks. By increasing the excitation rate and embedding the dots in state-of-the-art photonic structures, key generation rates in the gigabits per second range are in principle at reach.