Quantum key distribution (QKD) allows two users to communicate with theoretically provable secrecy by encoding information on photonic qubits. Current encoders are complex, however, which reduces their appeal for practical use and introduces potential vulnerabilities to quantum attacks. Distributed-phase-reference (DPR) systems were introduced as a simpler alternative, but have not yet been proven practically secure against all classes of attack. Here we demonstrate the first DPR QKD system with information-theoretic security. Using a novel light source, where the coherence between pulses can be controlled on a pulse-by-pulse basis, we implement a secure DPR system based on the differential quadrature phase shift protocol. The system is modulator-free, does not require active stabilization or a complex receiver, and also offers megabit per second key rates, almost three times higher than the standard Bennett-Brassard 1984 (BB84) protocol. This enhanced performance and security highlights the potential for DPR protocols to be adopted for real-world applications. Quantum key distribution (QKD) has developed strongly since the proposal of the first protocol in 1984 1-3. The future could see widespread quantum networks similar to those in Tokyo 4 and Vienna 5 and global secure communication enabled by QKD over satellites 6. These advances depend on the development of simple, cost-effective and high performance implementations. Innovations in both protocols and system hardware are required to achieve this. Nearly two decades after the inception of Bennett-Brassard 1984 (BB84) 1 , distributed phase reference (DPR) QKD was proposed, allowing for much simpler experimental implementations. The class includes the differential phase shift 7,8 and coherent-one-way 9,10 protocols. One advantage is that the transmitters needed to realize these DPR protocols can be made using off-the-shelf telecom lasers and modulators. However the benefit of their simpler implementation is outweighed by a seriously degraded performance when full security is taken into account 3,11,12. To plug the security gap, two further DPR protocols were proposed: round-robin differential phase shift and differential quadrature phase shift (DQPS). The former simplifies the estimation of Eve's information, but requires an overly complicated QKD receiver setup 13-16 , making it impractical. The latter separates the signal from the differential phase shift protocol into blocks, each having a global phase that varies randomly, ensuring the protocol is immune against coherent attacks 17,18. It does, however, stray from the main goal of DPR protocols to provide simpler QKD implementations, due to the phase randomization requirement that would ordinarily require extra system components. a) Electronic mail: glr28@cam.ac.uk In this work we show it is possible to produce phase coherent and phase randomized pulses from a single device. This device is based on optical injection of one laser diode into another, removing the need for a phase-randomization component in D...

The use of quantum bits (qubits) in cryptography holds the promise of secure cryptographic quantum key distribution schemes. Unfortunately, the implemented schemes can be totally insecure. We provide a thorough investigation of security issues for practical quantum key distribution, taking into account channel losses, a realistic detection process, and modifications of the "qubits" sent from the sender to the receiver. We first show that even quantum key distribution with perfect qubits cannot be achieved over long distances when fixed channel losses and fixed dark count errors are taken into account. Then we show that existing experimental schemes (based on "weak-pulse") are usually totally insecure. Finally we show that parametric downconversion offers enhanced performance compared to its weak coherent pulse counterpart.Pacs: 03.67. Dd, 42.50.Dv, 03.65.Bz, 89.80.+h Quantum information theory suggests the possibility of accomplishing tasks which are beyond the capability of classical computer science, such as information-secure cryptographic key distribution [1,2]. The lack of security proofs for standard (secret-and public-) key distribution schemes, and the insecurity of the strongest classical schemes against "quantum attacks" [3], emphasizes the need for information-secure key distribution. Whereas the security of idealized quantum key distribution (qkd) schemes has been investigated against very sophisticated collective and joint attacks (e.g., [4,5]), the experimental qkd schemes have been proven secure against the simple individual attack only recently [6] (via the application of ideas presented here).In the four-state scheme [1], usually referred to as Bennett-Brassard-84 (BB84), the sender (Alice) and the receiver (Bob) use two conjugate bases (say, the rectilinear basis, +, and the diagonal basis, ×) for the polarization of single photons. In basis + they use the two orthogonal basis states |0 + and |1 + to represent "0" and "1" respectively. In basis × they use the two orthogonal basis states2)[|0 + − |1 + ] to represent "0" and "1". The basis is revealed later on via an unjammable and insecure classical channel. The signals where Bob used the same basis as Alice form the sifted key on which Bob can decode the bit value. The remaining signals are being discarded. Finally, they test a few bits to estimate the error-rate, and if the test passes (the tested error-rate is less than some pre-agreed threshold), they use errorcorrection and privacy amplification to obtain a potentially secure final key [7,8].The security of that scheme, which assumes a source of perfect qubits as well as losses and errors which are bounded by some small threshold, has been investigated in various works. Very simple attacks already render realistic qkd impossible, as we show here.The experiments are usually based on weak coherent pulses (wcp) as signal states with a low probability of containing more than one photon [7,9]. Initial security analysis of such weak-pulse schemes were done [7,10], and evidence of some potentia...

We prove the security of the Bennett-Brassard (BB84) quantum key distribution protocol in the case where the source and detector are under the limited control of an adversary. Our proof applies when both the source and the detector have small basis-dependent flaws, as is typical in practical implementations of the protocol. We derive a general lower bound on the asymptotic key generation rate for weakly basis-dependent eavesdropping attacks, and also estimate the rate in some special cases: sources that emit weak coherent states with random phases, detectors with basis-dependent efficiency, and misaligned sources and detectors.

I prove the security of quantum key distribution against individual attacks for realistic signals sources, including weak coherent pulses and downconversion sources. The proof applies to the BB84 protocol with the standard detection scheme (no strong reference pulse). I obtain a formula for the secure bit rate per time slot of an experimental setup which can be used to optimize the performance of existing schemes for the considered scenario.