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...

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The use of linearly independent signal states in realistic implementations of quantum key distribution (qkd) enables an eavesdropper to perform unambiguous state discrimination. We explore quantitatively the limits for secure qkd imposed by this fact taking into account that the receiver can monitor to some extend the photon number statistics of the signals even with todays standard detection schemes. We compare our attack to the beamsplitting attack and show that security against beamsplitting attack does not necessarily imply security against the attack considered here.03.67. Dd, 03.65.Bz, 42.79.Sz

We report on the experimental verification of quantum non-Gaussian character of a heralded single-photon state with a positive Wigner function. We unambiguously demonstrate that the generated state cannot be expressed as a mixture of Gaussian states. Sufficient information to witness the quantum non-Gaussian character is obtained from a standard photon anticorrelation measurement.

We propose a protocol for conditional suppression of losses in direct quantum state transmission over a lossy quantum channel. The method works by noiselessly attenuating the input state prior to transmission through a lossy channel followed by noiseless amplification of the output state. The procedure does not add any noise hence it keeps quantum coherence. We experimentally demonstrate it in the subspace spanned by vacuum and single-photon states, and consider its general applicability.PACS numbers: 03.67. Hk, 42.50.Ex Quantum communication holds the promise of unconditionally secure information transmission [1]. However, the distance over which quantum states of light can be distributed without significant disturbance is limited due to unavoidable losses and noise in optical links. Losses, as well as errors or decoherence, may in principle be overcome by the sophisticated techniques of quantum error correction [2][3][4], entanglement distillation [5][6][7], and quantum repeaters [8,9]. However, these techniques typically require encoding information into complex multimode entangled states, processing many copies of an entangled state, and -even more challenging -using quantum memories [10,11]. In stark contrast with the situation for classical communication, losses in quantum communication cannot be compensated by amplifying the signal, because the laws of quantum mechanics imply that any deterministic phase-insensitive signal amplification is unavoidably accompanied by the addition of noise [12].Very recently, however, the concept of heralded noiseless amplification of light [13] was proposed as a way out, relaxing the deterministic requirement. The noiseless amplification is formally described by a quantum filter g n , where n is the photon number operator and g > 1 denotes the amplification gain. The noiseless amplifier thus modulates amplitudes of Fock states |n by factor g n . This filtration can conditionally increase amplitude of a coherent state |α without adding any noise, g n |α ∝ |gα . Although this cannot be done perfectly because g n is unbounded, faithful noiseless amplification is possible in any finite subspace spanned by the Fock states |n with n ≤ N , albeit with a correspondingly low probability scaling as g −2N in the worst case of input vacuum state. With current technology, it has been proven possible to faithfully noiselessly amplify weak coherent states containing mostly vacuum and single-photon contributions [14][15][16][17].The noiseless amplifier can improve the performance of quantum key distribution protocols [18][19][20][21] and it can also be used to distribute high-quality entanglement over a lossy channel [13,22]. Beyond that, the noiseless amplifier is not useful to suppress losses in direct transmission of arbitrary quantum states because it is not the inverse map of a lossy channel L. As a matter of fact, any superposition of Fock states that is not a coherent state is mapped by L onto a mixed state, and this added noise cannot be eliminated by noiseless amplification.He...