How to remove detector side channel attacks has been a notoriously hard problem in quantum cryptography. Here, we propose a simple solution to this problem-measurement device independent quantum key distribution. It not only removes all detector side channels, but also doubles the secure distance with conventional lasers. Our proposal can be implemented with standard optical components with low detection efficiency and highly lossy channels. In contrast to the previous solution of full device independent QKD, the realization of our idea does not require detectors of near unity detection efficiency in combination with a qubit amplifier (based on teleportation) or a quantum non-demolition measurement of the number of photons in a pulse. Furthermore, its key generation rate is many orders of magnitude higher than that based on full device independent QKD. The results show that long-distance quantum cryptography over say 200km will remain secure even with seriously flawed detectors.Quantum key distribution (QKD) allows two parties (typically called Alice and Bob) to generate a common string of secret bits, called a secret key, in the presence of an eavesdropper, Eve [1]. This key can be used for tasks such as secure communication and authentication. Unfortunately, there is a big gap between the theory and practice of QKD. In principle, QKD offers unconditional security guaranteed by the laws of physics [2-4]. However, real-life implementations of QKD rarely conform to the assumptions in idealized models used in security proofs. Indeed, by exploiting security loopholes in practical realizations, especially imperfections in the detectors, different attacks have been successfully launched against commercial QKD systems [5, 6], thus highlighting their practical vulnerabilities.To connect theory with practice again, several approaches have been proposed. The first one is the presumably hard-verifiable problem of trying to characterize real devices fully and account for all side channels. The second approach is a teleportation trick [2,7]. The third solution is (full) device independent QKD (DI-QKD) [9]. This last technique does not require detailed knowledge of how QKD devices work and can prove security based on the violation of a Bell inequality. Unfortunately, DI-QKD is highly impractical because it needs near unity detection efficiency together with a qubit amplifier or a quantum non-demolition (QND) measurement of the number of photons in a pulse, and even then generates an extremely low key rate (of order 10 −10 bits per pulse) at practical distances [10].

Secure communication plays a crucial role in the Internet Age. Quantum mechanics may revolutionise cryptography as we know it today. In this Review Article, we introduce the motivation and the current state of the art of research in quantum cryptography. In particular, we discuss the present security model together with its assumptions, strengths and weaknesses. After a brief introduction to recent experimental progress and challenges, we survey the latest developments in quantum hacking and counter-measures against it.With the rise of the Internet, the importance of cryptography is growing every day. Each time we make an on-line purchase with our credit cards, or we conduct financial transactions using Internet banking, we should be concerned with secure communication. Unfortunately, the security of conventional cryptography is often based on computational assumptions. For instance, the security of the RSA scheme [1], the most widely used public-key encryption scheme, is based on the presumed hardness of factoring. Consequently, conventional cryptography is vulnerable to unanticipated advances in hardware and algorithms, as well as to quantum code-breaking such as Shor's efficient algorithm [2] for factoring. Government and trade secrets are kept for decades. An eavesdropper, Eve, may simply save communications sent in 2014 and wait for technological advances. If she is able to factorise large integers in say 2100, she could retroactively break the security of data sent in 2014.In contrast, quantum key distribution (QKD), the best-known application of quantum cryptography, promises to achieve the Holy Grail of cryptographyunconditional security in communication. By unconditional security or, more precisely, -security, as it will be explained shortly (see section discussing the security model of QKD), Eve is not restricted by computational assumptions but she is only limited by the laws of physics. QKD is a remarkable solution to long-term security since, in principle, it offers security for eternity. Unlike conventional cryptography, which allows Eve to store a classical transcript of communications, in QKD, once a quantum transmission is done, there is no classical transcript for Eve to store. See Box 1 for background information on secure communication and QKD.Box 1 | Secure communication and QKD. Secure Communication: Suppose a sender, Alice, would like to send a secret message to a receiver, Bob, through an open communication channel. Encryption is needed. If they share a common string of secret bits, called a key, Alice can use her key to transform a plain-text into a cipher-text, which is unintelligible to Eve. In contrast, Bob, with his key, can decrypt the cipher-text and recover the plain-text. In cryptography, the security of a crypto-system should rely solely on the secrecy of the key. The question is: how to distribute a key securely? In conventional cryptography, this is often done by trusted couriers. Unfortunately, in classical physics, couriers may be brided or compromised without the users noti...

Due to its ability to tolerate high channel loss, decoy-state quantum key distribution (QKD) has been one of the main focuses within the QKD community. Notably, several experimental groups have demonstrated that it is secure and feasible under real-world conditions. Crucially, however, the security and feasibility claims made by most of these experiments were obtained under the assumption that the eavesdropper is restricted to particular types of attacks or that the finite-key effects are neglected. Unfortunately, such assumptions are not possible to guarantee in practice. In this work, we provide concise and tight finite-key security bounds for practical decoy-state QKD that are valid against general attacks.

Quantum key distribution promises unconditionally secure communications. However, as practical devices tend to deviate from their specifications, the security of some practical systems is no longer valid. In particular, an adversary can exploit imperfect detectors to learn a large part of the secret key, even though the security proof claims otherwise. Recently, a practical approach-measurement-device-independent quantum key distribution-has been proposed to solve this problem. However, so far its security has only been fully proven under the assumption that the legitimate users of the system have unlimited resources. Here we fill this gap and provide a rigorous security proof against general attacks in the finite-key regime. This is obtained by applying large deviation theory, specifically the Chernoff bound, to perform parameter estimation. For the first time we demonstrate the feasibility of long-distance implementations of measurement-device-independent quantum key distribution within a reasonable time frame of signal transmission.

We demonstrate that a necessary precondition for unconditionally secure quantum key distribution is that sender and receiver can use the available measurement results to prove the presence of entanglement in a quantum state that is effectively distributed between them. One can thus systematically search for entanglement using the class of entanglement witness operators that can be constructed from the observed data. We apply such analysis to two well-known quantum key distribution protocols, namely the 4-state protocol and the 6-state protocol. As a special case, we show that, for some asymmetric error patterns, the presence of entanglement can be proven even for error rates above 25% (4-state protocol) and 33% (6-state protocol). PACS numbers:Quantum key distribution (QKD) [1,2] QKD protocols distinguish typically two phases to establish a key. In the first phase, an effective bi-partite quantum mechanical state is distributed between the legitimate users, which establishes correlations between them. A (restricted) set of measurements is used to measure these correlations, and the measurement results are described by a joint probability distribution P (A, B). In the second phase, called key distillation, Alice and Bob use an authenticated public channel to process the correlated data in order to obtain a secret key. This procedure involves, typically, postselection of data, error correction to reconcile the data, and privacy amplification to decouple the data from a possible eavesdropper [4].In this Letter we consider the first phase of QKD and demonstrate that a necessary precondition for successful key distillation is that Alice and Bob can detect the presence of entanglement in a quantum state that is effectively distributed between them. Such detection may involve available observed data only; it can be realized by using the class of entanglement witness operators that can be constructed from these data.Two types of schemes are typically used to create correlated data. In prepare&measure schemes (P&M schemes) Alice prepares a random sequence of pre-defined nonorthogonal states that are sent to Bob through an untrusted channel (controlled by Eve). Generalizing the ideas of Bennett et al. [5], the signal preparation can be thought of as follows: Alice prepares an entangled bipartite state of the form |Ψ source AB = i √ p i |e i |ϕ i . If she measures the first system in the canonical orthonormal basis |e i , she effectively prepares the (nonorthogonal) signal states |ϕ i with probabilites p i . The action of the quantum channel on the state |Ψ source AB leads to an effective bi-partite state shared by Alice and Bob. One important characteristic of the P&M schemes is that the reduced density matrix ρ A of Alice is fixed [6]. In entanglement based schemes a bi-partite state is distributed to Alice and Bob by an, in general, untrusted third party. This party may be an eavesdropper who is in possession of a third sub-system that may be entangled with those given to Alice and Bob. While the subsystems measured by...