Quantum cryptography (QC) can provide unconditional secure communication between two authorized parties based on the basic principles of quantum mechanics. However, imperfect practical conditions limit its transmission distance and communication speed. Here we implemented the differential phase shift (DPS) quantum key distribution (QKD) with up-conversion assisted hybrid photon detector (HPD) and achieved 1.3 M bits per second secure key rate over a 10-km fiber, which is tolerant against the photon number splitting (PNS) attack, general collective attacks on individual photons, and any other known sequential unambiguous state discrimination (USD) attacks.
The security of conventional cryptography systems is threatened in the forthcoming era of quantum computers. Quantum key distribution (QKD) features fundamentally proven security and offers a promising option for quantum-proof cryptography solution. Although prototype QKD systems over optical fiber have been demonstrated over the years, the key generation rates remain several orders-of-magnitude lower than current classical communication systems. In an effort towards a commercially viable QKD system with improved key generation rates, we developed a discrete-variable QKD system based on time-b…
Quantum key distribution (QKD), a technology that enables perfectly secure communication, has evolved to the stage where many different protocols are being used in real-world implementations. Each protocol has its own advantages, meaning that users can choose the one best-suited to their application, however each often requires different hardware. This complicates multi-user networks, in which users may need multiple transmitters to communicate with one another. Here, we demonstrate a direct-modulation based transmitter that can be used to implement most weak coherent pulse based QKD protocols…
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 …
Quantum continuous variables are being explored as an alternative means to implement quantum key distribution, which is usually based on single photon counting. The former approach is potentially advantageous because it should enable higher key distribution rates. Here we propose and experimentally demonstrate a quantum key distribution protocol based on the transmission of gaussian-modulated coherent states (consisting of laser pulses containing a few hundred photons) and shot-noise-limited homodyne detection; squeezed or entangled beams are not required. Complete secret key extraction is ach…
A theorem is proved which states that no classical key generating protocol could ever be provably secure. Consequently, candidates for provably secure protocols must rely on some quantum effect. Theorem relies on the fact that BB84 Quantum key distribution protocol has been proven secure.
We present a quantum-public-key identification protocol and show that it is secure against a computationally-unbounded adversary. This demonstrates for the first time that unconditionally-secure and reusable public-key authentication is possible in principle with (pure-state) public keys.
We demonstrate a weak pulse quantum key distribution system using the BB84 protocol which is secure against all individual attacks, including photon number splitting. By carefully controlling the weak pulse intensity we demonstrate the maximum secure bit rate as a function of the fibre length. Unconditionally secure keys can be formed for standard telecom fibres exceeding 50 km in length.
A new cryptographic tool, anonymous quantum key technique, is introduced that leads to unconditionally secure key distribution and encryption schemes that can be readily implemented experimentally in a realistic environment. If quantum memory is available, the technique would have many features of public-key cryptography; an identification protocol that does not require a shared secret key is provided as an illustration. The possibility is also indicated for obtaining unconditionally secure quantum bit commitment protocols with this technique.
A new scheme of Quantum Key Distribution is proposed using three entangled particles in a GHZ state. Alice holds a 3-particle source and sends two particles to Bob, keeping one with herself. Bob uses one particle to generate a secure key, and the other to generate a master-key. This scheme should prove to be harder to break in non-ideal situations as compared to the standard protocols BB84 and Eckert. The scheme uses the concept of Quantum Disentanglement Eraser. Extension to multi-partite scheme has also been investigated.
We report the first gigahertz clocked decoy-protocol quantum key distribution (QKD). Record key rates have been achieved thanks to the use of self-differencing InGaAs avalanche photodiodes designed specifically for high speed single photon detection. The system is characterized with a secure key rate of 1.02 Mbit/s for a fiber distance of 20 km and 10.1 kbit/s for 100 km. As the present advance relies upon compact non-cryogenic detectors, it opens the door towards practical and low cost QKD systems to secure broadband communication in future.
Protecting secure random key from eavesdropping in quantum key distribution protocols has been well developed. In this letter, we further study how to detect and eliminate eavesdropping on the random base string in such protocols. The correlation between the base string and the key enables Alice and Bob to use specific privacy amplification to distill and reuse the previously shared base string with unconditional security and high efficiency. The analysis of the unconditional secure reusable base string brings about new concept and protocol design technique.
In all existing protocols of private communication with encryption and decryption, the pre-shared key can be used for only one time. We give a deterministic quantum key expansion protocol where the pre-shared key can be recycled. Our protocol is exponentially secure. Our protocol costs less qubits and almost zero classical communications with authentication steps being included. Since our protocol can distribute the deterministic bits, it can also be used for direct communication.
We use a recent numerical security proof approach to calculate tight secure key rates in the presence of collective attacks for a continuous-variable (CV) eight-state phase-shift keying (8PSK) protocol with heterodyne detection in the asymptotic limit. The results are compared to achievable secure key rates of a QPSK protocol obtained with the same security proof technique. Furthermore, we investigate the influence of radial postselection on the secure key rate and examine a recently suggested strategy to reduce the computational requirements on the error-correction phase for four-state phase-…
We report two key distribution schemes achieved by swapping quantum entanglement. Using two Bell states, two bits of secret key can be shared between two distant parties that play symmetric and equal roles. We also address eavesdropping attacks against the schemes.
We propose a class of quantum no-key protocols for private communication of classical message based on quantum computing of random Boolean permutations, and demonstrate that they are information-theoretic secure. These protocols are designed to resist middleman attack for two parties preshared authentication key, and achieve perfect mutual data origin authentication which ensures the permanent reusing of authentication key. Finally, we simplify the protocol to a 4-round one, and show that any protocol with 3 or less rounds cannot achieve perfect security without consuming preshared key.
In this report we discuss the insecurity with present implementations of the Ekert protocol for quantum-key distribution based on the Wigner Inequality. We propose a modified version of this inequality which guarantees safe quantum-key distribution.
We present a quantum no-key protocol for direct and secure transmission of quantum and classical messages based on simple Boolean function computation with several quantum gates and Shamir's interactive idea of classical message encryption. This protocol has inherent personal identification and message authentication. It probably is the first quantum protocol that can resist the man-in-the-middle attack by itself.
In this report, we simulate practical feature of Yuen-Kim protocol for quantum key distribution with unconditional secure. In order to demonstrate them experimentally by intensity modulation/direct detection(IMDD) optical fiber communication system, we use simplified encoding scheme to guarantee security for key information(1 or 0). That is, pairwise M-ary intensity modulation scheme is employed. Furthermore, we give an experimental implementation of YK protocol based on IMDD.
Quantum private states are the states that represent some amount of perfect secure key. A simple symmetry of any generalised private quantum state (ie. the states that represent perfect key but not fully random) is provided and extended on Devetak-Winter so called ccq (classical-classical-quantum) and cqq (classical-quantum-quantum) lower bound on secure key. This symmetry is used to develop a practical method of estimating the Alice measurement that is optimal form the perspective of single shot Devetak-Witner lower bound on secure key. The method is particularly good when the noise does not …
Quantum key distribution (QKD) is theoretically secure using the principle of quantum mechanics; therefore, QKD is a promising solution for the future of secure communication. Although several experimental demonstrations of QKD have been reported, they have not considered the polarization-dependent loss in state preparation in the key-rate estimation. In this study, we experimentally characterized polarization-dependent loss in realistic state-preparation devices and verified that a considerable PDL exists in fiber- and silicon-based polarization modulators. Hence, the security of such QKD sys…
Quantum key distribution (QKD) allows two distant parties to share encryption keys with security based on physical laws. Experimentally, it has been implemented with optical means, achieving key rates of 1.26 Megabit/s over 50 kilometres (km) of standard optical fibre and 1.16 bit/hour over 404 km of ultralow-loss fibre in a measurement-device-independent configuration. Increasing the bit rate and range of QKD is a formidable, but important, challenge. A related target, currently considered unfeasible without quantum repeaters, is overcoming the fundamental rate-distance limit of point-to-poin…
We construct multipartite entangled states with underlying W-type structure satisfying positive partial transpose (PPT) condition under any (N-1)|1 partition. Then we show how to distill N-partite secure key form the states using two different methods: direct application of local filtering and novel random key distillation scheme in which we adopt the idea form recent results on entanglement distillation. Open problems and possible implications are also discussed.
Precise quantum key distribution (QKD) secure bound analysis is essential for practical QKD systems. The effect of uniformity of random number seed for privacy amplification is not considered in existing secure bound analysis. In this paper, we propose and prove the quantum leftover hash lemma with non-uniform random number seeds based on the min-entropy, and we give a precise QKD secure bound analysis with non-uniform random number seeds on this basis. We take the two-decoy BB84 protocol as an example to simulate the effect of random number seed uniformity on the secure bound of a QKD system.…
We present a protocol for quantum cryptographic network consisting of a quantum network center and many users, in which any pair of parties with members chosen from the whole users on request can secure a quantum key distribution by help of the center. The protocol is based on the quantum authentication scheme given by Barnum et al. [Proc. 43rd IEEE Symp. FOCS'02, p. 449 (2002)]. We show that exploiting the quantum authentication scheme the center can safely make two parties share nearly perfect entangled states used in the quantum key distribution. This implies that the quantum cryptographic …
The four-state continuous-variable quantum key distribution (CVQKD) protocol has a long practical secure distance \cite{Leverrier_PRL_2009}, while it has the difficulty of parameter estimation. We propose an improved four-state protocol, where the covariance matrix can be estimated from experimental data without using the linear channel assumption, and thus ensuring its unconditional security in the asymptotical limit. Our new scheme keeps the advantages of high reconciliation efficiency and long secure distance of the four-state protocol, and it can be implemented under current technology.
We present a quantum key distribution experiment in which keys that were secure against all individual eavesdropping attacks allowed by quantum mechanics were distributed over 100 km of optical fiber. We implemented the differential phase shift quantum key distribution protocol and used low timing jitter 1.55 um single-photon detectors based on frequency up-conversion in periodically poled lithium niobate waveguides and silicon avalanche photodiodes. Based on the security analysis of the protocol against general individual attacks, we generated secure keys at a practical rate of 166 bit/s over…
We propose an unconditionally secure quantum key distribution (QKD) protocol, which uses a relatively strong signal pulse. While our protocol shares similar security bases as the Bennett 1992 protocol with a strong reference pulse (B92), our scheme uses a smaller number of detectors and it is robust against Rayleigh scattering in an optical fibre. We derive a lower bound of secret key generation rate of our protocol and show that our protocol can cover relatively long distances, assuming precise phase modulations and stable interferometers.
Secure key rate of decoy-state quantum key distribution protocols has been improved with biased basis choice, however, the security standards and parameters of current protocols are different. As a result, we cannot give an accurate key rate comparison between different kinds of protocols. Taking the schemes based on different formula of secure key rate as examples, we give a fair comparison between typical protocols under universal composable security standard. Through analyzing the relationship of security parameters in post-processing stage and final secure key, we achieve the unified quant…
The single photon detection efficiency of the detector unit is crucial for the security of common quantum key distribution protocols like Bennett-Brassard 1984 (BB84). A low value for the efficiency indicates a possible eavesdropping attack that exploits the photon receiver's imperfections. We present a method for estimating the detection efficiency, and calculate the corresponding secure key generation rate. The estimation is done by testing gated detectors using a randomly activated photon source inside the receiver unit. This estimate gives a secure rate for any detector with non-unity sing…
Random generation and confidential distribution of cryptographic keys are fundamental building blocks of secure communication. Using quantum states in which the transmitted quantum bit is entangled with a stationary memory quantum bit allows the secure generation and distribution of keys to be based on fundamental properties of quantum mechanics. At the same time, the reach of secure communication networks can be enhanced, in particular, since this architecture would be compatible with quantum repeaters which are an integral part for scaling quantum networks. Here, we realize a true single-pho…
In search of a quantum key distribution scheme that could stand up for more drastic eavesdropping attack, I discover a prepare-and-measure scheme using $N$-dimensional quantum particles as information carriers where $N$ is a prime power. Using the Shor-Preskill-type argument, I prove that this scheme is unconditional secure against all attacks allowed by the laws of quantum physics. Incidentally, for $N = 2^n > 2$, each information carrier can be replaced by $n$ entangled qubits. And in this case, I discover an eavesdropping attack on which no unentangled-qubit-based prepare-and-measure quantu…
The importance of quantum key distribution as a cryptographic method depends upon its purported strong security guarantee. The following gives reasons on why such strong security guarantee has not been validly established and why good QKD security is difficult to obtain.
We report here a complete experimental realization of one-way decoy-pulse quantum key distribution, demonstrating an unconditionally secure key rate of 5.51 kbps for a 25.3 km fibre length. This is two orders of magnitudes higher than the value that can be obtained with a non-decoy system. We introduce also a simple test for detecting the photon number splitting attack and highlight that it is essential for the security of the technique to fully characterize the source and detectors used.
The method of decoy-state quantum key distribution (QKD) requests different intensities of light pulses. Existing theory has assumed exact control of intensities. Here we propose a simple protocol which is secure and efficient even there are errors in intensity control. In our protocol, decoy pulses and signal pulses are generated from the same father pulses with a two-value attenuation. Given the upper bound of fluctuation of the father pulses, our protocol is secure provided that the two-value attenuation is done exactly. We propose to use unbalanced beam-splitters for a stable attenuation. …
Semi-quantum protocols that allow some of the users to remain classical are proposed for a large class of problems associated with secure communication and secure multiparty computation. Specifically, first time semi-quantum protocols are proposed for key agreement, controlled deterministic secure communication and dialogue, and it is shown that the semi-quantum protocols for controlled deterministic secure communication and dialogue can be reduced to semi-quantum protocols for e-commerce and private comparison (socialist millionaire problem), respectively. Complementing with the earlier propo…
Based on the detection loophole-free photon key distribution (PKD) compatible with classical optical systems, an optical key distribution (OKD) protocol is presented for unconditionally secured cryptography in fiber-optic communications networks using addressable continuous phase basis, where each communication channel is composed of paired transmission lines. The unconditional security in OKD lies in quantum superposition between the paired lines of each channel. The continuous phase basis in OKD can be applied for one-time-pad optical cryptography in networks, whose network address capacity …
Long-distance quantum communication requires quantum repeaters to overcome photon loss in optical fibers. Here we demonstrate a repeater node with two memory atoms in an optical cavity. Both atoms are individually and repeatedly entangled with photons that are distributed until each communication partner has independently received one of them. An atomic Bell-state measurement followed by classical communication serves to establish a key. We demonstrate scaling advantage of the key rate, increase the effective attenuation length by a factor of two, and beat the error-rate threshold of 11\% for …
Authentication provides the trust people need to engage in transactions. The advent of physical keys that are impossible to copy promises to revolutionize this field. Up to now, such keys have been verified by classical challenge-response protocols. Such protocols are in general susceptible to emulation attacks. Here we demonstrate Quantum-Secure Authentication ("QSA") of an unclonable classical physical key in a way that is inherently secure by virtue of quantum-physical principles. Our quantum-secure authentication operates in the limit of a large number of channels, represented by the more …
We analyse the distribution of secure keys using quantum cryptography based on the continuous variable degree of freedom of entangled photon pairs. We derive the information capacity of a scheme based on the spatial entanglement of photons from a realistic source, and show that the standard measures of security known for quadrature-based continuous variable quantum cryptography (CV-QKD) are inadequate. A specific simple eavesdropping attack is analysed to illuminate how secret information may be distilled well beyond the bounds of the usual CV-QKD measures.
In this paper, we investigate the long-standing gap of quantum key rate between the Weak Coherent Pulse (WCP) and Heralded Single Photon Sources(HSPS) implementation of quantum cryptographical protocol. We prove that, by utilizing the Heralded Pair Coherent State (HPCS) photon sources, such a gap can be actually filled in both BB84 and SARG quantum key distribution. Thus, a universal photon source which achieves the up-to-date optimal key rate for each transmission distance is obtained.
We present a scheme for direct and confidential communication between Alice and Bob, where there is no need for establishing a shared secret key first, and where the key used by Alice even will become known publicly. The communication is based on the exchange of single photons and each and every photon transmits one bit of Alice's message without revealing any information to a potential eavesdropper.
We present a quantum key distribution system with a 2.5 GHz repetition rate using a three-state time-bin protocol combined with a one-decoy approach. Taking advantage of superconducting single-photon detectors optimized for quantum key distribution and ultra low-loss fiber, we can distribute secret keys at a maximum distance of 421 km and obtain secret key rates of 6.5 bps over 405 km.
Twin field quantum key distribution promises high key rates at long distance to beat the rate distance limit. Here, applying the sending or not sending TF QKD protocol, we experimentally demonstrate a secure key distribution breaking the absolute key rate limit of repeaterless QKD over 509 km, 408 km ultra-low loss optical fibre and 350 km standard optical fibre. Two independent lasers are used as the source with remote frequency locking technique over 500 km fiber distance; Practical optical fibers are used as the optical path with appropriate noise filtering; And finite key effects are consi…
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.
We characterize the set of shared quantum states which contain a cryptographically private key. This allows us to recast the theory of privacy as a paradigm closely related to that used in entanglement manipulation. It is shown that one can distill an arbitrarily secure key from bound entangled states. There are also states which have less distillable private key than the entanglement cost of the state. In general the amount of distillable key is bounded from above by the relative entropy of entanglement. Relationships between distillability and distinguishability are found for a class of stat…
We propose a two-way secure-communication protocol in which Alice uses an amplified spontaneous emission source while Bob employs binary phase-shift keying and an optical amplifier. Against an eavesdropper who captures all the light lost in fibers linking Alice and Bob, this protocol is capable of 3.5 Gbps quantum-secured direct communication at 50 km range. If Alice augments her terminal with a spontaneous parametric downconverter and both Alice and Bob add channel monitors, they can realize 2 Gbps quantum key distribution at that range against an eavesdropper who injects her own light into B…
By carrying out measurements on entangled states, two parties can generate a secret key which is secure not only against an eavesdropper bound by the laws of quantum mechanics, but also against a hypothetical "post-quantum" eavesdroppers limited by the no-signalling principle only. We introduce a family of quantum key distribution protocols of this type, which are more efficient than previous ones, both in terms of key rate and noise resistance. Interestingly, the best protocols involve large number of measurements. We show that in the absence of noise, these protocols can yield one secret bit…
Secure quantum key distribution can be achieved with an imperfect single-photon source through implementing the decoy-state method. However, security of all those theoretical results of decoy-state method based on the original framework raised by Hwang needs monitoring the source state very carefully, because the elementary proposition that the counting rates of the same state from different sources are equal does not hold in general when the source is unstable. Source intensity monitoring greatly decreases the system efficiency. Here without using Hwang's proposition for stable source, we pre…
This paper presents a prepare-and-measure scheme using $N$-dimensional quantum particles as information carriers where $N$ is a prime power. One of the key ingredients used to resist eavesdropping in this scheme is to depolarize all Pauli errors introduced to the quantum information carriers. Using the Shor-Preskill-type argument, we prove that this scheme is unconditionally secure against all attacks allowed by the laws of quantum physics. For $N = 2^n > 2$, each information carrier can be replaced by $n$ entangled qubits. In this case, there is a family of eavesdropping attacks on which no u…
We study the problem of secret key distillation from bipartite states in the scenario where Alice and Bob can only perform measurements at the single-copy level and classically process the obtained outcomes. Even with these limitations, secret bits can be asymptotically distilled by the honest parties from any two-qubit entangled state, under any individual attack. Our results point out a complete equivalence between two-qubit entanglement and secure key distribution: a key can be established through a one-qubit channel if and only if it allows to distribute entanglement. These results can be …
Semiquantum key distribution allows a quantum party to share a random key with a "classical" party who only can prepare and measure qubits in the computational basis or reorder some qubits when he has access to a quantum channel. In this work, we present a protocol where a secret key can be established between a quantum user and an almost classical user who only needs the quantum ability to access quantum channels, by securely delegating quantum computation to a quantum server. We show the proposed protocol is robust even when the delegated quantum server is a powerful adversary, and is experi…
Afterpulsing is a factor limiting the distance over which discrete-variable quantum key distribution systems are secure, and a common feature in single-photon detectors. The relevance of this phenomenon stems from its stochastic, self-interacting nature and the fact that its rate rises with the number of avalanche events, which increases the quantum bit error rate. Here we introduce an effective analytic model, including dead-time and afterpulsing corrections, where afterpulsing correction depends on dead-time value. This model is useful to evaluate the performance of prepare and measure quant…
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 asymmet…
Device-independent quantum key distribution (DIQKD) provides the strongest form of secure key exchange, using only the input-output statistics of the devices to achieve information-theoretic security. Although the basic security principles of DIQKD are now well-understood, it remains a technical challenge to derive reliable and robust security bounds for advanced DIQKD protocols that go beyond the previous results based on violations of the CHSH inequality. In this work, we present a framework based on semi-definite programming that gives reliable lower bounds on the asymptotic secret key rate…
