Since several papers appeared in 2000, the quantum key distribution (QKD) community has been well aware that photon number splitting (PNS) attack by Eve severely limits the secure key distribution distance in BB84 QKD systems with Poissonian photon sources. In attempts to solve this problem, entanglement-based QKD, single-photon based QKD, and entanglement swapping-based QKD, have been studied in recent years. However, there are many technological difficulties that must be overcome before these schemes can become practical systems. Here we report a very simple QKD system, in which secure keys were generated over >100 km fibre for the first time. We used an alternative protocol of differential phase shift keying (DPSK) but with a Poissonian source. We analysed the security of the DPSK protocol and showed that it is robust even against hybrid attacks including collective PNS attack over consecutive pulses, intercept-and-resend (I-R) attack and beamsplitting (BS) attack, because of the non-deterministic collapse of a wavefunction in a quantum measurement. To implement this protocol, we developed a new detector for the 1.5 um band based on frequency up-conversion in a periodically poled lithium niobate (PPLN) waveguide followed by a Si avalanche photodiode (APD). The use of the new detectors increased the sifted key generation rate up to > 1 Mbit/s over 30 km fibre, which is two orders of magnitude larger than the previous record.
We introduce a differential geometric framework for describing families of quantum error-correcting codes and for understanding quantum fault tolerance. This work unifies the notion of topological fault tolerance with fault tolerance in other kinds of quantum error-correcting codes. In particular, we use fibre bundles with a natural flat projective connection to study the transformation of codewords under unitary fault-tolerant evolutions. We show that the fault-tolerant logical operations are given by the monodromy group for either of two bundles, both of which have flat projective connection…
A new definition and interpretation of geometric phase for mixed state cyclic unitary evolution in quantum mechanics are presented. The pure state case is formulated in a framework involving three selected Principal Fibre Bundles, and the well known Kostant-Kirillov-Souriau symplectic structure on (co) adjoint orbits associated with Lie groups. It is shown that this framework generalises in a natural and simple manner to the mixed state case. For simplicity, only the case of rank two mixed state density matrices is considered in detail. The extensions of the ideas of Null Phase Curves and Panc…
The performances of optical fibre-based quantum information systems are limited by the intrinsic properties of silica glass materials, e.g. high latency, Rayleigh-scattering loss wavelength scaling law, and cross-coupling induced modal impurity. Hollow-core optical fibre (HCF) promises to unify air-borne light propagation and non-line-of-sight transmission, thus holding great potentials for versatile photonics-based quantum infor-mation applications. The early version of HCF based on photonic-bandgap guidance has not proven itself as a reliable quantum channel because of the poor modal purity …
The differential-phase-shift quantum key distribution protocol is formalised as a prepare-and-measure scheme and translated into an equivalent entanglement-based protocol. A necessary condition for security is that Bob's measurement can detect the entanglement of the distributed state in the entanglement-based translation, which implies that his measurement is described by non-commuting POVM elements. This condition is shown to be met.
Quantum secret sharing (QSS) is essential for multiparty quantum communication, which is one of cornerstones in the future quantum internet. However, a linear rate-distance limitation severely constrains the secure key rate and transmission distance of QSS. Here, we present a practical QSS protocol among three participants based on the differential phase shift scheme and twin field ideas for the solution of high-efficiency multiparty communication task. In contrast to formerly proposed differential phase shift QSS protocol, our protocol can break the linear rate-distance bound, theoretically i…
We derive a proof of security for the Differential Phase Shift Quantum Key Distribution (DPSQKD) protocol under the assumption that Eve is restricted to individual attacks. The security proof is derived by bounding the average collision probability, which leads directly to a bound on Eve's mutual information on the final key. The security proof applies to realistic sources based on pulsed coherent light. We then compare individual attacks to sequential attacks and show that individual attacks are more powerful.
We propose a scheme for quantum key distribution (QKD) in a passive optical network (PON) based on differential phase shift (DPS) coding. A centralized station including all expensive components serves many users, making it suitable for a true multi-user network in a local environment with moderate distances on the order of a few kilometers. The emphasis lies on an asymmetric design for cost effective implementation of network end points.
A cost-effective QKD transmitter is evaluated in a 16km reach, 2:16-split PON and yields 5.10-7secure bits/pulse. Co-existence with 20 down-and 1 upstream channel is possible at low QBER degradation of 0.93% and 1.1%.
We investigate limitations imposed by sequential attacks on the performance of differential-phase-shift quantum key distribution protocols that use pulsed coherent light. In particular, we analyze two sequential attacks based on unambiguous state discrimination and minimum error discrimination, respectively, of the signal states emitted by the source. Sequential attacks represent a special type of intercept-resend attacks and, therefore, they do not allow the distribution of a secret key.
We prove the unconditional security of coherent-state-based differential phase shift quantum key distribution protocol (DPSQKD) with block-wise phase randomization. Our proof is based on the conversion of DPSQKD to an equivalent entanglement-distillation protocol where the estimated phase error rate determines the amount of the privacy amplification. The generated final key has a contribution from events where the sender emits two or more photons, indicating the robustness of DPSQKD against photon-number-splitting attacks.
We investigate limitations imposed by detector dead-times on the performance of sequential attacks against a differential-phase-shift (DPS) quantum key distribution (QKD) protocol with weak coherent pulses. In particular, we analyze sequential attacks based on unambiguous state discrimination of the signal states emitted by the source and we obtain ultimate upper bounds on the maximal distance achievable by a DPS QKD scheme both in the so-called trusted and untrusted device scenarios, respectively.
The differential phase shift quantum key distribution protocol is of high interest due to its relatively simple practical implementation. This protocol uses trains of coherent pulses and allows the legitimate users to resist individual attacks. In this paper, a new attack on this protocol is proposed which is based on the idea of information extraction from the part of each coherent state and then making decision about blocking the rest part depending on the amount of extracted information.
We propose an auto-compensating differential phase shift scheme for quantum key distribution with a high key-creation efficiency, which skillfully makes use of automatic alignment of the photon polarization states in optical fiber with modified Michelson interferometers composed of unequal arms with Faraday mirrors at the ends. The Faraday-mirrors-based Michelson interferometers not only function as pulse splitters, but also enable inherent compensation of polarization mode dispersion in the optic-fiber paths at both Alice's and Bob's sites. The sequential pulses encoded by differential phase …
In this paper, we investigate limitations imposed by sequential attacks on the performance of a differential-phase-shift (DPS) quantum key distribution (QKD) protocol with weak coherent pulses. Specifically, we analyze a sequential attack based on optimal unambiguous discrimination of the relative phases between consecutive signal states emitted by the source. We show that this attack can provide tighter upper bounds for the security of a DPS QKD scheme than those derived from sequential attacks where the eavesdropper aims to identify the state of each signal emitted by the source unambiguousl…
In this Letter, we prove the unconditional security of single-photon differential phase shift quantum key distribution (DPS-QKD) protocol, based on the conversion to an equivalent entanglement-based protocol. We estimate the upper bound of the phase error rate from the bit error rate, and show that DPS-QKD can generate unconditionally secure key when the bit error rate is not greater than 4.12%. This proof is the first step to the unconditional security proof of coherent state DPS-QKD.
Round-robin differential-phase-shift quantum key distribution (RRDPS QKD) has been proposed to raise the noise tolerability of the channel. However, in practice, the measurement device in RRDPS QKD may be imperfect. Here, we show that, with these imperfections, the security of RRDPS may be damaged, by proposing two attacks for RRDPS systems with uncharacterized measurement devices. One is valid even for a system with unit total efficiency, while the other is valid even when a single photon state is sent. To prevent these attacks, either security arguments need to be fundamentally revised or fu…
We present a stable and deterministic quantum key distribution (QKD) system based on differential phase shift. With three cascaded Mach-Zehnder interferometers with different arm-length differences for creating key, its key creation efficiency can be improved to be 7/8, more than other systems. Any birefringence effects and polarization-dependent losses in the long-distance fiber are automatically compensated with a Faraday mirror. Added an eavesdropping check, this system is more secure than some other phase-coding-based QKD systems. Moreover, the classical information exchanged is reduces la…
Recently, a new type of quantum key distribution, called the round-robin differential phase-shift (RRDPS) protocol [Nature 509, 475 (2014)], was proposed, where the security can be guaranteed without monitoring any statistics. In this Letter, we investigate source imperfections and side-channel attacks on the source of this protocol. We show that only three assumptions are needed for the security, and no detailed characterizations of the source or the side-channel attacks are needed. This high robustness is another striking advantage of the RRDPS protocol over other protocols.
As a promising quantum key distribution (QKD), most of the existing round-robin differential-phase-shift quantum key distribution (RRDPS-QKD) protocols have adopted the decoy-state method and have assumed the source states are exactly controlled. However, the precise manipulation of source states is impossible for any practical experiment, and the RRDPS-QKD with source errors has an unignorable impact on the performance of the protocol. In the paper, we study the three-intensity decoy-state RRDPS-QKD protocol with source errors, formulate the secure generation key rate of the proposed protocol…
We give a security proof of the `Round Robin Differential Phase Shift' Quantum Key Distribution scheme, and we give a tight bound on the required amount of privacy amplification. Our proof consists of the following steps. We construct an EPR variant of the scheme. We identify Eve's optimal way of coupling an ancilla to an EPR qudit pair under the constraint that the bit error rate between Alice and Bob should not exceed a value beta. As a function of beta we derive, for finite key size, the trace distance between the real state and a state in which no leakage exists. For asymptotic key size we…
One of the simplest methods for implementing quantum key distribution over fiber-optic communication is the Bennett-Brassard 1984 protocol with phase encoding (PE-BB84 protocol), in which the sender uses phase modulation over double pulses from a laser and the receiver uses a passive delayed interferometer. Using essentially the same setup and by regarding a train of many pulses as a single block, one can carry out the so-called differential quadrature phase shift (DQPS) protocol, which is a variant of differential phase shift (DPS) protocols. Here we prove the security of the DQPS protocol ba…
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…
The interplay between electronic properties and optical response enables the realization of novel types of materials with tunable responses. Superconductors are well known to exhibit profound changes in the electronic structure related to the formation of Cooper pairs, yet their influence on the electromagnetic response in the optical regime has remained largely unstudied. Photonics metamaterials offer new opportunities to enhance the light-matter interaction, boosting the influence of subtle effects on the optical response. The combination of photonic metamaterials and superconducting quantum…
We demonstrate near-complete cancellation of the differential light shift of a two-photon magnetic-field-insensitive microwave hyperfine (clock) transition in $^{87}$Rb atoms trapped in an optical lattice. Up to $95(2)%$ of the differential light shift is canceled while maintaining magnetic-field insensitivity. This technique should have applications in quantum information and frequency metrology.
We demonstrate a phase-selective and colorless quantum receiver assisted by a silicon-on-insulator microring, enabling a low 1.3% QBER at 5.3kb/s secure-key rate. No penalty incurs compared to a delay interferometer. BiCMOS 3D-integration is proven feasible.
This paper analyzes the information-theoretical security of the Differential Phase Shift (DPS) Quantum Key Distribution (QKD) protocol, using efficient computational information geometric algorithms. The DPS QKD protocol was introduced for practical reasons, since the earlier QKD schemes were too complicated to implement in practice. The DPS QKD protocol can be an integrated part of current network security applications, hence it's practical implementation is much easier with the current optical devices and optical networks. The proposed algorithm could be a very valuable tool to answer the st…
