Two-way Quantum Research Articles

An improved two-way continuous-variable quantum key distribution protocol with added noise in homodyne detection

We propose an improved two-way continuous-variable quantum key distribution (CV QKD) protocol by adding proper random noise on the receiver’s homodyne detection, the security of which is analysed against general collective attacks. The simulation result under the collective entangling cloner attack indicates that despite the correlation between two-way channels decreasing the secret key rate relative to the uncorrelated channels slightly, the performance of the two-way protocol is still far beyond that of the one-way protocols. Importantly, the added noise in detection is beneficial for the secret key rate and the tolerable excess noise of this two-way protocol. With the reasonable reconciliation efficiency of 90%, the two-way CV QKD with added noise allows the distribution of secret keys over 60 km fibre distance.

On the Security of Quantum Key Distribution Ping-Pong Protocol

Computational based cryptography might not guarantee long term security if computational algorithms, computers, and so on are made remarkable progress. Therefore, quantum cryptography with unconditionally security attracts attention. In this paper, we consider security of a two-way quantum key distribution protocol, so called Ping-Pong protocol. As a result, we introduce not only robustness but also a different information disturbance theorem, which denotes a trade-off relationship between information gain for an eavesdropper and error rate, from the related works for an attack model.

Improved two-way six-state protocol for quantum key distribution

A generalized version for a qubit based two-way quantum key distribution scheme was first proposed in the paper [Phys. Lett. A 358 (2006) 85] capitalizing on the six quantum states derived from three mutually unbiased bases. While boasting of a higher level of security, the protocol was not designed for ease of practical implementation. In this work, we propose modifications to the protocol, resulting not only in improved security but also in a more efficient and practical setup. We provide comparisons for calculated secure key rates for the protocols in noisy and lossy channels.

Two-way and one-way quantum cryptography protocols

We present two quantum cryptography protocols. The first one generalizes the concept of the two-way deterministic protocol to work with qudits in prime d-dimensional system where d is odd. The second protocol makes use of the tomographically complete set construction for odd d-dimensional systems where d=p1p2 to modify the BB84 protocol to work with qudits of such systems. The securities of the two protocols are analyzed according to the intercept and resend attack.

Implementation of two-way free space quantum key distribution

We report an implementation of a two-way, four-state quantum key distribution (QKD) protocol (namely, the LM05 protocol) over a free space medium. A stable quantum bit error rate of 3.34% and maximum tolerable channel loss for secure key generation of 5.68 dB in the absence of a photon number splitting attack were recorded. The result strongly indicates the feasibility of a two-way QKD protocol implementation over a free space medium.

Quantum key distribution with delayed privacy amplification and its application to the security proof of a two-way deterministic protocol

Privacy amplification (PA) is an essential post-processing step in quantum key distribution (QKD) for removing any information an eavesdropper may have on the final secret key. In this paper, we consider delaying PA of the final key after its use in one-time pad encryption and prove its security. We prove that the security and the key generation rate are not affected by delaying PA. Delaying PA has two applications: it serves as a tool for significantly simplifying the security proof of QKD with a two-way quantum channel, and also it is useful in QKD networks with trusted relays. To illustrate the power of the delayed PA idea, we use it to prove the security of a qubit-based two-way deterministic QKD protocol which uses four states and four encoding operations.

A new multi-wavelength two-way quantum key distribution system with a single optical source

Low-efficiency is an important issue in "plug and play" quantum key distribution system for practical application. The inefficient reasons of system and the current solutions are analyzed. And then a new quantum key distribution scheme with multi-wavelength pulses is proposed, in which a broadband optical source and the WDMs are combined to generate the optical pulses with four wavelengths. Moreover, the WDMs can also be used as a filter to extract the signal in the proposed scheme. Compared with other schemes, this scheme is highly stable, more securate and independent of the response speed of the control systems. The scheme can be used in high efficiency "plug and play" quantum key distribution system in practice.

Unconditional security proof of a deterministic quantum key distribution with a two-way quantum channel

In a deterministic quantum key distribution (DQKD) protocol with a two-way quantum channel, Bob sends a qubit to Alice who then encodes a key bit onto the qubit and sends it back to Bob. After measuring the returned qubit, Bob can obtain Alice's key bit immediately, without basis reconciliation. Since an eavesdropper may attack the qubits traveling on either the Bob-Alice channel or the Alice-Bob channel, the security analysis of DQKD protocol with a two-way quantum channel is complicated and its unconditional security has been controversial. This paper presents a security proof of a single-photon four-state DQKD protocol against general attacks.

Efficient and Secure Two-Way Asynchronous Quantum Secure Direct Communication Protocol by Using Entangled States

An efficient and secure two-way asynchronous quantum secure direct communication protocol by using entangled states is proposed in this paper. Decoy photons are utilized to check eavesdropping; the securities of the protocol are equal to BB84 protocol. After ensuring the security of the quantum channel, both parties encode the secret message by using CNOT operation and local unitary operation separately. The two-way asynchronous direct transition of secret message can be realized by using Bell measurement and von Neumann measurement, combined with classical communication. Different from the present quantum secure direct communication protocols, the two parties encode secret message through different operations which is equivalent to sharing two asymmetric quantum channels, and the protocol is secure for a noise quantum protocol. The protocol is efficient in that all entangled states are used to transmit secret message.

RELATION BETWEEN INFORMATION AND DISTURBANCE IN QUANTUM KEY DISTRIBUTION PROTOCOL WITH CLASSICAL ALICE

The "semiquantum" key distribution protocol introduced by Zou et al. is examined. The protocol while using two-way quantum communication requires only Bob to be fully quantum. We derive a trade-off inequality between information gained by Eve and the disturbance observed by legitimate users. It guarantees that Eve cannot obtain large information if the disturbance is sufficiently small.

Duplex quantum communication through a spin chain

Data multiplexing within a quantum computer can allow for the simultaneous transfer of multiple streams of information over a shared medium thereby minimizing the number of channels needed for requisite data transmission. Here, we investigate a two-way quantum communication protocol using a spin chain placed in an external magnetic field. In our scheme, Alice and Bob each play the role of a sender and a receiver as two states cos({\theta}1/2)|0> + sin({\theta}1/2)exp(i{\phi}1)|1>, cos({\theta}2/2)|0> + sin({\theta}2/2)exp(i{\phi}2)|1> are transferred through one channel simultaneously. We find that the transmission fidelity at each end of a spin chain can usually be enhanced by the presence of a second party. This is an important result for establishing the viability of duplex quantum communication through spin chain networks.

Two-way quantum communication: ‘secure quantum information exchange’

In this paper, we present a new idea of two-way quantum communication called ‘secure quantum information exchange’ (SQIE). If there are two arbitrary unknown quantum states |ξ⟩IA and |η⟩IB, initially with Alice and Bob, respectively, then SQIE protocol leads to the simultaneous exchange of these states between Alice and Bob with the aid of the special kind of six-qubit entangled (SSE) state and classical assistance of the third party, Charlie. The term ‘secure’ signifies the fact that SQIE protocol either faithfully exchanges the unknown quantum states proceeding in a prescribed way or, in case of any irregularity, the process generates no results. For experimental realization of the SQIE protocol, we have suggested an efficient scheme for generating SSE states using the interaction between highly detuned Λ-type three-level atoms and the optical coherent field. By theoretical calculations, we found that SSE states of almost unit fidelity with perfect success rates for appreciable mean photon numbers (Fav ⩾ 0.999 for |α|2 ⩾ 1.5) can be generated by our scheme. Further, we have discussed possible experimental imperfections, such as atomic-radiative time, cavity damping time, atom–cavity interaction time, and the efficiency of discrimination between the coherent field and the vacuum state shows that our SQIE protocol is within the reach of technology presently available.

Attack with Hong–Ou–Mandel interferometer to quantum key distribution

We suggest a sophisticated attack protocol, which can be applied to the two-way deterministic quantum key distribution protocol. When more than two photons are used for a qubit, Eve picks out a photon each from the forward and the backward channel and measures whether the states of the two picked-out photons are orthogonal or the same by using the fourth order quantum interference. We describe the attack protocol, which uses an active arrangement of two Hong–Ou–Mandel interferometers combined with further quantum nondemolition measurements, beam splitters, and polarization rotator.

Upper bound on key generation rate of quantum key distribution with two-way or two-step quantum channels

We present an asymptotic security proof of deterministic quantum key distribution (DQKD) with a two-way quantum channel. The security proof of DQKD with a two-way quantum channel is different from that of BB84, because Eve can attack the travel qubits twice, both in line Bob to Alice and in line Alice to Bob. With the no-signaling principle and the property of mutual information, we obtain an upper bound of the final key generation of entanglement-based DQKD and hence single-photon four-state DQKD. Our results can be applied to the protocol of QKD with two-step quantum channels.

Independent attacks in imperfect settings: A case for a two-way quantum key distribution scheme

We review the study on a two-way quantum key distribution protocol given imperfect settings through a simple analysis of a toy model and show that it can outperform a BB84 setup. We provide the sufficient condition for this as a ratio of optimal intensities for the protocols.

Quantum key distribution based on phase encoding in long-distance communication fiber

A robust two-way quantum key distribution system based on phase encoding is demonstrated in 50 km and 100 km commercial communication fiber. The system can automatically compensate for birefringence effects and remain stable over 23 h. A low quantum bit error rate and high visibility are obtained. Furthermore, the storage fiber is unnecessary and train of pulses is only needed in the test with 100 km fiber.

Improvement of two-way continuous variable quantum cryptography by using additional noise

The performance of quantum key distribution such as one-way continuous variable protocols, can be increased by adding some noise on the reference side of error correction in the error-correction phase. For this reason, we here study this possibility in the case of two-way continuous variable system. Finally, the numerical results show that the using of additional noise gives two-way schemes better security performance in terms of secret key rates and resistance to channel excess noise.

Succinctness of two-way probabilistic and quantum finite automata

special issue dedicated to the second edition of the conference AutoMathA: from Mathematics to Applications We introduce a new model of two-way finite automaton, which is endowed with the capability of resetting the position of the tape head to the left end of the tape in a single move during the computation. Several variants of this model are examined, with the following results: The weakest known model of computation where quantum computers recognize more languages with bounded error than their classical counterparts is identified. We prove that two-way probabilistic and quantum finite automata (2PFAs and 2QFAs) can be considerably more concise than both their one-way versions (1PFAs and 1QFAs), and two-way nondeterministic finite automata (2NFAs). For this purpose, we demonstrate several infinite families of regular languages which can be recognized with some fixed probability greater than 1 2 by just tuning the transition amplitudes of a 2QFA (and, in one case, a 2PFA) with a constant number of states, whereas the sizes of the corresponding 1PFAs, 1QFAs and 2NFAs grow without bound. We also show that 2QFAs with mixed states can support highly efficient probability amplification.

Passive switching scheme for two-way quantum key distribution setups

A simple design for Bob in two-way quantum key distribution schemes that employs only passive linear optical components to perform the necessary switching operations is presented. It is shown that, for some specific protocols and/or practical qubit implementations, the losses in Bob's station can be made negligible, hence increasing the maximum distance for secret key generation.

Interview

A new practical proposal for two-way quantum key distribution links is presented in this issue in ‘Passive switching scheme for two-way quantum key distribution setups’ on page 512. The author, Dr Guilherme Temporão from PUC-Rio in Brazil, tells us more.