International Journal of Quantum Information

A chaos-based arbitrated quantum signature (AQS) scheme is designed on the basis of an improved quantum chaotic encryption algorithm whose security is ensured due to the implementation of the quantum one-time pad that embraces the key-dependent chaotic operation string. It involves in a small-scale quantum computation network with three participants in three phases, i.e. initializing phase, signing phase and verifying phase. The signatory signs the encrypted message and then the receiver verifies the signature is valid with the aid of an arbitrator who plays a crucial role when a dispute arises. Analysis shows that the signature can neither be forged nor disavowed by the malicious attackers.

In this paper, we first study a generalized protocol of discrete modulation for continuous-variable quantum key distribution with N coherent states in a Gaussian lossy and noisy channel and investigate its performance against collective attacks. We find that discrete modulation protocols with more than eight states do not perform better than the eight-state protocol. Then, we study the improvement of this protocol by using a nondeterministic noiseless linear amplifier (NLA) on Bob's detection stage. Results indicate that a NLA with gain g can extend the maximum transmission distance by 50 log₁₀g² km and can increase the maximal tolerable excess noise. With the reconciliation efficiency β, we find the gain of NLA has a maximal value defined as g_max and by adjusting the gain to about βg_max one can have the best improvement on secret key rate.

We study the dynamical properties of a cavity field coupling to a Cooper pair box (CPB). We assumed that the CPB is prepared initially in a mixed state with a coherent state for the field. By solving the time-dependent equations using the evolution operator, it shows that mean numbers of Cooper pairs is affected by the detuning. The mean number of Cooper pairs is further enhanced by the multi-photon processes in commonly used cavity field.

In this paper, we carry out an in-depth analysis of the quantum private comparison (QPC) protocol with the semi-honest third party (TP). The security of QPC protocol using the EPR pairs is re-examined. Unfortunately, we find that TP can use the fake EPR pairs to steal all the secret information. Furthermore, we give two simple and feasible solutions to improve the original QPC protocol. It is shown that the improved protocol is secure, which can resist various kinds of attacks from both the outside eavesdroppers and the inside participants, even the semi-honest TP.

We propose a scheme of realization of fast quantum information transfer and two-qubit entangled state with two different superconducting flux qubits coupled to a single-mode resonator. In this approach, the two lowest levels of each Λ-type flux qubit serve as the logical states and a higher-energy intermediate level is used as the gate manipulation. This proposal does not require equal device parameters for the two flux qubits that is extremely difficult to achieve experimentally. In addition, neither a second-order detuning nor auxiliary qubits are required in this scheme. Moreover, there is no need to change the Rabi frequencies to satisfy the adiabatic passage. Therefore, the operation time can be shortened to 20–30 ns, which is faster by one order of magnitude compared to the schemes employing the second-order detuning technique.

The concept of quantum-secure readout of Physical Unclonable Functions (PUFs) has recently been realized experimentally in an optical PUF system. We analyze the security of this system under the strongest type of classical attack: the challenge estimation attack. The adversary performs a measurement on the challenge quantum state in order to learn as much about it as he can. Using this knowledge, he then tries to reconstruct the challenge and to emulate the PUF. We consider quadrature measurements, which are the most informative practical measurements known to us. We prove that even under this attack the expected number of photons detected in the verification mechanism is approximately a factor S + 1 too low; here S is the Quantum Security Parameter, defined as the number of modes in the optical system divided by the number of photons in the challenge. The photon count allows for a reliable distinction between an authentic PUF and a challenge estimation attack.

Two novel schemes via different entanglement resources are put forward to remotely preparing a five-qubit Brown-type state with high probabilities. To design the schemes, some useful measurement bases with the aid of Hurwitz matrix equation are constructed, which have no restrictions on the coefficients of the prepared state. Through a four-particle projective measurement and two-step three-particle projective measurement under the novel sets of mutually orthogonal basis vectors, the remote preparation can be realized with the probability 50% and 100% in general case, respectively. To improve the probability, the special ensembles of the prepared state that the success probability reaches up to 100% are discussed for the first scheme by the permutation group. Furthermore, the present schemes are extended to the nonmaximally entangled quantum channel.

Beyond the simplest case of bipartite qubits, the composite Hilbert space of multipartite systems is largely unexplored. In order to explore such systems, it is important to derive analytic expressions for parameters which characterize the system's state space. Two such parameters are the degree of genuine multipartite entanglement and the degree of mixedness of the system's state. We explore these two parameters for an N-qubit system whose density matrix has an X form. We derive the class of states that has the maximum amount of genuine multipartite entanglement for a given amount of mixedness. We compare our results with the existing results for the N = 2 case. The critical amount of mixedness above which no N-qubit X-state possesses genuine multipartite entanglement is derived. It is found that as N increases, states with higher mixedness can still be entangled.

We provide a new joint remote preparation scheme for some states. By designing some appropriate measurement bases, two separate parties can jointly prepare a three-qubit state for a remote receiver. The receiver may be assigned during the remote preparation progress. This scheme is extended to M > 2 senders for special two-qubit state. Our scheme succeeds with unit probability and is applicable to a certain realistic situation.