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Quantum systems can be used to certify the existence of true randomness in nature, and semi-device-independent (SDI) randomness expansion has been used to achieve this goal under the prepare-measure configuration. Usually, randomness is certified with the degree of violation of some inequalities. However, the observed statistics characterizing SDI protocols contain much richer information than just a single value of the inequality. Thus, there might be a gap between the randomness in observed statistics and the randomness we certify. The purpose of this paper is to optimize the use of the full observed statistics to reduce that gap. It shows that certifying randomness directly from the full observed statistics but not some inequality is available in the SDI protocols. Besides, average of guessing probabilities should be considered as randomness quantitative standards, since it certifies more randomness than the previous maximal guessing probability method. In the analysis, we take into account the assumption that the devices and the potential adversary are classically correlated. [ABSTRACT FROM AUTHOR]

Semi-device-independent randomness expansion is the protocol which can generate and certify perfect free random numbers, without making detailed assumptions on the internal working of quantum devices, except the bound on the dimension of the Hilbert space. The two-dimensional (2D) quantum witness violation has been used to generate guaranteed fresh randomness from untrusted devices in the prepare-and-measure configuration. In this paper we first show that the 2D quantum witness violation can be simulated by a classical deterministic process by relaxing the assumption of the dimension. A deterministic process to simulate the generation of random numbers is given, under the hidden variables model, with just the high dimensional measurement device and one qubit communication. This makes the generated number essentially deterministic, which is said to be pseudorandom. Then we also show that when we relax the freedom of choice, there is the exact region for which quantum correlations can be simulated by the classical process in the task of randomness expansion. [ABSTRACT FROM AUTHOR]

As a new model for signing quantum messages, arbitrated quantum signature (AQS) has recently received a lot of attention. In this paper we study the cryptanalysis of previous AQS protocols from the aspects of forgery and disavowal. We show that in these protocols the receiver, Bob, can realize existential forgery of the sender's signature under known message attack. Bob can even achieve universal forgery when the protocols are used to sign a classical message. Furthermore, the sender, Alice, can successfully disavow any of her signatures by simple attack. The attack strategies are described in detail and some discussions about the potential improvements of the protocols are given. Finally we also present several interesting topics on AQS protocols that can be studied in future. [ABSTRACT FROM AUTHOR]

Analog quantum simulators can be used to study quantum correlation in novel many-body systems by emulating the Hamiltonian of these systems. One essential question in quantum simulation is to probe the properties of an emulated many-body system. Here we present a circuit QED scheme for probing such properties by measuring the spectrum of a superconducting resonator coupled to a quantum simulator. We first study a general framework of this approach and show that the spectrum of the resonator is directly related to the correlation function of the coupling operator between the resonator and the simulator. We then apply this scheme to a simulator of the transverse field Ising model implemented with superconducting qubits, where the resonance peaks in the resonator spectrum correspond to the frequencies of the elementary excitations. The effects of resonator damping, qubit decoherence, and resonator backaction are also discussed. This setup can be used to probe a broad range of many-body models. [ABSTRACT FROM AUTHOR]

In this paper, we mainly study the local indistinguishability of mutually orthogonal product basis quantum states in d ⊗ d. In 3 ⊗ 3, Bennett et al. [Phys. Rev. A 59, 1070 (1999)] presented nine orthogonal product basis quantum states which cannot be distinguished by local operations and classical communication (LOCC). In the work by Zhang et al. [Z.-C. Zhang et al., Phys. Rev. A 90, 022313 (2014)], this result was generalized in d ⊗ d, where d is odd. In this paper, we aim to construct locally indistinguishable orthogonal product basis quantum states in d ⊗ d. For the general d ⊗ d(d > 2) quantum system, we first construct 4d-4 orthogonal product states, and prove these states are locally indistinguishable using a very simple but quite effective method. Then, based on these states, we construct some classes of locally indistinguishable orthogonal product basis quantum states (OPBS) in d ⊗ d(d > 2). Finally, we construct some LOCC indistinguishable OPBS in multipartite quantum systems. All of the above results demonstrate the phenomenon of nonlocality without entanglement. [ABSTRACT FROM AUTHOR]

Recently, Kalev et al. [A. Kalev, A. Mann, and M. Revzen, Phys. Rev. Lett. 110, 260502 (2013).] proposed a quantum (public) communication protocol with an interesting encoding style, where the signal is encoded in the choice of the measurement basis of one of the communicating patties (Bob) and Bob's measurement results are irrelevant for the communication. As we know, one of the main advantages of quantum technique in communication over the classical one is the higher security. Therefore, as Kalev et al. have mentioned in their paper, an interesting open question is whether one could and how to utilize this new encoding style to design a secure communication protocol, i.e., translate the choice of measurement into the secret. Here we propose a quantum key distribution protocol, which is a secure communication protocol, with this encoding style, where Bob's measurement results are still irrelevant. Furthermore, the security of the proposed protocol in the zero-error case has been proved. [ABSTRACT FROM AUTHOR]

In a recent paper [K. Loukopoulos and D. E. Browne, Phys. Rev. A 81, 062336 (2010)], five schemes for secure multiparty computation utilizing the Greenberger-Horne-Zeilinger (GHZ) quantum correlation were presented, which were claimed to be secure in some kinds of security models. However, this study points out that schemes C and D2 could be attacked by corrupted parties replacing photon and exchanging classical information. [ABSTRACT FROM AUTHOR]

Local distinguishability of orthogonal quantum states is an area of active research in quantum information theory. However, most of the relevant results are about local distinguishability in bipartite Hilbert space and very little is known about the multipartite case. In this paper we present a generic method to construct a completable n-partite (n = 3) product basis with only 2n members, which exhibits nonlocality without entanglement with n parties, each holding a system of any finite dimension. We give an effective proof of the nonlocality of the completable multipartite product basis. In addition, we construct another incomplete multipartite product basis with a smaller number of members that cannot be distinguished by local operations and classical communication in a d1 ⊗ d2 ⊗...⊗ dn quantum system, where n = 3 and di = 2 for i=1,2,...,n. The results can lead to a better understanding of the phenomenon of nonlocality without entanglement in any multipartite quantum system. [ABSTRACT FROM AUTHOR]

It has been proven by Rana et al. that the n-qubit generic Dicke states |GD(n, ℓ) are uniquely determined, among arbitrary states, by their (ℓ + l )-partite reduced density matrices, and (n-1/ℓ) number of them which have one party common to all are sufficient. We show that among arbitrary states, Dicke-type states are also uniquely determined by their (ℓ + l)-partite reduced density matrices. Here, by Dicke-type states, we mean all multiqubit pure states that are stochastic local operations and classical communication equivalent to the n-qubit standard Dicke state |D(n, ℓ). Moreover, we find two kinds of subsets of the (ℓ + l)-partite reduced density matrices set, both of which can determine the Dicke-type states. More importantly, the number of elements in these two subsets is much smaller than (n-1/ℓ). [ABSTRACT FROM AUTHOR]

The quantum Zeno effect (QZE) is the phenomenon in which the unitary evolution of a quantum state is suppressed, e.g., due to frequent measurements. Here, we investigate the use of the QZE in a class of communication complexity problems (CCPs). Quantum entanglement is known to solve certain CCPs beyond classical constraints. However, recent developments have yielded CCPs for which superclassical results can be obtained using only communication of a single ¿-level quantum state (qudit) as a resource. In the class of CCPs considered here, we show quantum reduction of complexity in three ways: using (i) entanglement and the QZE, (ii) a single qudit and the QZE, and (iii) a single qudit. We have performed a proof of concept experimental demonstrations of three party CCP protocol based on single-qubit communication with and without QZE. [ABSTRACT FROM AUTHOR]

It has been known that, among pure states, N-qubit W states cannot be uniquely determined by their arbitrary (N - 1) bipartite reduced density matrices. Parashar and Rana proved that among arbitrary states, (N - 1) bipartite reduced density matrices that the pairs of qubits constitute a star graph or a line graph can uniquely determine stochastic local operations and classical communication (SLOCC) equivalent W states, and we generalize this conclusion into tree graph. In this paper, we show that all SLOCC equivalent W states can be uniquely determined (among pure, mixed states) by their (N - 1) bipartite reduced density matrices, if the (N - 1) pairs of qubits constitute a tree graph on N vertices, where each pair of qubits represents an edge. [ABSTRACT FROM AUTHOR]

Anonymous ranking is a kind of privacy-preserving ranking whereby each of the involved participants can correctly and anonymously get the rankings of his data. It can be utilized to solve many practical problems, such as anonymously ranking the students' exam scores. We investigate the issue of how quantum mechanics can be of use in maintaining the anonymity of the participants in multiparty ranking and present a series of quantum anonymous multiparty, multidata ranking protocols. In each of these protocols, a participant can get the correct rankings of his data and nobody else can match the identity to his data. Furthermore, the security of these protocols with respect to different kinds of attacks is proved. [ABSTRACT FROM AUTHOR]

In the general bipartite quantum system m ⊗ n(m, n ≥ 3), Y.-L. Wang et al. [Phys. Rev. A 92, 032313 (2015)] presented 3(m + n)--9 orthogonal product states which cannot be distinguished by local operations and classical communication (LOCC). In this paper, we aim to construct less locally indistinguishable orthogonal product states in m ⊗ n. First, in the 3 ⊗ n(3