International Journal of Quantum Information

Quantum entanglement is an important resource for quantum information processing tasks. However, realistic multipartite entangled state production is very difficult. In this paper, we propose an efficient single-step scheme for generating many body Greenberger–Horne–Zeilinger (GHZ) states on superconducting circuits by using a superconducting transmission-line resonator (TLR) interact with $N$ superconducting transmon qubits. The distinct merit of our proposal is that it does not require the qubit-resonator coupling strengths to be the same, which is usually impractical experimentally, and thus is one of the main reasons for entanglement generation infidelity in previous single-step schemes. The removing of the uniform interaction requirement is achieved by modulating the qubits splitting frequencies with ac microwave fields, which results in tunable individual qubit-resonator coupling strength, and thus effective uniform qubit–qubit interaction Hamiltonian can be obtained. Since microwave control is conventional nowadays, our proposal can be directly tested experimentally, which makes previous multipartite entangled states generation schemes more efficient.

A scheme is proposed for cyclic-controlled quantum operation teleportation (CCQOT) for three sides with EPR and cluster states. Under the control of David, Alice can implement an unknown single-qubit unitary operation on the remote Bob’s quantum system, while Bob can execute a single-qubit unitary operation on Charlie’s quantum system and Charlie can also perform an unknown single-qubit unitary operation on Alice’s quantum system. Our scheme can be generalized to $n(n\ge 3$) agents involved in the cycle to realize the transmission of single-qubit operations. Moreover, by replacing the quantum channels, we can change the cyclic direction of controlled qunatum operation teleportation (CQOT) from clockwise to counterclockwise. In addition, we discuss our scheme in four types of noisy environments (amplitude-damping, phase-damping, bit-flip and phase-flip noisy environment), and use fidelity to analyze the amount of information lost in the process of CCQOT due to noise. The results show that the fidelity is determined by decoherence rate and amplitude parameters of the final state.

We establish the existence of optimal entanglement-assisted quantum ${[[n,k,d;n-k]]}_2$ codes for $(n,k,d)=(14,6,7)$, $(15,7,7)$, $(17,6,9)$, $(17,7,8)$, $(19,7,9)$ and $(20,7,10)$. These codes are obtained from quaternary Hermitian linear complementary dual codes. We also give some observations on the largest minimum weights.

We study the dynamic of quantum speed limit (QSL) time of a qubit coupled to a bath of noninteracting spins. The investigations are carried under both exact and approximate solutions of the model and the results of the two approaches are compared with each other. Under the exact solution and in the low temperature limit, QSL time becomes independent of the number of spins in the environment with potential capacity for quantum speed up. Beyond a critical value of temperature, there exists a frequency of the spins that reduces the capacity for speeding up quantum evolution. The effects of driving time, nonuniform frequencies of the bath’s spins and coupling strength on QSL time are also investigated.

We address the superposition of causal orders in the quantum switch as a convenient framework for quantum process discrimination in the presence of noise in qubit systems, using Bayes strategy. We show that, for different kinds of qubit noises, the indefinite causal order between the unitary to be discriminated and noise gives enhancement compared to the definite causal order case without reaching the ultimate bound of discrimination in general. Whereas, for entanglement breaking channels, the enhancement is significant, where the quantum switch allows for the attainability of the ultimate bound for discrimination posed by quantum mechanics. Memory effects escorting the superposition of causal orders are discussed, where we point out that processes describing an indefinite causal order, violate the notion of Markov locality. Accordingly, a suggestion for the simulation of indefinite causal orders in more generic scenarios beyond the quantum switch is given.

The efficient quantum circuits of arithmetic operations are important to perform quantum algorithms. To implement efficient matrix operations, we first modify the generalized model of the novel enhanced quantum representation of digital images (GNEQR) to store unsigned and signed integer matrices. Next, we design the circuits of the circuits of quantum addition, quantum modulo addition, quantum subtraction, and quantum modulo subtraction, these operations all keeping two operands unchanged. Then, we propose the circuits of quantum matrix addition, quantum matrix modulo addition, quantum matrix subtraction, and quantum matrix modulo subtraction for the first time. Furthermore, we present a simulation method to verify the correctness of the proposed arithmetic operations of matrix. The results of simulation experiment show that the propose arithmetic operations of matrix are efficient and correct.

By studying the properties of $q^2$-cyclotomic cosets, the maximum designed distances of Hermitian dual-containing constacyclic Bose–Chaudhuri–Hocquenghem (BCH) codes with length $n=2(q^m+1)$ are determined, where $q$ is an odd prime power and $m\ge 3$ is an integer. Further, their dimensions are calculated precisely for the given designed distance. Consequently, via Hermitian Construction, many new quantum codes could be obtained from these codes, which are not covered in the literature.

In this paper, we introduce a new quantum key distribution protocol, which we refer to as polarization-phase (PoP) protocol. In this protocol, two degrees of photonic freedom of the same particle in a hybrid manner are used to encode the information in the format of two- or high-dimensional quantum states (qubits and qudits, respectively). Here, we only discuss the qubit version of the general PoP protocol (we refer to two-dimensional PoP (TD-PoP) protocol) as an interesting extension for the standard BB84 protocol. We investigate the performance of the TD-PoP protocol using infinite-key analysis against restricted individual attacks, i.e. the intercept-resend and photon number splitting attacks, in both ideal single-photon and Poisson (attenuated laser) sources. In addition, we demonstrate that this protocol, despite using two physical dimensions, is simple and fully has empirical implementation capability. Ability to extract two bits of information from each detection event, and increasing the sifting parameter, the secure key rate, and the likelihood of detection of an eavesdropper, are the advantages of the TD-PoP protocol compared to the standard polarization- or phase-encoded BB84 protocol.

Single-photon detection concept is the most crucial and often difficult factor to determine the performance of quantum key distribution (QKD) systems. One solution to facilitate understanding this concept is to create a virtual environment for modeling, analyzing, and investigating the performance of single photon detectors. In this paper, a simulator for superconducting single photon detectors with time domain visualizer and configurable parameters is presented. The widely used ID281SNSPD in the QKD area was theoretically modeled in terms of pulse analysis, the impact of biasing current and the temperature on the dark counts rate and single photon-detection efficiency and influence of the number of photons per pulse on the single photon-detection efficiency. The simulated results were in good agreement with the theoretical results and the simulator demonstrated its adaptability.

Hanoi network (HN) has a one-dimensional periodic lattice as its main structure with additional long-range edges, which allow having efficient quantum walk algorithm that can find a target state on the network faster than the exhaustive classical search. In this paper, we use regular quantum walks and lackadaisical quantum walks, respectively, to search for a target state. From the curve fitting of the numerical results for HN of degrees three and four, we find that their running time for the regular quantum walks are followed by amplitude amplification scales as ${𝒪}(N^{0.79}\sqrt{logN})$ and ${𝒪}(N^{0.65}\sqrt{logN})$, respectively. For the search by lackadaisical quantum walks, the running time scales are as ${𝒪}(N^{0.57}logN)$ and ${𝒪}(N^{0.50}logN)$, respectively.