Single-qubit Unitary Operations Research Articles

Experimental Quantum Message Authentication with Single Qubit Unitary Operation

We have developed a quantum message authentication protocol that provides authentication and integrity of an original message using single qubit unitary operations. Our protocol mainly consists of two parts: quantum encryption and a correspondence check. The quantum encryption part is implemented using linear combinations of wave plates, and the correspondence check is performed using Hong–Ou–Mandel interference. By analyzing the coincidence counts of the Hong–Ou–Mandel interference, we have successfully proven the proposed protocol experimentally, and also showed its robustness against an existential forgery.

Enhancing entanglement of two-qubit states from amplitude damping channel using single-qubit unitary operation and local filtering operation

We propose a scheme to enhance entanglement from amplitude damping or correlated amplitude damping decoherence. We show that entanglement sudden death time can be prolonged by the initial single-qubit operation combined with local filtering operation. For the amplitude damping channel case, we give the optimal single-qubit operation for arbitrary pure state [Formula: see text]. For the correlated amplitude damping channel case, we find that single-qubit operation on the initial state can not only enhance the final entanglement but also avoid entanglement sudden death. Compared to the previous schemes, the optimal operations and local filtering operations used in our scheme are independent with the decay parameters of the environment.

Two-Qubit Bloch Sphere

Three unit spheres were used to represent the two-qubit pure states. The three spheres are named the base sphere, entanglement sphere, and fiber sphere. The base sphere and entanglement sphere represent the reduced density matrix of the base qubit and the non-local entanglement measure, concurrence, while the fiber sphere represents the fiber qubit via a simple rotation under a local single-qubit unitary operation; however, in an entangled bipartite state, the fiber sphere has no information on the reduced density matrix of the fiber qubit. When the bipartite state becomes separable, the base and fiber spheres seamlessly become the single-qubit Bloch spheres of each qubit. Since either qubit can be chosen as the base qubit, two alternative sets of these three spheres are available, where each set fully represents the bipartite pure state, and each set has information of the reduced density matrix of its base qubit. Comparing this model to the two Bloch balls representing the reduced density matrices of the two qubits, each Bloch ball corresponds to two unit spheres in our model, namely, the base and entanglement spheres. The concurrence–coherence complementarity is explicitly shown on the entanglement sphere via a single angle.

Deterministic quantum cyclic controlled teleportation of arbitrary multi-qubit states using multi-qubit partially entangled channel

In this paper, we present a deterministic four-party quantum cyclic controlled teleportation (QCYCT) scheme, by using a multi-qubit partially entangled state as the quantum channel. In this scheme, Alice can teleport an arbitrary [Formula: see text]-qubit state to Bob, Bob can teleport an arbitrary [Formula: see text]-qubit state to Charlie and Charlie can teleport an arbitrary [Formula: see text]-qubit state to Alice under the control of the supervisor David. We utilize rotation gate, Hadamard gates and controlled-NOT (CNOT) gates to construct the multi-qubit partially entangled channel. Only Bell-state measurements, single-qubit von-Neumann measurement and proper unitary operations are required in this scheme, which can be realized in practice easily based on the present quantum experiment technologies. The direction of cyclic controlled teleportation of arbitrary multi-qubit states can also be changed by altering the quantum channel. Analysis demonstrates that the success probability of the proposed scheme can still reach 100% although the quantum channel is non-maximally entangled. Furthermore, the proposed four-party scheme can be generalized into the case involving [Formula: see text] correspondents, which is more suitable for quantum communication networks. We also calculate the intrinsic efficiency and discuss the security of the proposed scheme. Compared with the existing QCYCT schemes which realized cyclic controlled teleportation of arbitrary single-qubit states, specific two-qubit and three-qubit states, the proposed scheme is of general significance.

Controlled Teleportation of a Three-Qubit State by Using a Five-Qubit Cluster State

We propose a scheme of controlled teleportation of an arbitrary three-qubit state by using a five-qubit cluster state. In our scheme, the sender Alice performs five sets of single-qubit measurements and the controller Charlie needs only take a single-qubit measurement. The receiver Bob can reconstruct the arbitrary three-qubit state by performing the single-qubit unitary operations and a controlled-NOT operation if he obtains the help of the controller. We show that the efficiency in this scheme approaches 100%

Cyclic-controlled quantum operation teleportation in noisy environment

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 [Formula: see text]) 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.

Quantum homomorphic encryption scheme with flexible number of evaluator based on (k, n)-threshold quantum state sharing

Quantum homomorphic encryption provides the ability to perform calculations on encrypted data without decrypting them. The number of evaluators in most previous schemes is 1, and in other schemes it is n. In this paper, we propose a novel quantum homomorphic encryption scheme with flexible number of evaluator. One of our main contributions is that the number of evaluators is d (k ⩽ d ⩽ n), here n ≥ 2. These evaluators are allowed to alternately complete the evaluation of single-qubit unitary operations on the shared encrypted sequence. Other work is that for n = 1, we also give the quantum homomorphic encryption scheme that one evaluator can complete the evaluation. Then, for (3, 5)-case and (2, 2)-case, we give two examples (i.e. example-I and example-II) to illustrate the flexibility of evaluators. Finally, it is shown that the scheme is secure from analyzing the private key, plaintext sequence and encrypted sequence.

Single-photon heralded two-qubit unitary gates for pairs of nitrogen-vacancy centers in diamond

The implementation of a high-fidelity two-qubit quantum logic gate remains an outstanding challenge for isolated solid-state qubits such as nitrogen-vacancy (NV) centers in diamond. In this work, we show that by driving pairs of NV centers to undergo photon scattering processes that flip their qubit states simultaneously, we can achieve a unitary two-qubit gate conditioned upon a single photon-detection event. Further, by exploiting quantum interference between the optical transitions of the NV centers' electronic states, we realize the existence of two special drive frequencies: a ``magic'' point where the spin-preserving elastic scattering rates are suppressed and a ``balanced'' point where the state-flipping scattering rates are equal. We analyzed four different gate operation schemes that utilize these two special drive frequencies, and various combinations of polarizations in the drive and collection paths. Our theoretical and numerical calculations show that the gate fidelity can be as high as 98%. The proposed unitary gate, combined with available single-qubit unitary operations, forms a universal gate set for quantum computing.

Bidirectional quantum operation teleportation with different states

A new concept of bidirectional quantum operation teleportation (BQOT) is proposed, which is essentially a union of the idea of quantum operation teleportation (QOT) and bidirectional quantum teleportation (QT). This means that Alice can transmit an unknown single-qubit unitary operation [Formula: see text] on the remote Bob’s quantum system; and at the same time, Bob can also transmit an arbitrary single-qubit unitary operation [Formula: see text] on Alice’s quantum system. In this paper, we present three BQOT schemes via different quantum channels. Furthermore, using these quantum channels, we investigate the BQOT in noisy environments, such as phase-damping noise and amplitude-damping noise. We considered the influence of the noises on the process of these three BQOT protocols through analytical derivation of the fidelity. Moreover, these three schemes are amply compared with each other from five aspects, i.e. quantum resource consumption, operation complexity, classical resource consumption, success probability and efficiency. It is found that these schemes can be realized deterministically and the first scheme is better than the other two schemes.

Multi-party quantum key agreement with five-qubit brown states

In this paper, we propose a multi-party quantum key agreement protocol with five-qubit brown states and single-qubit measurements. Our multi-party protocol ensures each participant to contribute equally to the agreement key. Each party performs three single-qubit unitary operations on three qubits of each brown state. Finally, by measuring brown states and decoding the measurement results, all participants can negotiate a shared secret key without classical bits exchange between them. With the analysis of security, our protocol demonstrates that it can resist against both outsider and participant attacks. Compared with other schemes, it also possesses a higher information efficiency. In terms of physical operation, it requires single-qubit measurements only which weakens the hardware requirements of participant and has a better operating flexibility.

A note on cohering power and de-cohering power

Cohering power and de-cohering power have recently been proposed to quantify the ability of a quantum operation to produce and erase coherence respectively. In this paper, we investigate the properties of cohering power and de-cohering power. First, we prove the equivalence between two different kinds of cohering power for any quantum operation on single qubit systems, which implies that l1 norm of coherence is monotone under Maximally incoherent operation (MIO) and Dephasing-covariant operation (DIO) in 2-dimensional space. In higher dimensions, however, we show that the monotonicity under MIO or DIO does not hold. Besides, we compare the set of quantum operations with zero cohering power with Maximally incoherent operation (MIO) and Incoherent operation (IO). Moreover, two different types of de-cohering power are defined and we find that they are not equal in single qubit systems. Finally, we make a comparison between cohering power and de-cohering power for single qubit unitary operations and show that cohering power is always larger than de-cohering power.

Bidirectional Quantum Teleportation of a Class of n-Qubit States by Using (2n + 2)-Qubit Entangled States as Quantum Channel

A new protocol of bidirectional quantum teleportation (BQT) is proposed in which the users can transmit a class of n-qubit state to each other simultaneously, by using (2n + 2)-qubit entangled states as quantum channel. The state of the art approaches can only transmit two-qubit states in each round. This scheme is based on control-not operation, single-qubit measurements and appropriate single-qubit unitary operations. It is shown that the protocol is secure in preparation phase.

Bidirectional Teleportation of a Two-Qubit State by Using Eight-Qubit Entangled State as a Quantum Channel

In this paper, a new scheme of bidirectional quantum teleportation (BQT) making use of an eight-qubit entangled state as the quantum channel is presented. This scheme is the first protocol without controller by which the users can teleport an arbitrary two-qubit state to each other simultaneously. This protocol is based on the ControlledNOT operation, appropriate single-qubit unitary operations and single-qubit measurement in the Z-basis and X-basis.

Probabilistic cloning of a single-atom state via cavity QED

We propose a scheme for probabilistically cloning a two-level state of an atom to a polarization photon via cavity QED system combined with linear optics elements. By choosing appropriate parameters, a controlled phase flip (CPF) gate between the atom and the probe photon is realized. Then we can judge that the cloning process should be continued (with the optimal probability) or interrupted by detecting the probe photon. If the cloning can be continued, the original atom state is deterministically cloned to the cloning photon by performing two more CPF gates and three single-qubit unitary operations. Otherwise, if the detection shows that the cloning should be interrupted, the cloning photon and the relevant operations are omitted.

Bidirectional Controlled Remote Implementation of an Arbitrary Single Qubit Unitary Operation with EPR and Cluster States

We present a scheme for bidirectional controlled implementation of arbitrary single qubit operations by using two EPR states and a five-qubit cluster state as the quantum channel. Alice can implement an unknown single-qubit unitary operation on the remote Bob’s quantum system and at the same time, Bob can also remotely implement an arbitrary single-qubit unitary operation on Alice’s quantum system if they cooperate with the controller. We also study the bidirectional controlled implementation of unitary operations belonging to the restricted sets. It is shown that receivers can prepare the target state by performing corresponding unitary operations on their entangled particles if they cooperate with the controller.

Revisiting “Quantum State Sharing of an Arbitrary Two-Atom State by Using a Six-Atom Cluster State in Cavity QED”

In Nie et al. (Int. J. Theor. Phys. 50: 2526, 2011), authors put forward a cavity QED scheme for deterministic quantum state sharing (QSTS) of an arbitrary two-atom state. They claimed that, the quantum channel of the QSTS scheme is a six-atom cluster state. After simple calculation, one can see that the quantum channel they used is a direct product of two three-atom GHZ states. In this paper, we propose a cavity QED scheme for QSTS of an arbitrary two-atom state via a six-atom cluster state channel. In our scheme, two two-atom Bell state measurements are transformed into the discrimination of single-atom product states. Moreover, the two-atom unitary operation is changed to single-qubit unitary operations. Our scheme is insensitive to the cavity decay. The necessary time for the scheme is much shorter than the Rydberg-atom lifespan, therefore atom decays do not need to be considered.

Single-qubit unitary gates by graph scattering

We consider the effects of plane-wave states scattering off finite graphs, as an approach to implementing single-qubit unitary operations within the continuous-time quantum walk framework of universal quantum computation. Four semi-infinite tails are attached at arbitrary points of a given graph, representing the input and output registers of a single qubit. For a range of momentum eigenstates, we enumerate all of the graphs with up to $n=9$ vertices for which the scattering implements a single-qubit gate. As $n$ increases, the number of new unitary operations increases exponentially, and for $n>6$ the majority correspond to rotations about axes distributed roughly uniformly across the Bloch sphere. Rotations by both rational and irrational multiples of $\pi$ are found.

Single-Photon Multiparty Quantum Cryptographic Protocols With Collective Detection

In order to reduce the requirements on quantum devices, Shih proposed two three-party quantum key distribution (QKD) protocols . However, Gao presented a simple and effective attack strategy (i.e., dense-coding attack) upon the two protocols . In this paper, we make a deep analysis on the reason why Shih three-party QKD protocols are not secure. Meanwhile, we improve the three-party QKD protocols so that they can resist the Gao attack. On this basis, we present a formalism of single-photon multiparty quantum cryptographic protocol with collective detection. In this formalism, single photons are employed and all participants, except one whom we call as the center, only need to have the ability to perform single qubit unitary operations. Considering the expensive quantum devices, this formalism is low-priced which means it is easy to realize. Further more, this formalism can be widely used in various quantum cryptographic protocols, such as quantum secret sharing, quantum coin flipping, and quantum secure direct communication.

QUANTUM STATE SPLITTING WITH YEO–CHUA GENUINE ENTANGLED STATE

Utilizing the four-qubit genuine entangled state presented by Yeo and Chua [Phys. Rev. Lett.96 (2006) 060502], we propose a tripartite quantum state splitting scheme for a sender to achieve the bipartition of his/her arbitrary two-qubit pure state between two sharers. During the scheme design, two novel and important ideas originated, respectively, from Phys. Rev. A74 (2006) 054303 and J. Phys. B41 (2008) 145506 are adopted to enhance the security and optimize resource consumption, operation complexity, and intrinsic efficiency. In the scheme, first the sender performs two Bell-state measurements and publishes the results. Afterwards, if and only if the two sharers cooperate together, they can perfectly restore the sender's quantum pure state by executing first a two-qubit collective unitary operation and then two single-qubit unitary operations.

Threshold Multiparty Controlled Teleportation of Arbitrary m-Qubit Quantum Information

We present a (t, n) threshold multiparty controlled quantum teleportation protocol of an arbitrary m-qubit quantum state between two remote parties. The unknown m-qubit quantum state can be recovered by the receiver under control of a subset of the n controllers if the number of the subset is larger than or equal to a threshold, say, t, but not for any t – 1 or fewer controllers. Our scheme seems to be more practical and more flexible than other existing protocols. The quantum resource required is just m Einstein–Podolsky–Rosen (EPR) pairs plus some single photons. The techniques required are only Bell state measurement, single-qubit unitary operation and von Neumann measurement. So our scheme is also feasible with present-day technique.