Quantum state discrimination and enhancement by noise

Discrimination between two quantum states is addressed as a quantum detection process where a measurement with two outcomes is performed and a conclusive binary decision results about the state. The performance is assessed by the overall probability of decision error. Based on the theory of quantum detection, the optimal measurement and its performance are exhibited in general conditions. An application is realized on the qubit, for which generic models of quantum noise can be investigated for their impact on state discrimination from a noisy qubit. The quantum noise acts through random application of Pauli operators on the qubit prior to its measurement. For discrimination from a noisy qubit, various situations are exhibited where reinforcement of the action of the quantum noise can be associated with enhanced performance. Such implications of the quantum noise are analyzed and interpreted in relation to stochastic resonance and enhancement by noise in information processing.     

基于六光子量子避错码的量子密钥分发方案

量子信道中不可避免存在的噪声将扭曲被传输的信息,对通信造成危害。目前克服量子信道噪声的较好方案是量子避错码（QEAC）。将量子避错码思想用于量子密钥分发,能有效克服信道中的噪声,且无需复杂的系统。用六光子构造了量子避错码,提出了一种丛于六光子避错码的量子密钥分发（QDK）方案。以提高量子密钥分发的量子比特效率和安全性为前提,对六光子避错码的所有可能态进行组合,得到一种六光子避错码的最优组合方法,可将两比特信息编码在一个态中,根据测肇结果和分组信息进行解码,得到正确信息的平均概率为7／16。与最近的基于四光子避错码的克服量子信道噪声的量子密钥分发方案相比,该方案的量子比特效率提高了16．67％,密钥分发安全性足它的3．5倍。     

Remote interactions on two distributed quantum systems： nonlocal unambiguous quantum-state discrimination

Remote quantum-state discrimination is a critical step for the implementation of quantum communication network and distributed quantum computation. We present a protocol for remotely implementing the unambiguous discrimination between nonorthogonal states using quantum entanglements, local operations, and classical communications. This protocol consists of a remote generalized measurement described by a positive operator valued measurement （POVM）. We explicitly construct the required remote POVM. The remote POVM can be realized by performing a nonlocal controlled-rotation operation on two spatially separated qubits, one is an ancillary qubit and the other is the qubit which is encoded by two nonorthogonal states to be distinguished, and a conventional local Von Neumann orthogonal measurement on the ancilla. The particular pair of states that can be remotely and unambiguously distinguished is specified by the state of the ancilla. The probability of successful discrimination is not optimal for all admissible pairs. However, for some subset it can be very close to an optimal value in an ordinary local POVM.     

Effects of Quantum Noise on Quantum Clock Synchronization

In laboratory environment, the channel apparatus will generate particular dominant quantum noise. The noise then will give rise to some errors during synchronization. In this work, the accuracies of one qubit transport protocol and entangled states transport protocol in the presence of noise have been studied. With the help of three important and familiar noise models, the quantum noise will degrade the accuracy has been proved. Due to the influence of quantum noise, the accuracy of entangled qubits decrease faster than that of one qubit. The entangled states will improve the accuracy in noise-free channel, and will degrade the accuracy in noise channel.     

Classical and quantum communication without a shared reference frame

We show that communication without a shared reference frame is possible using entangled states. Both classical and quantum information can be communicated with perfect fidelity without a shared reference frame at a rate that asymptotically approaches one classical bit or one encoded qubit per transmitted qubit. We present an optical scheme to communicate classical bits without a shared reference frame using entangled photon pairs and linear optical Bell state measurements.     

Improving Quantum Metrology via Purifying and Distilling Noisy Probe States

In quantum metrology, the precision of unknown parameter estimation is studied in the quantum regime, and the choice of the probe state plays an important role in determining the precision of the parameter to be estimated. The quality of quantum metrology will be reduced in the presence of quantum noise during the memory time of probe states after preparation. Meanwhile the noisy probe state can be manipulated by different protocols such as single‐qubit purification, entanglement purification, and entanglement distillation etc. In this paper, the effects of these manipulations on the usefulness, that is, quantum Fisher information (QFI), of the noisy probe state in quantum metrology are studied. The results show that joint operations in single‐qubit purification and entanglement purification processes play positive roles in enhancing the QFI of the probe states, and local measurements in entanglement purification and entanglement distillation processes play both positive and negative roles in enhancing the QFI of the probe states. In this sense, single‐qubit purification will always be helpful in parameter estimation by using single qubits as probe, and entanglement purification process maybe more suitable for improving the estimation precision when entangled‐state probe is adopted.     

Fault tolerant channel-encrypting quantum dialogue against collective noise

In this paper,two fault tolerant channel-encrypting quantum dialogue(QD)protocols against collective noise are presented.One is against collective-dephasing noise,while the other is against collective-rotation noise.The decoherent-free states,each of which is composed of two physical qubits,act as traveling states combating collective noise.Einstein-Podolsky-Rosen pairs,which play the role of private quantum key,are securely shared between two participants over a collective-noise channel in advance.Through encryption and decryption with private quantum key,the initial state of each traveling two-photon logical qubit is privately shared between two participants.Due to quantum encryption sharing of the initial state of each traveling logical qubit,the issue of information leakage is overcome.The private quantum key can be repeatedly used after rotation as long as the rotation angle is properly chosen,making quantum resource economized.As a result,their information-theoretical efficiency is nearly up to 66.7%.The proposed QD protocols only need single-photon measurements rather than two-photon joint measurements for quantum measurements.Security analysis shows that an eavesdropper cannot obtain anything useful about secret messages during the dialogue process without being discovered.Furthermore,the proposed QD protocols can be implemented with current techniques in experiment.     

Quantum state manipulation of single-Cesium-atom qubit in a micro-optical trap

Based on single Cesium atoims trapped in a 1064 nm microscopic optical trap we have exhibited a single qubit encoded in the Cesium ＂clock states＂. The single qubit initialization, detection and the fast state rotation with high efficiencies are demonstrated and this state manipulation is crucial for quantmn information processing. The ground ~ates Rabi flopping rate of 229.0 ± 0.6 kHz is realized hy a two-photon Raman process. A clock states dephasing time of 3.0 ± 0.7 ms is measured, while all irreversible homogeneous dephasing time of 124 ± 17 ms is achieved by using the spin-echo technique. This well-controlled single atom provides an ideal quantmn qubit and quantmn node for quantum information processing.     

Quantum nondemolition measurements of a flux qubit coupled to a noisy detector

We theoretically study the quantum nondemolition measurements of a flux qubit coupled to a noisy superconduct-ing quantum interference device (SQUID).The obtained analytical results indicate that the measurement probability is frequency-dependent in a short time scale and has a close relationship with the measurement-induced dephasing.Furthermore,when the detuning between the driven and bare resonator equals the coupling strength,we can obtain the maximum measurement rate that is determined by the character of the noise in the SQUID.Finally,we analysed the mixed effect caused by coupling between the non-diagonal term and the external variable.It is found that the initial information of the qubit is destroyed due to quantum tunneling between the qubit states.     

All-optical generation of states for "Encoding a qubit in an oscillator"

Most quantum computation schemes propose encoding qubits in two-level systems. Others exploit the use of an infinite-dimensional system. In "Encoding a qubit in an oscillator" [Phys. Rev. A 64, 012310 (2001)], Gottesman, Kitaev, and Preskill (GKP) combined these approaches when they proposed a fault-tolerant quantum computation scheme in which a qubit is encoded in the continuous position and momentum degrees of freedom of an oscillator. One advantage of this scheme is that it can be performed by use of relatively simple linear optical devices, squeezing, and homodyne detection. However, we lack a practical method to prepare the initial GKP states. Here we propose the generation of an approximate GKP state by using superpositions of optical coherent states (sometimes called "Schr?dinger cat states"), squeezing, linear optical devices, and homodyne detection.     

Quantum Information Transfer from a Double Quantum Dot System to a Cavity Field＊

In this paper, we propose a scheme to realize quantum information transfer from a double quantum dot （DQD） system to a quantized cavity field. The DQD and the cavity field are treated as a two-state charge qubit and a continuous-variable system, respectively. It is shown that quantum information encoded in the two-state DQD system can be transferred to quantum states of the cavity field with a continuous-variable basis through appropriate projective measurements with respect to the DQD.     

Interaction-Induced Chiral Quantum States of the Ultracold Polar Molecules

The ultracold polar molecules with the tunable dipole-dipole interaction, not only would enable explorations of a large class of exotic many-body physics phenomena, but also could be used for quantum information processing. In the present paper we demonstrate that this dipole-dipole interaction can generate the degenerate chiral quantum states acting as a qubit robust against noise when the ultracold polar molecules are confined by a triangular lattice. Moreover, we also find two first-order quantum phase transitions by controlling an external driving field. One is the transition with the change of the different degenerate chiral quantum states. The other is the transition with the breaking of the degenerate quantum chiral states to the nondegenerate state. In experiment, these first-order quantum phase transitions can be detected by measuring the collective molecular population.     

Interaction-Induced Chiral Quantum States of the Ultracold Polar Molecules

The ultracold polar molecules with the tunable dipole-dipole interaction, not only would enable explorations of a large class of exotic many-body physics phenomena, but also could be used for quantum information processing. In the present paper we demonstrate that this dipole-dipole interaction can generate the degenerate chiral quantum states acting as a qubit robust against noise when the ultracold polar molecules are confined by a triangular lattice. Moreover, we also find two first-order quantum phase transitions by controlling an external driving field. One is the transition with the change of the different degenerate chiral quantum states. The other is the transition with the breaking of the degenerate quantum chiral states to the nondegenerate state. In experiment, these first-order quantum phase transitions can be detected by measuring the collective molecular population.     

Quantum Teleportation of Three and Four-Qubit State Using Multi-qubit Cluster States

We provide various schemes for quantum teleportation by using the four and five qubit cluster states. Explicit protocols for the perfect quantum teleportation of three and four qubit states are illustrated. It is found that the four-qubit cluster state can be used for perfect quantum teleportation of a special form of three-qubit state and the five-qubit cluster state can be used for perfect quantum teleportation of a special form of four-qubit state.     

Valley-based noise-resistant quantum computation using Si quantum dots

We devise a platform for noise-resistant quantum computing using the valley degree of freedom of Si quantum dots. The qubit is encoded in two polarized (1,1) spin-triplet states with different valley compositions in a double quantum dot, with a Zeeman field enabling unambiguous initialization. A top gate gives a difference in the valley splitting between the dots, allowing controllable interdot tunneling between opposite valley eigenstates, which enables one-qubit rotations. Two-qubit operations rely on a stripline resonator, and readout on charge sensing. Sensitivity to charge and spin fluctuations is determined by intervalley processes and is greatly reduced as compared to conventional spin and charge qubits. We describe a valley echo for further noise suppression.     

Rydberg quantum computation with nuclear spins in two-electron neutral atoms

Alkaline-earth-like (AEL) atoms with two valence electrons and a nonzero nuclear spin can be excited to Rydberg state for quantum computing. Typical AEL ground states possess no hyperfine splitting, but unfortunately a GHz-scale splitting seems necessary for Rydberg excitation. Though strong magnetic fields can induce a GHz-scale splitting, weak fields are desirable to avoid noise in experiments. Here, we provide two solutions to this outstanding challenge with realistic data of well-studied AEL isotopes. In the first theory, the two nuclear spin qubit states |0〉 and |1〉 are excited to Rydberg states |r〉 with detuning Δ and 0, respectively, where a MHz-scale detuning Δ arises from a weak magnetic field on the order of 1 G. With a proper ratio between Δ and Ω, the qubit state |1〉 can be fully excited to the Rydberg state while |0〉 remains there. In the second theory, we show that by choosing appropriate intermediate states a two-photon Rydberg excitation can proceed with only one nuclear spin qubit state. The second theory is applicable whatever the magnitude of the magnetic field is. These theories bring a versatile means for quantum computation by combining the broad applicability of Rydberg blockade and the incomparable advantages of nuclear-spin quantum memory in two-electron neutral atoms.     

The remote implementation of all possible generalized quantum measurement on single atomic qubit in a quantum network

To implement generalized quantum measurement (GQM) one has to extend the original Hilbert space. Generally speaking, the additional dimensions of the ancilla space increase as the number of the operators of the GQM n increases. This paper presents a scheme for deterministically implementing all possible n-operator GQMs on a single atomic qubit by using only one 2-dimensional ancillary atomic qubit repeatedly, which remarkably reduces the complexity of the realistic physical system. Here the qubit is encoded in the internal states of an atom trapped in an optical cavity and single-photon pulses are employed to provide the interaction between qubits. It shows that the scheme can be performed remotely, and thus it is suitable for implementing GQM in a quantum network. What is more, the number of the total ancilla dimensions in our scheme achieves the theoretic low bound.     

Preparation of Cluster States of Atomic Qubits in Cavity QED

We propose an optical scheme to generate cluster states of atomic qubits, with each trapped in separate optical cavity, via atom-cavity-laser interaction. The quantum information of each qubit is encoded on the degenerate ground states of the atom, hence the entanglement between them is relatively stable against spontaneous emission. A single-photon source and two classical fields are employed in the present scheme. By controlling the sequence and time of atom-cavity-laser interaction, we show that the atomic cluster states can be produced deterministically.     

Holographic quantum computing

We propose to use a single mesoscopic ensemble of trapped polar molecules for quantum computing. A "holographic quantum register" with hundreds of qubits is encoded in collective excitations with definite spatial phase variations. Each phase pattern is uniquely addressed by optical Raman processes with classical optical fields, while one- and two-qubit gates and qubit readout are accomplished by transferring the qubit states to a stripline microwave cavity field and a Cooper pair box where controllable two-level unitary dynamics and detection is governed by classical microwave fields.     

Harnessing Quantumness of States using Discrete Wigner Functions under (non)-Markovian Quantum Channels

The negativity of the discrete Wigner functions (DWFs) is a measure of non-classicality and is often used to quantify the degree of quantum coherence in a system. The study of Wigner negativity and its evolution under different quantum channels can provide insight into the stability and robustness of quantum states under their interaction with the environment, which is essential for developing practical quantum computing systems. The variation of DWF negativity of qubit, qutrit, and two-qubit systems under the action of (non)-Markovian random telegraph noise (RTN) and amplitude damping (AD) quantum channels is investigated. Different negative quantum states that can be used as a resource for quantum computation and quantum teleportation are constructed. The success of quantum computation and teleportation is estimated for these states under (non)-Markovian evolutions.