Unselective ground-state blockade of Rydberg atoms for implementing quantum gates

A dynamics regime of Rydberg atoms, unselective ground-state blockade (UGSB), is proposed in the context of Rydberg antiblockade (RAB), where the evolution of two atoms is suppressed when they populate in an identical ground state. UGSB is used to implement a SWAP gate in one step without individual addressing of atoms. Aiming at circumventing common issues in RAB-based gates including atomic decay, Doppler dephasing, and fluctuations in the interatomic coupling strength, we modify the RAB condition to achieve a dynamical SWAP gate whose robustness is much greater than that of the nonadiabatic holonomic one in the conventional RAB regime. In addition, on the basis of the proposed SWAP gates, we further investigate the implementation of a three-atom Fredkin gate by combining Rydberg blockade and RAB. The present work may facilitate to implement the RAB-based gates of strongly coupled atoms in experiment.     

Generation of Perfectly Entangled Two and Three Qubits States by Classical Random Interaction

This study examines the possibility of finding perfect entanglers for a Hamiltonian which corresponds to several quantum information platforms of interest at the present time. However, in this study, a superconducting circuit is used that stands out from other quantum-computing devices, especially because transmon qubits can be coupled via capacitors or microwave cavities, which enables to combine high coherence, fast gates, and high flexibility in its design parameters. There are currently two factors limiting the performance of superconducting processors: timing mismatch and the limitation of entangling gates to two qubits. In this work, a two-qubit SWAP and a three-qubit Fredkin gate is presented, additionally, a perfect adiabatic entanglement generation between two and three programmable superconducting qubits is also demonstrated. Furthermore, the impact of random dephasing, emission, and absorption noises on the quantum gates and entanglement is also demonstrated in this study. It is demonstrated by numerical simulation that CSWAP gate and W-state generation can be achieved perfectly in one step with high reliability under weak coupling conditions. Hence, this scheme could contribute to quantum teleportation, quantum communication, and some other areas of quantum information processing.     

Remote Implementation of a Fredkin Gate via Virtual Excitation of an Atom-Cavity-Fiber System

Simple operations and robust results are always of interest for any quantum tasks. Herein, a novel scheme is proposed for implementing a Fredkin gate via the virtual excitation of an atom-cavity-fiber system. The scheme is to control the nonlocal state-swap of two spatially separated target atoms according to the state of the control atom at hand. In the scheme, only the control atom at hand needs the laser to drive and the virtual excitation of the atom-cavity-fiber system effectively suppresses the decoherence. By numerical simulations, appreciated parameters are chosen and it is shown that the Fredkin gate can be implemented with high fidelity. Although the operation time error has slightly stronger influence on the fidelity than atom-cavity coupling strength error, the robustness of the scheme can be effectively improved against the operation time error by adopting Gaussian pulse to replace the constant pulse. In addition, the scheme can be generalized to implement alternative Fredkin gates by controlling the non-local state-swap of two remote atoms or of two remote and spatially separated atoms, which will be undoubtedly of benefit to the distributed quantum computation and remote quantum information processing.     

Heralded linear optical quantum Fredkin gate based on one auxiliary qubit and one single photon detector

Linear optical quantum Fredkin gate can be applied to quantum computing and quantum multi-user communication networks. In the existing linear optical scheme, two single photon detectors （SPDs） are used to herald the success of the quantum Fredkin gate while they have no photon count. But analysis results show that for non-perfect SPD, the lower the detector efficiency, the higher the heralded success rate by this scheme is. We propose an improved linear optical quantum Fredkin gate by designing a new heralding scheme with an auxiliary qubit and only one SPD, in which the higher the detection efficiency of the heralding detector, the higher the success rate of the gate is. The new heralding scheme can also work efficiently under a non-ideal single photon source. Based on this quantum Fredkin gate, large-scale quantum switching networks can be built. As an example, a quantum Bene~ network is shown in which only one SPD is used.     

Note on Implementation of Three-Qubit SWAP Gate

In this paper, the synthesis and implementation of three-qubit SWAP gate is discussed. The three-qubit SWAP gate can be decomposed into product of 2 two-qubit SWAP gates, and it can be realized by 6 CNOT gates. Research illustrated that although the result is very simple, the current methods of matrix decomposition for multi-qubit gate can not get that. Then the implementation of three-qubit SWAP gate in the three spin system with Ising interaction is investigated and the sequence of control pulse and drift process to implement the gate is given. It needs 23 control pulses and 12 drift processes. Since the interaction can not be switched on and off at will, the realization of three-qubit SWAP gate in specific quantum system also can not simply come down to 2 two-qubit SWAP gates.     

A scheme for realizing a distant two-qubit controlled-U gate with nearest qubit-qubit interaction

We propose a new scheme for realizing a distant two-qubit controlled-U gate with nearest qubit-qubit interaction. The present scheme does not need measurement. Furthermore, it is noted that the two-qubit CNOT gates required by the scheme are greatly reduced when compared with the conventional method based on SWAP operations. The scheme is useful in quantum computing with solid-state systems where only interaction between nearest systems is available.     

Shortcut-based quantum gates on superconducting qubits in circuit QED

Construction of optimal gate operations is significant for quantum computation. Here an efficient scheme is proposed for performing shortcut-based quantum gates on superconducting qubits in circuit quantum electrodynamics (QED). Two four-level artificial atoms of Cooper-pair box circuits, having sufficient level anharmonicity, are placed in a common quantized field of circuit QED and are driven by individual classical microwaves. Without the effect of cross resonance, one-qubit NOT gate and phase gate in a decoupled atom can be implemented using the invariant-based shortcuts to adiabaticity. With the assistance of cavity bus, a one-step SWAP gate can be obtained within a composite qubit-photon-qubit system by inversely engineering the classical drivings. We further consider the gate realizations by adjusting the microwave fields. With the accessible decoherence rates, the shortcut-based gates have high fidelities. The present strategy could offer a promising route towards fast and robust quantum computation with superconducting circuits experimentally.     

构造Fredkin量子门的一种简易方法

在Ｄａｒｅｎｃｏ的工作基础上，提出用基本的两位量子门完成Ｆｒｅｄｋｉｎ门的功能．与Ｃｈａｕ的方案比较，该方案结构简单、便于理解，并且实现的可能性大． 关键词：     

Quantum logic gates operation using SQUID qubits in bimodal cavity

We present a scheme to realize the basic two-qubit logic gates such as the quantum phase gate and SWAP gate using a detuned microwave cavity interacting with three-level superconducting-quantum-interference-device (SQUID) qubit(s), by placing SQUID(s) in a two-mode microwave cavity and using adiabatic passage methods. In this scheme, the two logical states of the qubit are represented by the two lowest levels of the SQUID, and the cavity fields are treated as quantized. Compared with the previous method, the complex procedures of adjusting the level spacing of the SQUID and applying the resonant microwave pulse to the SQUID to create transformation are not required. Based on superconducting device with relatively long decoherence time and simplified operation procedure, the gates operate at a high speed, which is important in view of decoherence.     

量子控制交换门的光学实现

提出一种量子控制交换门（Fredkin gate）的光学实现方案.在方案中,利用光子与辅助相干光之间的交叉Kerr效应来实现一定的光子路径的控制,进而实现量子控制交换门.这一方案只需要一个相干光脉冲的辅助,并且不需要任何的单光子探测器和符合测量技术,因而这一量子门是可扩展的量子门操作.同时方案的概率可以达到1/133,并在目前的实验条件下是可行的. 关键词： 控制交换门 交叉Kerr效应     

Quantum SWAP gate with atomic ensembles in two distant cavities

A scheme is presented to realize remote quantum SWAP gate with atomic ensembles. During the process, the fiber mode is only virtually excited. The quantum information is encoded in two degenerate ground states, so the atoms' spontaneous emission can be omitted approximately. Moreover, the time needed to complete the gate is proportional to the inverse of the number of atoms and thus the gate is greatly speeded up as the number of atoms increases.     

飞秒激光直写光量子逻辑门

量子比特在同一时刻可处于所有可能状态上的叠加特性使得量子计算机具有天然的并行计算能力,在处理某些特定问题时具有超越经典计算机的明显优势.飞秒激光直写技术因其具有单步骤高效加工真三维光波导回路的能力,在制备通用型集成光量子计算机的基本单元—量子逻辑门中发挥着越来越重要的作用.本文综述了飞秒激光直写由定向耦合器构成的光量子比特逻辑门的进展.主要包括定向耦合器的功能、构成、直写和性能表征,集成波片、哈达玛门和泡利交换门等单量子比特逻辑门、受控非门和受控相位门等两量子比特逻辑门的直写加工,并对飞秒激光加工三量子比特逻辑门进行了展望.     

激光选态激发的量子逻辑功能

基于三模激光与三能级分子相互作用时态的布居依赖于激光的位相和振幅,提出利用激光选态激发,实现Ｆｒｅｄｋｉｎ量子逻辑门的功能。     

Simultaneous implementation of n SWAP gates using superconducting charge qubits coupled to a cavity

Based on superconducting quantum interference devices (SQUIDs) coupled to a cavity, we propose a scheme for implementing n SWAP gates simultaneously. In our scheme, the SQUID works in the charge regime, the quantum logic gate operations are performed in the subspace spanned by two charge states |0〉 and |1〉. The interaction between the qubits and the cavity field can be achieved by turning the gate voltage and the external flux. Especially, the gate operation time is independent of the number of the qubits, and the gate operation is insensitive to the initial state of the cavity mode. We also analyze the experimental feasibility that the conditions of the large detuning can be achieved by adjusting the frequency of the cavity mode, and the operation time satisfies the requirement for the designed experiment by choosing suitable detuning and the quality factor of the cavity. Based on the simple operation, our scheme may be realized in this solid-state system, and our idea may be realized in other systems.     

Scheme for implementing quantum logic gates for two atomstrapped in different cavities

A scheme is presented for realizing quantum logic gates for two atoms localized in two distant optical cavities. Our scheme works in a regime in which the atom--cavity coupling strength is smaller than the cavity decay rate. Thus the requirement on the quality factor of the cavities is greatly relaxed. Furthermore, the fidelity of our scheme is not affected by detection inefficiency and atomic decay. These advantages are important in view of experiment.     

激光对分子振动态的控制与量子Fredkin逻辑门

基于激光相干合成分子的局域模振动态,提出利用激光对分子振动态的控制实现量子Fredkin逻辑门的功能,并对方案的特点及可行性进行分析.     

Universal quantum control based on parametric modulation in superconducting circuits

As superconducting quantum circuits are scaling up rapidly towards the noisy intermediate-scale quantum(NISQ)era, the demand for electronic control equipment has increased significantly. To fully control a quantum chip of N qubits,the common method based on up-conversion technology costs at least 2 × N digital-to-analog converters(DACs) and N IQ mixers. The expenses and complicate mixer calibration have become a hinderance for intermediate-scale quantum control.Here we propose a universal control scheme for superconducting circuits, fully based on parametric modulation. To control N qubits on a chip, our scheme only requires N DACs and no IQ mixer, which significantly reduces the expenses. One key idea in the control scheme is to introduce a global pump signal for single-qubit gates. We theoretically explain how the universal gates are constructed using parametric modulation. The fidelity analysis shows that parametric single-qubit(two-qubit) gates in the proposed scheme can achieve low error rates of 10~(4), with a gate time of about 60 ns(100 ns).     

Three‐Photon Polarization‐Spatial Hyperparallel Quantum Fredkin Gate Assisted by Diamond Nitrogen Vacancy Center in Optical Cavity

The construction of a near‐deterministic photonic hyperparallel quantum Fredkin (hyper‐Fredkin) gate is investigated for a three‐photon system with the optical property of a diamond nitrogen vacancy center embedded in an optical cavity (cavity‐NV center system). This hyper‐Fredkin gate can be used to perform double Fredkin gate operations on both the polarization and spatial‐mode degrees of freedom (DOFs) of a three‐photon system with a near‐unit success probability, compared with those on the double three‐photon systems in one DOF. In this proposal, the hybrid quantum logic gate operations are the key elements of the hyper‐Fredkin gate, and only two cavity‐NV center systems are required. Moreover, the possibility of constructing a high‐fidelity and high‐efficiency hyper‐Fredkin gate in the experimental environment of a cavity‐NV center system is discussed, which may be used to implement high‐fidelity photonic computational tasks in two DOFs with a high efficiency.     

Experimental realization of linear-optical partial swap gates

We present a linear-optical implementation of a class of two-qubit partial SWAP gates for polarization states of photons. Different gate operations, including the SWAP and entangling sqrt[SWAP], can be obtained by changing a classical control parameter, namely, the path difference in the interferometer. Reconstruction of output states, full quantum process tomography, and an evaluation of entanglement of formation prove very good performance of the gates.     

High‐Fidelity Hybrid Quantum Gates between a Flying Photon and Diamond Nitrogen‐Vacancy Centers Assisted by Low‐Q Single‐Sided Cavities

High‐fidelity universal quantum gates are crucial in quantum computing. Three high‐fidelity universal quantum gates, namely the hybrid controlled NOT gate, the hybrid Toffoli gate, and the hybrid Fredkin gate, on a flying photon qubit and diamond nitrogen‐vacancy (NV) centers, assisted by low‐Q single‐sided cavities, are presented. Errors due to the imperfection of the practical input–output process are detected to improve the fidelity of these quantum gates, which therefore relaxes the requirement on their implementation, since strong coupling is no longer mandatory. In addition, quantum gates have the advantage that they can work faithfully even when the resonant condition among the NV center, the photon, and the cavity is not strictly satisfied, or the NV centers are not identical. The performance and success probability of these quantum gates are analyzed, finding that these schemes are feasible with current technology.