Programmable quantum processor implemented with superconducting circuit

A quantum processor might execute certain computational tasks exponentially faster than a classical processor. Here, using superconducting quantum circuits we design a powerful universal quantum processor with the structure of symmetric all-to-all capacitive connection. We present the Hamiltonian and use it to demonstrate a full set of qubit operations needed in the programmable universal quantum computations. With the device the unwanted crosstalk and ZZ-type couplings between qubits can be effectively suppressed by tuning gate voltages, and the design allows efficient and high-quality couplings of qubits. Within available technology,the scheme may enable a practical programmable universal quantum computer.     

高维辅助的普适量子线路优化

受到Lanyon等(Lanyon B P et al 2008 Nature Physics. 5 134)利用高维Hilbert空间成功简化Toffoli门的启发, 本文将辅助维度应用到普适量子线路中, 结合Cosine-Sine Decomposition(CSD), Quantum Shannon Decomposition(QSD)等矩阵分解方法, 优化了两比特和三比特普适幺正量子线路, 给出了计算n比特普适量子线路复杂度的公式, 并利用线性光学和腔QED系统设计了实验方案. 结果表明, 两比特和三比特量子线路的复杂度已分别接近和优于目前最优结果, 且随着比特数的增加, 本方案的优势愈加明显.     

Robust Quantum Computing in Decoherence-Free Subspaces with Double-Dot Spin Qubits

We present a scheme for implementing robust quantum gates in decoherence-free subspaces （DFSs） with double-dot spin qubits. Through the resonator-assisted interaction, the controllable interqubit couplings can be achieved only by adjusting the qubit transition frequencies. We construct a set of logic gates on the DFS-encoded qubits to eliminate the collective noise effects, and thus the gate fidelities can be enhanced remarkably. This proposal may offer a potential approach to realize the robust quantum computing with spin qubits.     

Generation of an N-qubit phase gate via atom--cavity nonidentical coupling

A scheme for approximate generation of an N-qubit phase gate is proposed in cavity QED based on nonidentical coupling between the atoms and the cavity. The atoms interact with a highly detuned cavity-field mode, but quantum information does not transfer between the atoms and cavity field, and thus the cavity decay is negligible. The gate time does not rise with an increase in the number of qubits. With the choice of a smaller odd number l （related to atom-cavity coupling constants）, the phase gate can be generated with a higher fidelity and a higher success probability in a shorter time （the gate time is much shorter than the atomic radiative lifetime and photon lifetime）. When the number of qubits N exceeds certain small values, the fidelity and success probability rise slowly with an increase in the number of qubits N. When N→∞, the fidelity and success probability infinitely approach 1, but never exceed 1.     

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.     

Polarization-Dependent Mode Analyzer and Controlled-Z Gate in Ti:LiNbO3 Quantum Photonic Circuits

Abstract

On-chip integrated photonic circuits are crucial for further progress toward quantum technologies and in the science of quantum optics. The quantum controlled-Z gate is an example of the maximally entangling gate, which is universal for quantum computing when coupled with single-qubit gates. This article demonstrates a deterministic controlled-Z photonic quantum gate based on titanium in-diffused channel waveguides in which polarization and modal degrees of freedom of a single photon are used for encoding the control and target qubits, respectively.     

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).     

Quantum computation and simulation with superconducting qubits

Superconducting circuits based on Josephson junctions are regarded as one of the most promising technologies for the implementation of scalable quantum computers. This review presents the basic principles of superconducting qubits and shows the progress of quantum computing and quantum simulation based on superconducting qubits in recent years.The experimental realization of gate operations, readout, error correction codes, as well as some quantum algorithms are summarized, followed by an introduction of quantum simulation. And then some important applications in fields including condensed matter physics, quantum annealing, and quantum chemistry are discussed.     

Superconducting quantum bits

Superconducting quantum bits(qubits) and circuits are the leading candidate for the implementation of solid-state quantum computation. They have also been widely used in a variety of studies of quantum physics, atomic physics, quantum optics, and quantum simulation. In this article, we will present an overview of the basic principles of the superconducting qubits, including the phase, flux, charge, and transmon(Xmon) qubits, and the progress achieved so far concerning the improvements of the device design and quantum coherence property. Experimental studies in various research fields using the superconducting qubits and circuits will be briefly reviewed.     

Demonstration of local expansion toward large-scale entangled webs

We demonstrate an optical gate that increases the size of polarization-based W states by accessing only one of the qubits. Using this gate, we have generated three-photon and four-photon W states with fidelities 0.836 ± 0.042 and 0.784 ± 0.028, respectively. We also confirmed the existence of pairwise entanglement in every pair of qubits, including the one that was left untouched by the gate. The gate is applicable to any size of W states and hence is a universal tool for expanding entanglement.     

Quantum information processing with fiber optics: Quantum Fourier transform of 1024 qubits

Among a number of candidates, photons have advantages for implementing qubits: very weak coupling to the environment, the existing single photon measurement technique, and so on. Moreover, commercially available fiber-optic devices enable us to construct quantum circuits that consist of one-qubit operations (including classically controlled gates). Fiber optics resolves the mode matching problems in conventional optics and provides mechanically stable optical circuits. A quantum Fourier transform (QFT) followed by measurement was demonstrated with a simple circuit based on fiber optics. The circuit was shown to be robust against imperfections in the rotation gate. The error probability was estimated to be 0.01 per qubit, which corresponded to error-free operation for 100 qubits. The error probability can be further reduced to achieve successful QFT of 1024 qubits by taking the majority of the accumulated results. As is well known, QFT is a key function in quantum computations such as the final part of Shor’s factorization algorithm. The present QFT circuit, in combination with controlled unitary gates, would make possible practical quantum computers. Possible schemes of realizing quantum computers in this line are explored.     

Implementation of the Fredkin gate with a three-qubit mixed-spin Heisenberg model

We show that a local unitary(LU) equivalent Fredkin gate can be obtained from the free evolution of three mixed-spin qubits by virtue of numerical simulation with only one step.The spin-1 qubit acts as the control qubit,and two spin-1/2 qubits,which interact with the spin-1 qubit via the first neighbor spin interaction,respectively,play the role of target qubits.We also examine the imperfect Fredkin gate operation by considering the effects of nonidentical coupling constants,uniform and inhomogeneous magnetic fields.     

Suppressing decoherence of quantum algorithms by jump codes

The stabilizing properties of one-error correcting jump codes are explored under the realistic non-ideal condition of finite recovery times. For this purpose the quantum algorithm of the tent-map is decomposed into a universal set of Hamiltonian quantum gates which ensure perfect correction of spontaneous decay processes under ideal circumstances even if they occur during a gate operation. An entanglement gate is presented which is capable of entangling any two logical qubits of different one-error correcting code spaces. With the help of this gate simultaneous spontaneous decay processes affecting physical qubits of different code spaces can be corrected and decoherence can be suppressed significantly.     

Consideration for Affects of an XOR in a Random Number Generator Using Ring Oscillators

A cloud service to offer entropy has been paid much attention to. As one of the entropy sources, a physical random number generator is used as a true random number generator, relying on its irreproducibility. This paper focuses on a physical random number generator using a field-programmable gate array as an entropy source by employing ring oscillator circuits as a representative true random number generator. This paper investigates the effects of an XOR gate in the oscillation circuit by observing the output signal period. It aims to reveal the relationship between inputs and the output through the XOR gate in the target generator. The authors conduct two experiments to consider the relevance. It is confirmed that combining two ring oscillators with an XOR gate increases the complexity of the output cycle. In addition, verification using state transitions showed that the probability of the state transitions was evenly distributed by increasing the number of ring oscillator circuits.     

Observation of high coherence in Josephson junction qubits measured in a three-dimensional circuit QED architecture

Superconducting quantum circuits based on Josephson junctions have made rapid progress in demonstrating quantum behavior and scalability. However, the future prospects ultimately depend upon the intrinsic coherence of Josephson junctions, and whether superconducting qubits can be adequately isolated from their environment. We introduce a new architecture for superconducting quantum circuits employing a three-dimensional resonator that suppresses qubit decoherence while maintaining sufficient coupling to the control signal. With the new architecture, we demonstrate that Josephson junction qubits are highly coherent, with T2 ～ 10 to 20 μs without the use of spin echo, and highly stable, showing no evidence for 1/f critical current noise. These results suggest that the overall quality of Josephson junctions in these qubits will allow error rates of a few 10(-4), approaching the error correction threshold.     

Classical simulation of limited-width cluster-state quantum computation

We present a classical protocol, using the matrix product-state representation, to simulate cluster-state quantum computation at a cost polynomial in the number of qubits in the cluster and exponential in d---the width of the cluster. We use this result to show that any log-depth quantum computation in the gate array model, with gates linking only nearby qubits, can be simulated efficiently on a classical computer.     

Distributed quantum computation with superconducting qubit via LC circuit using dressed states

A scheme is proposed where two superconducting qubits driven by a classical field interacting separately with two distant LC circuits connected by another LC circuit through mutual inductance,are used for implementing quantum gates.By using dressed states,quantum state transfer and quantum entangling gate can be implemented.With the help of the time-dependent electromagnetic field,any two dressed qubits can be selectively coupled to the data bus (the last LC circuit),then quantum state can be transferred from one dressed qubit to another and multi-mode entangled state can also be formed.As a result,the promising perspectives for quantum information processing of mesoscopic superconducting qubits are obtained and the distributed and scalable quantum computation can be implemented in this scheme.     

Implementing a Multi-Qubit Quantum Phase Gate Encoded by Photonic Qubit

A protocol is proposed to implement a three-qubit phase gate for photonic qubits in a three-mode cavity. The idea can be extended to directly implement a N-qubit phase gate. We also show that the interaction time remains unchanged with the increasing number of qubits. In addition, the influence of cavity decay and atomic spontaneous emission on the gate fidelity and photon loss probability is also discussed by numerical calculation.     

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.     

Efficient scheme for realizing a multiplex-controlled phase gate with photonic qubits in circuit quantum electrodynamics

We propose an efficient scheme to implement a multiplex-controlled phase gate with multiple photonic qubits simultaneously controlling one target photonic qubit based on circuit quantum electrodynamics (QED). For convenience, we denote this multiqubit gate as MCP gate. The gate is realized by using a two-level coupler to couple multiple cavities. The coupler here is a superconducting qubit. This scheme is simple because the gate implementation requires only one step of operation. In addition, this scheme is quite general because the two logic states of each photonic qubit can be encoded with a vacuum state and an arbitrary non-vacuum state |φ> (e.g., a Fock state, a superposition of Fock states, a cat state, or a coherent state, etc.) which is orthogonal or quasi-orthogonal to the vacuum state. The scheme has some additional advantages: because only two levels of the coupler are used, i.e., no auxiliary levels are utilized, decoherence from higher energy levels of the coupler is avoided; the gate operation time does not depend on the number of qubits; and the gate is implemented deterministically because no measurement is applied. As an example, we numerically analyze the circuit-QED based experimental feasibility of implementing a three-qubit MCP gate with photonic qubits each encoded via a vacuum state and a cat state. The scheme can be applied to accomplish the same task in a wide range of physical system, which consists of multiple microwave or optical cavities coupled to a two-level coupler such as a natural or artificial atom.