利用超导量子干涉仪制备N比特团簇态（英文）

利用两个超导量子干涉仪与腔场的相互作用,提出一种实现标准两比特量子相位门的方案。利用构造的两比特相位门,还提出了一种制备N比特团簇态的方案。在此方案中,量子信息被编码在两个超导量子干涉仪的相对稳定的基态上。在两个超导量子干涉仪与单模腔场的相互作用过程中,由于超导量子干涉比特的激发态被绝热地消去,激发态所引起的消相干得到了有效的抑制。此外,还讨论方案的实验可行性。     

超导非绝热几何量子计算及其优化控制

为实现量子门的高保真度和强鲁棒性,提出基于超导量子电路体系的非绝热几何量子计算方案.仅通过对超导比特施加含时共振微波驱动的方式,可以在超导比特上实现任意的单比特几何量子门.同时,在2个电容耦合的超导比特体系中,非平庸的2比特几何量子门也可以类似地实现.结果表明:提出的非绝热几何量子计算方案不仅对几何量子操作具有较好的鲁棒性,还可以与优化控制技术兼容,进一步增强量子门的鲁棒性.该方案的提出使容错固态量子计算的研究与发展向前迈出了重要的一步.     

腔中利用人工原子实现多量子比特相位门

在量子信息研究中,多量子比特相位门具有非常重要的意义。本文介绍了实现多量子比特相位门的两种模型,提出了一个利用腔中四能级超导人工原子实现多量子比特相位门的方案。该方案采用一个量子场和两个经典场控制各能级之间的跃迁,提高了多量子比特系统抵抗退相干的能力。     

超导电荷量子比特研究综述

随着量子计算机以及量子算法的提出，人们开始寻找可以实现量子计算机的真实物理体系。超导量子电路以其丰富的可设计性和优良的易集成性成为最有潜力实现量子计算机的人造量子体系。文章介绍了超导电荷量子比特的基本原理、超导电荷量子比特的耦合以及耗散和退相干问题，展望了超导电荷量子比特在量子计算和量子信息科学中的应用前景。     

最优1→3对称型经济实态量子克隆的固态电路实现方案

提出一种最优1→3对称型经济实态量子克隆的固态电路实现方案.在方案中,通过利用磁脉冲控制磁通量、电荷和穿过约瑟夫森结的相位差来实现最优1→3对称型经济实态量子克隆.经济实态量子克隆只需要对比特进行操作,不需要其它辅助操作,实验上简化了量子克隆的实现过程.由于超导比特具有耦合强度强,又没有非线性电感和能量的耗散等优点.相比于腔QED系统,固态系统在实验上便于集成和操作.     

通过双Raman作用在超导量子干涉器件中实现多比特GHZ态

在腔中通过双Raman作用,在超导量子干涉器件中实现多比特GHZ（Greenberger-Horne-Zeilinger）态的制备.在制备过程中,由于腔场只是被虚激发的,所以腔模的衰减可以忽略.GHZ态的实现只用到了超导系统的两个基态,有效地避免了超导系统激发态的弛豫.     

利用耦合超导量子比特实现受控U门

运用两量子比特非局域操作的几何表示理论,提出了利用射频脉冲作用下的耦合超导量子比特构建受控逻辑门(受控U门)的一个理论方案,并进一步推导出在电容耦合和自感耦合系统中构建受控U门时,其哈密顿量中的拉比频率所需要满足的条件.最后通过两量子比特控制相位门的实现说明该方案的可行性.     

超导量子比特中的Landau-Zener-Stckelberg干涉

超导量子比特是人工固态量子系统,在低温和弱耗散条件下,具有量子化的能级结构.这些能级之间的间隔可以随外加偏置电磁场连续变化,不同能级在某些偏置场可能形成量子系统中所特有的免交叉.最近研究成果表明,利用这些免交叉,可以在固态量子比特中实现Landau-Zener-Stckelberg（LZS）干涉,为测量系统的能谱、表征系统与环境的耦合、实现量子逻辑门、产生量子纠缠等提供了一个便捷的新手段.     

二项式光场与负二项式光场对超导量子比特读出电流的影响

研究了二项式态光场和负二项式态光场与耦合超导量子比特相互作用下的超导电流的时间演化规律.研究结果显示:在确定的二项式态或负二项式态光场中,耦合超导量子比特的初始量子态对穿过自身的超导电流的动力学行为几乎没有影响;但光场的量子态,尤其是光场参数对超导电流的动力学行为具有调控作用.因此,通过测量超导量子比特的输出电流,能够探测量子光场的统计性质.     

基于腔场中氮空穴色心的几何量子计算（英文）

利用纯几何操作,提出腔场中的氮空穴色心作为量子比特实现几何量子计算的方案.在暗态子空间中,选取合适的闭合演化回路,可以构造出单比特和双比特控制相位门.因此,该文提出了一种很有前景的实现容错量子计算的方案.     

Coherent quantum state storage and transfer between two phase qubits via a resonant cavity

As with classical information processing, a quantum information processor requires bits (qubits) that can be independently addressed and read out, long-term memory elements to store arbitrary quantum states, and the ability to transfer quantum information through a coherent communication bus accessible to a large number of qubits. Superconducting qubits made with scalable microfabrication techniques are a promising candidate for the realization of a large-scale quantum information processor. Although these systems have successfully passed tests of coherent coupling for up to four qubits, communication of individual quantum states between superconducting qubits via a quantum bus has not yet been realized. Here, we perform an experiment demonstrating the ability to coherently transfer quantum states between two superconducting Josephson phase qubits through a quantum bus. This quantum bus is a resonant cavity formed by an open-ended superconducting transmission line of length 7 mm. After preparing an initial quantum state with the first qubit, this quantum information is transferred and stored as a nonclassical photon state of the resonant cavity, then retrieved later by the second qubit connected to the opposite end of the cavity. Beyond simple state transfer, these results suggest that a high-quality-factor superconducting cavity could also function as a useful short-term memory element. The basic architecture presented here can be expanded, offering the possibility for the coherent interaction of a large number of superconducting qubits.     

Coherent dynamics of a flux qubit coupled to a harmonic oscillator

In the emerging field of quantum computation and quantum information, superconducting devices are promising candidates for the implementation of solid-state quantum bits (qubits). Single-qubit operations, direct coupling between two qubits and the realization of a quantum gate have been reported. However, complex manipulation of entangled states-such as the coupling of a two-level system to a quantum harmonic oscillator, as demonstrated in ion/atom-trap experiments and cavity quantum electrodynamics-has yet to be achieved for superconducting devices. Here we demonstrate entanglement between a superconducting flux qubit (a two-level system) and a superconducting quantum interference device (SQUID). The latter provides the measurement system for detecting the quantum states; it is also an effective inductance that, in parallel with an external shunt capacitance, acts as a harmonic oscillator. We achieve generation and control of the entangled state by performing microwave spectroscopy and detecting the resultant Rabi oscillations of the coupled system.     

Climbing the Jaynes-Cummings ladder and observing its nonlinearity in a cavity QED system

The field of cavity quantum electrodynamics (QED), traditionally studied in atomic systems, has gained new momentum by recent reports of quantum optical experiments with solid-state semiconducting and superconducting systems. In cavity QED, the observation of the vacuum Rabi mode splitting is used to investigate the nature of matter-light interaction at a quantum-mechanical level. However, this effect can, at least in principle, be explained classically as the normal mode splitting of two coupled linear oscillators. It has been suggested that an observation of the scaling of the resonant atom-photon coupling strength in the Jaynes-Cummings energy ladder with the square root of photon number n is sufficient to prove that the system is quantum mechanical in nature. Here we report a direct spectroscopic observation of this characteristic quantum nonlinearity. Measuring the photonic degree of freedom of the coupled system, our measurements provide unambiguous spectroscopic evidence for the quantum nature of the resonant atom-field interaction in cavity QED. We explore atom-photon superposition states involving up to two photons, using a spectroscopic pump and probe technique. The experiments have been performed in a circuit QED set-up, in which very strong coupling is realized by the large dipole coupling strength and the long coherence time of a superconducting qubit embedded in a high-quality on-chip microwave cavity. Circuit QED systems also provide a natural quantum interface between flying qubits (photons) and stationary qubits for applications in quantum information processing and communication.     

Coupling superconducting qubits via a cavity bus

Superconducting circuits are promising candidates for constructing quantum bits (qubits) in a quantum computer; single-qubit operations are now routine, and several examples of two-qubit interactions and gates have been demonstrated. These experiments show that two nearby qubits can be readily coupled with local interactions. Performing gate operations between an arbitrary pair of distant qubits is highly desirable for any quantum computer architecture, but has not yet been demonstrated. An efficient way to achieve this goal is to couple the qubits to a 'quantum bus', which distributes quantum information among the qubits. Here we show the implementation of such a quantum bus, using microwave photons confined in a transmission line cavity, to couple two superconducting qubits on opposite sides of a chip. The interaction is mediated by the exchange of virtual rather than real photons, avoiding cavity-induced loss. Using fast control of the qubits to switch the coupling effectively on and off, we demonstrate coherent transfer of quantum states between the qubits. The cavity is also used to perform multiplexed control and measurement of the qubit states. This approach can be expanded to more than two qubits, and is an attractive architecture for quantum information processing on a chip.     

利用绝热方法制备类W纠缠态

提出一个利用绝热方法一步制备三比特类W纠缠态的简易方案。相比其他方案,该方案最突出优势在于没有应用任何带来相位影响的操作,这更利于实验的实现。在目前的实验技术条件下,该方案是可行的。     

多个超导磁通量子比特的可控耦合

通过在两个或者三个超导磁通量子比特中插入一个耦合线圈，使得超导磁通量子比特与线圈有一个强的相互作用。耦合线圈的磁通直接影响两个超导磁通量子比特的耦合强度J，因此可以通过对线圈磁通的控制来调控超导磁通量子比特的耦合强度J。     

Quantum oscillations in two coupled charge qubits

A practical quantum computer, if built, would consist of a set of coupled two-level quantum systems (qubits). Among the variety of qubits implemented, solid-state qubits are of particular interest because of their potential suitability for integrated devices. A variety of qubits based on Josephson junctions have been implemented; these exploit the coherence of Cooper-pair tunnelling in the superconducting state. Despite apparent progress in the implementation of individual solid-state qubits, there have been no experimental reports of multiple qubit gates--a basic requirement for building a real quantum computer. Here we demonstrate a Josephson circuit consisting of two coupled charge qubits. Using a pulse technique, we coherently mix quantum states and observe quantum oscillations, the spectrum of which reflects interaction between the qubits. Our results demonstrate the feasibility of coupling multiple solid-state qubits, and indicate the existence of entangled two-qubit states.     

基于STQ系统的一个四比特簇态制备方案

Transmon比特通过电容与一个超导TLR(transmission line resonator)耦合。本文采用Transmon比特与TLR之间有更强的耦合常量,提出了一个在STQ(superconducting transmon qubit)系统中制备四比特纠缠簇态(cluster state)的简单方案。与已有的方案相比,此方案有更长的消相干时间。又由于Transmon比特和TLR有以上的属性,此方案在实验上更可行。     

Coherent coupling of a superconducting flux qubit to an electron spin ensemble in diamond

During the past decade, research into superconducting quantum bits (qubits) based on Josephson junctions has made rapid progress. Many foundational experiments have been performed, and superconducting qubits are now considered one of the most promising systems for quantum information processing. However, the experimentally reported coherence times are likely to be insufficient for future large-scale quantum computation. A natural solution to this problem is a dedicated engineered quantum memory based on atomic and molecular systems. The question of whether coherent quantum coupling is possible between such natural systems and a single macroscopic artificial atom has attracted considerable attention since the first demonstration of macroscopic quantum coherence in Josephson junction circuits. Here we report evidence of coherent strong coupling between a single macroscopic superconducting artificial atom (a flux qubit) and an ensemble of electron spins in the form of nitrogen-vacancy colour centres in diamond. Furthermore, we have observed coherent exchange of a single quantum of energy between a flux qubit and a macroscopic ensemble consisting of about 3?×?10(7) such colour centres. This provides a foundation for future quantum memories and hybrid devices coupling microwave and optical systems.     

利用超导量子电路中的宏观量子相干性测量弱磁场

超导量子电路可以被等效地看作人造原子,其内部能级结构随外磁场而变化.这里根据这些超导量子电路中的宏观量子相干性,设计了一种新型的磁强计.这种磁强计不仅体积小,而且具有低耗散和高灵敏度的优点.正因为此,它在读取诸如超导磁通量子比特等量子纳米设备方面,具有很大的应用前景.