Scalable quantum computing with Josephson charge qubits

A goal of quantum information technology is to control the quantum state of a system, including its preparation, manipulation, and measurement. However, scalability to many qubits and controlled con-nectivity between any selected qubits are two of the major stumbling blocks to achieve quantum com-puting (QC). Here we propose an experimental method, using Josephson charge qubits, to efficiently solve these two central problems. The proposed QC architecture is scalable since any two charge qubits can be effectively coupled by an experimentally accessible inductance. More importantly, we formulate an efficient and realizable QC scheme that requires only one (instead of two or more) two-bit operation to implement conditional gates.     

Speed optimized two-qubit gates with laser coherent control techniques for ion trap quantum computing

We propose a new concept for a two-qubit gate operating on a pair of trapped ions based on laser coherent control techniques. The gate is insensitive to the temperature of the ions, works also outside the Lamb-Dicke regime, requires no individual addressing by lasers, and can be orders of magnitude faster than the trap period, which is presently the speed limit of all two-qubit proposals.     

The ion trap quantum information processor

Received: 16 October 1996/Revised version: 2 April 1997     

Scaling ion trap quantum computation through fast quantum gates

We propose a method to achieve scalable quantum computation based on fast quantum gates on an array of trapped ions, without the requirement of ion shuttling. Conditional quantum gates are obtained for any neighboring ions through spin-dependent acceleration of the ions from periodic photon kicks. The gates are shown to be robust to influence all the other ions in the array and insensitive to the ions' temperature.     

Process tomography of ion trap quantum gates

A crucial building block for quantum information processing with trapped ions is a controlled-NOT quantum gate. In this Letter, two different sequences of laser pulses implementing such a gate operation are analyzed using quantum process tomography. Fidelities of up to 92.6(6)% are achieved for single-gate operations and up to 83.4(8)% for two concatenated gate operations. By process tomography we assess the performance of the gates for different experimental realizations and demonstrate the advantage of amplitude-shaped laser pulses over simple square pulses. We also investigate whether the performance of concatenated gates can be inferred from the analysis of the single gates.     

Scalable quantum computing with Josephson charge-phase qubits inside a cavity

This Letter proposes a theoretical scheme for scalable quantum computing with charge-phase qubits inside a common cavity. Individually addressing the applied gate pulses, we obtain the switchable interqubit couplings mediated by the cavity mode, from which a universal set of logic gates can be constructed. In our scheme the interqubit couplings are completely feasible to perform conditional gates, and the classical microwaves cause negligible leakage errors.     

Scalable quantum computing in decoherence-free subspaces with Cooper-pair box qubits

We theoretically propose a feasible scheme to perform quantum computing in decoherence-free subspaces (DFSs) with Cooper-pair box (CPB) qubits arrayed in a circuit QED architecture. Based on the cavity-bus assisted interaction, the selective and controllable interqubit couplings occur only by adjusting the individual gate pulses, by which we obtain the scalable DFS-encoded universal quantum gates to resist certain collective noises. Further analysis shows the protocol may implement the scalable fault-tolerant quantum computing with current experimental means.     

Scalable geometric quantum computing with Cooper-pair box qubits in circuit QED

We theoretically present a scheme to realize the scalable geometric quantum computing with Cooper-pair box (CPB) qubits in circuit QED. A one-dimensional transmission line resonator in circuit QED acting as quantum data bus generates a common cavity mode and interacts with each CPB. It is found that the interqubit couplings between any pair of qubits are switchable by individually adjusting the gate pulses applied to the selected CPBs. In this proposed scheme, we can both controllably and selectively address logic gates in geometric scenarios, which opens the possibility to implement the scalable fault-tolerant quantum computing with Josephson qubits.     

A quantum dense coding implementation in an ion trap

This paper reports that a quantum dense coding can be implemented with ions confined in a linear trap and interacting with laser beams. The scheme is insensitive to the interaction between the quantum channel and the environment. The Bell-state measurement is not involved and the probability of success in our scheme is 1.0.     

Microfabricated surface-electrode ion trap for scalable quantum information processing

Individual laser-cooled 24Mg+ ions are confined in a linear Paul trap with a novel geometry where gold electrodes are located in a single plane and the ions are trapped 40 microm above this plane. The relatively simple trap design and fabrication procedure are important for large-scale quantum information processing (QIP) using ions. Measured ion motional frequencies are compared to simulations. Measurements of ion recooling after cooling is temporarily suspended yield a heating rate of approximately 5 motional quanta per millisecond for a trap frequency of 2.83 MHz, sufficiently low to be useful for QIP.     

A quantum dense coding implementation in an ion trap

This paper reports that a quantum dense coding can be implemented with ions confined in a linear trap and interacting with laser beams. The scheme is insensitive to the interaction between the quantum channel and the environment. The Bell-state measurement is not involved and the probability of success in our scheme is $1.0$.     

Penning阱存储离子的高分辨分析

根据Penning阱中存储和探测离子的原理,研究改善离子谱分辨率的有效措施,发现离子谱的分辨率与LC振荡回路的Q值、阱中离子的密度及LC回路的谐振频率等因素有关.实验中,谐振频率为480千赫兹,Q在120左右,电子束流在40微安左右,真空度合适时,得到了较高分辨率的离子谱.最后对离子谱进行了辨认,离子分别是H+3,H+5,H+6,H+7.     

共面两囚禁离子体系精确的量子运动

研究Paul阱中共面两离子体系精确的量子运动. 在考虑库仑关联的条件下，得到了系统Schr?dinger方程的精确解和以此为基础的微扰解，包括精确的离散本征态和本征能量，以及近似的分段连续谱和能带结构. 以低能级的几个态为例，用数值方法计算了两离子间的平均距离，并绘出了两离子的概率分布图. 关键词： Paul阱 共面两离子 平均距离 能带结构     

Sympathetic cooling rate of gas-phase ions in a radio-frequency-quadrupole ion trap

We theoretically investigated the mass dependence of the sympathetic cooling rate of gas-phase ions trapped in a linear radio-frequency-quadrupole ion trap. Using an a priori molecular dynamical calculation, tracing numerically with Newtonian equations of motion, we found that ions with a mass greater than 0.54±0.04 times that of the laser-cooled ions are sympathetically cooled; otherwise, they are heated. To understand the mass dependence obtained using the molecular-dynamical calculation, we made a heat-exchange model of sympathetic cooling, which shows that the factor of 0.54±0.04 is a consequence of absence of micro-motion along the axis of the linear ion trap. Received: 10 December 2001 / Revised version: 28 January 2002 / Published online: 14 March 2002     

A grooved planar ion trap design for scalable quantum information processing

We describe a new electrode design for a grooved surface-electrode ion trap,which is fabricated in printed-circuitboard technology with segmented electrodes.This design allows a laser beam to get through the central groove to avoid optical access blocking and laser scattering from the ion trap surface.The confining potentials are modeled both analytically and numerically.We optimize the radio frequency(rf) electrodes and dc electrodes to achieve the maximum trap depth for a given ion height above the trap electrodes.We also compare our design with the reality ion chip MI I for practical considerations.Comparison results show that our design is superior to MI I.This ion trap design may form the basis for large scale quantum computers or parallel quadrupole mass spectrometers.     

Two quantum oscillators coupled with a planar radio frequency ion trap

In this scheme,two quantum oscillators in a planar radio frequency ion trap are coupled by the trap electrodes.The ions motional states encode the quantum bits (qubits),and a swap gate could be achieved.Under different conditions of the experiments,the intensity of the coupling between two quantum oscillators and the dissipation of the system are calculated.We compute fidelities for a quantum swap gate and discuss experimental issues.     

Trapping of ions from high energy sources into a radiofrequency ion trap

An attempt is described to confine ions, created externally and accelerated to some energy, in an rf quadrupole trap. 4 keV Ba⁺ ions were stopped on a Ni foil, placed in an aperture of one trap electrode. The Ba then was evaporated from the heated foil and ionized by electron impact. At background pressure of about 10^–5 mbar of various light buffer gases (He, H₂, N₂), the trap was filled once with 10⁵ ions, at a minimum primary ion number of 10¹⁰. The storage time was 10 min. From the data obtained the possibility of spectroseopic experiments on rare isotopes, created with accelerators or nuclear reactors, is discussed.     

Observations of anomalous “quantum jumps” in a single ion trap

Cooled positive barium ions, proposed for new frequency standards, cease scattering cooling radiation for anomalously long periods in the presence of carbon dioxide or water. The probable mechanism is the attraction of a gas molecule to the ion as a result of the dipole induced in the molecule by the ion, followed by the formation of a weak chemical bond between the two. The mechanism seems applicable to any ion.     

线型离子阱中冷却离子所需高纯度氦气的渗透与实验

利用氦的渗透性质,向超高真空系统中充入高纯度氦缓冲气体,可以降低离子阱中囚禁离子的运动速度,减小二级多普勒频移,增加离子的存储时间,提高线型离子阱微波频标的稳定度.实验上得到了氦气渗透的实验参数、特性以及对系统真空度的影响、实现了用温度调节精确的控制充入氦气的分压强.