Dispersive manipulation of paired superconducting qubits

Abstract

We combine the ideas of qubit encoding and dispersive dynamics to enable robust and easy quantum information processing (QIP) on paired superconducting charge boxes sharing a common bias lead. We establish a decoherence free subspace on these and introduce universal gates by dispersive interaction with a LC resonator and inductive couplings between the encoded qubits. These gates preserve the code space and only require the established local symmetry and the control of the voltage bias.PACS numbers: 03.67. Lx, 74.50.+r Superconducting nano-circuits consisting of charge boxes (CB) [1] are among the most promising candidates for a quantum computer. Coherent control of a single charge qubit [2], long decoherence time [3] and, more recently, coupled two qubit systems [4] have been demonstrated. But despite this encouraging experimental progress, there are serious difficulties with superconducting QIP which may appear insurmountable. The first is the severe decoherence experienced by these macroscopic qubits, which are coupled to a large number of degrees of freedom in their environment including control circuitry [5]. A few methods have been employed to reduce the decoherence [6,7], but they usually require sophisticated manipulation or significant overhead in the control circuitry. The second major difficulty comes from the imperfect control realizable in solid state systems. Specifically, one finds it difficult to achieve controllable couplings between superconducting qubits, since the commonly used hard-wired inductive or capacitive couplings are untunable. Great effort has been exercised to realize controllable couplings [8,9,10]. Schemes allowing to compute with invariable couplings were also studied [11,12]. Others have recently discussed to use an LC circuit to actively mediate the interaction between superconducting qubits [13,14].The requirement to reduce decoherence and the desire for the easiest manipulation apply to all QIP implementations. Unfortunately, it is not always easy to accomplish both -actually the goals are often contradictory -since reducing decoherence may require extra complication in the manipulation.In this work we show how to achieve both goals. Using a closely placed pair of charge boxes (PCB) sharing a common bias lead as the logic qubit, we can encode information in a fashion immune to collective noise, which is the dominating decoherence source in our setting. We introduce LC resonators inductively coupled to the PCBs whose virtual excitations allow us to manipulate the PCB dispersively; all interactions will be off resonance, without energy transfer [15,16], and thus a logical qubit stays within its encoding space even during manipulation. By inductively coupling the * Electronic address: mikewulf@pas.rochester.edu CBs and taking advantage of dispersive dynamics again, controlled phases can be induced between logical qubits.Combining dispersive dynamics and encoding offers a new method for QIP. It overcomes the major difficulties of superconducting CBs in a realistic and very si...

Superconducting qubits coupled to nanoelectromechanical resonators: An architecture for solid-state quantum-information processing

“…15,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32 In particular, the promising design being developed at Yale, 15,28,29 using charge qubits coupled to superconducting transmission line resonators, is very similar to the architecture discussed here. Each junction is connected to a metallic plate on the surface of the resonator that covers about one quarter of the surface.…”

“…Several investigators have proposed the use of LC resonators, 17,18,19,20,21,22,23,24,25,26 superconducting cavities, 15,27,28,29 or other types of oscillators, 30,31,32 to couple JJs together. We note that although harmonic oscillators are ineffective as computational qubits, because the lowest pair of levels cannot be frequency selected by an external driving force, they are quite desirable as bus qubits or coupling elements.…”

Robust Quantum Gates in Decoherence-Free Subspaces with Josephson Charge Qubits

“…Additionally, the dephasing effects are usually stronger than the relaxation ones [33], the present encoded scheme is just immune to the σ ztype collective dephasing effects, which is helpful to enhance the fidelity. It is found that the dephasing time of the encoded logic qubits may be prolonged to two orders of magnitude longer using well-designed charge qubits with same device parameters [18,31].…”

“…In particular, encoding qubits into decoherence-free subspace (DFS) is an interesting way to avoid quantum errors [18][19][20][21]. Towards implementing the fault-tolerant schemes, the controllable interqubit couplings and the optimal quantum operations are two important aspects.…”

Full protection of superconducting qubit systems from coupling errors

“…Such encoding is also attractive for superconducting charge qubits, 6,7 which are subject to similar decoherence sources. 8 In this work, we show how to develop such protection for qubits coupled by the nearest neighbor XY-interaction that is encountered in both flux and charge qubit designs.…”