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arXiv:cond-mat/0403285 · Physical Review B

Josephson junction qubit network with current-controlled interaction

J. Lantz, Margareta Wallquist, Vitaly S. Shumeiko, Goran P. Wendin

Published Oct 14, 2004• 47 citations
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Abstract

We design and evaluate a scalable charge qubit chain network with controllable current-current coupling of neighboring qubit loops via local dc-current gates. The network allows construction of general $N$-qubit gates. The proposed design is in line with current main stream experiments.

DOI
10.1103/physrevb.70.140507

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•Aug 31, 2005•0 citations
We design and evaluate a scalable charge qubit chain network with controllable current-current coupling of neighboring qubit loops via local dc-current gates. The network allows construction of general NNN-qubit gates. The proposed design is in line with current main stream experiments.
We investigate the design and functionality of a network of loop-shaped charge qubits with switchable nearest-neighbour coupling. The qubit coupling is achieved by placing large Josephson junctions (JJs) at the intersections of the qubit loops and selectively applying bias currents. The network is scalable and makes it possible to perform a universal set of quantum gates. The coupling scheme allows gate operation at the charge degeneracy point of each qubit, and also applies to charge-phase qubits. Additional JJs included in the qubit loops for qubit readout can also be employed for qubit coupling.
We propose a coupling scheme for the three-Josephson junction qubits which uses a connecting loop, but not mutual inductance. Present scheme offers the advantages of a large and tunable level splitting in implementing the controlled-NOT (CNOT) operation. We calculate the switching probabilities of the coupled qubits in the CNOT operations and demonstrate that present CNOT gate can meet the criteria for the fault-tolerant quantum computing. We obtain the coupling strength as a function of the coupling energy of the Josephson junction and the length of the connecting loop which varies with selecting two qubits from the scalable design.
An improved architecture, which performs a universal set of gates by current biasing of coupling Josephson junction, has been proposed. This improvement is necessary to the realization of a functional and scalable quantum computer. The proposed architecture is in line with current technology. Secondly, we investigate a scheme for implementing the Deutsch–Jozsa algorithm via the improved architecture. It is a simple, scalable and feasible scheme for the implementation of the Deutsch–Jozsa algorithm based on the current-controlled superconducting charge qubit network.
Josephson junctions have been shown to be a promising solid-state system for implementation of quantum computation. The two-qubit gates are generally realized by the capacitive coupling between the nearest-neighbor qubits. We propose an effective Hamiltonian to describe charge qubits coupled through the microwave cavity. We find that nontrivial two-qubit gates may be achieved by this coupling. The ability to interconvert localized charge qubits and flying qubits in the proposed scheme implies that quantum network can be constructed using this large scalable solid-state system.
We propose an improved architecture for the physical implementation of two-qubit operations between arbitrary pairs of qubits by current biasing of coupled Josephson junctions. This improvement is necessary for the realization of a functional and scalable quantum computer. The proposed architecture is within the reach of current experimental technology. Secondly, we investigate a simple scheme for generating the graph states via the improved architecture. Since the controlled-phase gate between arbitrary pairs of qubits can be implemented easily, we can generate any multi-qubit graph states corresponding to various shape graphs, which meet the expectations of various quantum information processing schemes. It is a simple, scalable and feasible scheme for the generation of various graph states based on the current-controlled superconducting charge qubit network.
During this grant, we have invented, designed and successfully operated a new type of readout for charge-phase superconducting qubits. This new readout is based on the bifurcation of an RF-driven Josephson junction inserted in the loop of a split Cooper pair box. When the amplitude of the RF drive is in the vicinity of the bifurcation points a small change in the effective critical current of the junction is amplified into a large phase change in the reflected signal. The performance reached by the device is in agreement with theoretical predictions. The speed of this readout is unprecedented: we measure in 20ns the critical current increment corresponding to the transition between the 2 states of the qubit. The repetition rate is 10Mz. We have performed a series of experiments showing that the amplification mechanism behaves as expected. No quasiparticles are produced during the readout and the back-action on the qubit when the readout is OFF is minimal. We have performed decoherence measurements at dilution refrigerator temperatures on the quantronium qubit integrated with RF readout. These measurements involve all possible 1-qubit gate operations. Compared with the DC readout method we have gained in repetition rate (factor of 100), signal-to-noise ration on all coherence signals (factor of 10) but most importantly, on readout fidelity which now attains more than 60% (compared to 15%). This last improvement is very important for showing the entanglement produced by two-bit gate operations. The T1 has also been improved by 250%. We still have comparable T2 as before but we suspect this is due to insufficient filtering on the listening lines of our measurement apparatus and this problem is technically solvable. In parallel with this work on readout and decoherence of the quantronium qubit, we have designed a new 2-bit gate based on capacitive coupling between qubits and worked out the complete protocol of pulses.