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Superconducting nanocircuits for topologically protected qubits


For successful realization of a quantum computer, its building blocks—the individual qubits—should be simultaneously scalable and sufficiently protected from environmental noise. Recently, a novel approach to the protection of superconducting qubits has been proposed. The idea is to prevent errors at the hardware level, by building a fault-free logical qubit from ‘faulty’ physical qubits with properly engineered interactions between them. The decoupling of such a topologically protected logical qubit from local noises is expected to grow exponentially with the number of physical qubits. Here, we report on proof-of-concept experiments with a prototype device that consists of twelve physical qubits made of nanoscale Josephson junctions. We observed that owing to properly tuned quantum fluctuations, this qubit is protected against magnetic flux variations well beyond linear order, in agreement with theoretical predictions. These results suggest that topologically protected superconducting qubits are feasible.

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Figure 1: Protected qubit based on ‘c o s(2φ)’ Josephson elements.
Figure 2: The prototype of a protected superconducting qubit.
Figure 3: Coherent transport of pairs of Cooper pairs.
Figure 4: Characteristic energies 2E2 and Δ2 for the devices with different values of EJ/EC.
Figure 5: Gate voltage dependence of the switching current.

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We thank J. Sanchez and S. Pereverzev for help with experiments and O. Buisson, W. Guichard, L. Faoro, M. Feigelman, B. Pannetier and V. Schmidt for stimulating discussions. The work at Rutgers University was supported in part by the NSF grant ECS-0608842 and the Rutgers Academic Excellence Fund. The work at the University of Paris has been partly supported by the ANR grant ANR-06BLAN-0218-01.

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Correspondence to Michael E. Gershenson.

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Gladchenko, S., Olaya, D., Dupont-Ferrier, E. et al. Superconducting nanocircuits for topologically protected qubits. Nature Phys 5, 48–53 (2009).

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