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Superconducting Nanocircuits for Topologically Protected Qubits (0802.2295v1)

Published 15 Feb 2008 in cond-mat.mes-hall and cond-mat.supr-con

Abstract: For successful realization of a quantum computer, its building blocks (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 (topologically protected) logical qubit from "faulty" physical qubits with properly engineered interactions between them. It has been predicted that the decoupling of a protected logical qubit from local noises would grow exponentially with the number of physical qubits. Here we report on the proof-of-concept experiments with a prototype device which consists of twelve physical qubits made of nanoscale Josephson junctions. We observed that due to properly tuned quantum fluctuations, this qubit is protected against magnetic flux variations well beyond linear order, in agreement with theoretical predictions. These results demonstrate the feasibility of topologically protected superconducting qubits.

Citations (173)

Summary

Superconducting Nanocircuits for Topologically Protected Qubits

The paper "Superconducting Nanocircuits for Topologically Protected Qubits" presents an experimental paper aimed at establishing the feasibility of topologically protected superconducting qubits. This work is pivotal for the development of error-resistant quantum computational systems. The authors focus on utilizing nanoscale Josephson junctions to implement logical qubits that can intrinsically shield themselves from local noise through topological protection. The core challenge addressed is achieving a balance between enhanced noise decoupling and operational feasibility at small scales.

Key Contributions

The authors provide an insightful exploration into the creation of topologically protected qubits by engineering interactions between inherently faulty physical qubits. The paper details the use of a chain of Josephson junctions, each described as having an effective Josephson energy characterized by the term VRcos(2ϕ)V_R \propto \cos(2\phi), where ϕ\phi is the phase difference across each element. This structure, denoted as a "rhombus," is used to form a logical qubit.

A critical aspect of the implementation is utilizing quantum fluctuations to suppress first-order sensitivity to magnetic flux noise. The experiments demonstrate that the energy barriers for logical state transitions in a chain of Josephson junctions can be effectively heightened, providing protection from noise up to the fourth-order perturbation. The proof-of-concept device effectively isolates the logical states from flux variations, verifying theoretical predictions of exponential growth of noise decoupling with the number of constituent qubits.

Experimental Findings

The constructed prototype device contains 12 Josephson junctions grouped into chains. The researchers observed that the superconducting element could be effectively isolated from local noise, even with non-zero but minute relative phase fluctuations across junctions. Importantly, the logical qubit's energy showed expected suppression against deviations of magnetic flux, exceeding linear susceptibility to noise.

To measure the effectiveness of the protection, the authors conducted experiments measuring the switching current as a function of magnetic flux. They found reduced amplitude of the first harmonic of oscillations for devices with a Josephson energy-to-capacitance ratio EJ/ECE_J/E_C around 3-6, in line with their theoretical predictions.

Implications and Future Considerations

These results affirm the conceptual model where a logical qubit exhibits substantial noise immunity thanks to strategic engineering of Josephson junction arrays. Practically, this implies that even systems with relatively small numbers of physical qubits could support fault-tolerant quantum operations, lending credibility to the use of topologically protected qubits in scalable quantum computing architectures.

The advancements hint at a promising path forward where the operational temperature and coherence times of qubits can be further enhanced by refining material properties and device architectures. Specifically, optimizing JJ dimensions and their relative transparency could lead to even better operability at reduced scales, higher temperatures, and possibly, simpler fabrication protocols.

This work is a notable stride towards realizing robust quantum computing systems. The authors intend to expand on this foundation by investigating the direct spectroscopic measurement of the energy gap Δ12\Delta_{12} and examining Rabi oscillations to provide a comprehensive characterization of designed qubits. As the community pushes the boundaries of quantum error correction and protection mechanisms, these findings serve as a significant benchmark in the ongoing development of scalable, noise-resistant quantum computers.

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