Abstract
We study a flux qubit consisting of a symmetrical pair of superconducting loops, with two Josephson junctions in each, joined by a common Josephson junction---a `twin' flux qubit. The qubit is capacitively coupled to a transmission line, which allows us to characterize the spectrum of the device by measuring the scattering of propagated electromagnetic waves. We perform a detailed analytical analysis of the double-loop system, revealing its properties, and compare experimental results with numerical simulations. At half-flux quantum bias of both loops, the qubit is protected against global and local magnetic field fluctuations with much less sensitivity to the global field in the second order. The system selection rules allow even-odd transitions and prohibit transitions between even-even or odd-odd levels due to the symmetry of the device.
Highlights
Superconducting qubits are among the most promising platforms for quantum computing technology
We study a flux qubit consisting of a symmetrical pair of superconducting loops, with two Josephson junctions in each, joined by a common Josephson junction—a ‘twin’ flux qubit
Typical qubits are on-chip aluminium structures with Josephson junctions (JJs), whose geometry can be designed to select an operating energy, state transition rates, and sensitivity required for a particular environment
Summary
Superconducting qubits are among the most promising platforms for quantum computing technology. Typical qubits are on-chip aluminium structures with Josephson junctions (JJs), whose geometry can be designed to select an operating energy, state transition rates, and sensitivity required for a particular environment. One of the inherent limitations, which is encountered with superconducting qubits, is a coherence time τdec, beyond which quantum information becomes lost. Two main sources of decoherence are charge and flux fluctuations in the vicinity of the qubit. Charge fluctuations are particular harmful for the qubits, where the charging energy EC is large. The original investigation was motivated by the weak flux dependence of the system transition energy when it is biased to the degeneracy point ( 0/2) in each loop. Compared to the original flux qubit, the energy levels of the ‘twin’ qubit are very flat. Simulations further indicate some level of protection against local-flux fluctuations
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