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Four-junction superconducting circuit in both flux and phase regimes

Yueyin Qiu, Wei Xiong, Xiao-Ling He, Tiefu Li, J. Q. You

Published Jan 30, 2016• 0 citations
OpenAlex

Abstract

We develop a theory for the quantum circuit consisting of a superconducting loop interrupted by four Josephson junctions and pierced by a magnetic flux (either static or time-dependent). In addition to the similarity with the typical three-junction flux qubit, we demonstrate the difference of the four-junction circuit from its three-junction analogue, especially its distinct advantages over the latter. Moreover, the four-junction circuit in the phase regime is also investigated. Our theory provides a tool to explore the physical properties of this four-junction superconducting circuit.

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We develop a theory for the quantum circuit consisting of a superconducting loop interrupted by four Josephson junctions and pierced by a magnetic flux (either static or time-dependent). In addition to the similarity with the typical three-junction flux qubit, we demonstrate the difference of the four-junction circuit from its three-junction analogue, especially its distinct advantages over the latter. Moreover, the four-junction circuit in the phase regime is also investigated. Our theory provides a tool to explore the physical properties of this four-junction superconducting circuit.
We develop a theory for the quantum circuit consisting of a superconducting loop interrupted by four Josephson junctions and pierced by a magnetic flux (either static or time-dependent). In addition to the similarity with the typical three-junction flux qubit in the double-well regime, we demonstrate the difference of the four-junction circuit from its three-junction analogue, including its advantages over the latter. Moreover, the four-junction circuit in the single-well regime is also investigated. Our theory provides a tool to explore the physical properties of this four-junction superconducting circuit.
Superconducting quantum interference devices (SQUIDs), single flux-quantum (SFQ) logic circuits, and quantum Josephson junction circuits have been developed into a family of superconductor integrated circuit, and are widely applied for subtle magnetic-field measurements, energy-efficient computing, and quantum computing, respectively. They are Josephson junction networks composed of Josephson junctions and normal resistor-inductor-capacitor (RLC) components, working with the fluxoid-quantization principle and Josephson effects to achieve unique flux-modulated dynamics and characteristics; they react to the vector potential of magnetic fields rather than the electric potential. However, the conventional circuit diagrams and nodal analysis methods focus on the electric charges flowing though branches and nodes, ignoring dynamics of the magnetic fluxes flowing from loop to loop. This article introduces a general flux-based circuit theory to unify the analyses of Josephson junction circuits and normal RLC circuits. This theory presents a magnetic-flux-generator (MFG) concept to unify Josephson junctions and normal circuit elements, and abstract both Josephson junction circuits and normal RLC circuits as MFG network; it derives a general network equation to describe dynamics of Josephson junction circuits, and invents a kind of magnetic-flux flow (MFF) diagram to depict the working principles of magnetic-flux flows inside Josephson junction circuits. The flux-based theory is complementary to the conventional circuit theories in the design and analysis of superconductor integrated circuits.
We present the first experimental results on a device with more than two superconducting qubits. The circuit consists of four three-junction flux qubits, with simultaneous ferro- and antiferromagnetic coupling implemented using shared Josephson junctions. Its response, which is dominated by the ground state, is characterized using low-frequency impedance measurement with a superconducting tank circuit coupled to the qubits. The results are found to be in excellent agreement with the quantum-mechanical predictions.
Josephson junction circuits, such as superconducting quantum interference devices (SQUIDs) and single-flux-quantum circuits, have been successfully applied in both the analog and digital domains. Due to phase-based working principles, their analyses are not well supported by charge-based analysis methods and conventional circuit diagrams. This article introduces a type of magnetic-flux-flow (MFF) diagrams to simplify the design and analysis of Josephson junction circuits. MFF diagrams are the graphical expression of circuit equations derived by loop analysis methods. They vividly depict the functions of Josephson junctions and superconducting loops and visualize how Josephson junctions transfer flux quanta among superconducting loops. The applications of MFF diagrams in the design and analysis of typical Josephson junction circuits are demonstrated case by case; it is shown that MFF diagrams are complementary to conventional circuit diagrams for both Josephson junction circuits and normal electric circuits.
We analyze the influence of noise on magnetic properties of a superconducting loop which contains three Josephson junctions. This circuit is a classical analog of a persistent current (flux) qubit. A loop supercurrent induced by external magnetic field in the presence of thermal fluctuations is calculated. To connect with experiment, we calculate the impedance of a low-frequency tank circuit which is inductively coupled with the loop of interest. We compare the results with the results obtained in the quantum mode — when the three junction loop exhibits quantum tunneling of the magnetic flux. We demonstrate that the tank–loop impedance in the classical and quantum modes have different temperature dependence and can be easily distinguished experimentally.