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URSI General Assembly and Scientific Symposium

Circuit models in quantum electrodynamics

Johannes A. Russer, P. Russer

Published Aug 1, 2011• 7 citations
Publisher

Abstract

Network methods are applied to establish quantum mechanical models of distributed circuits. Based on the Hamiltonian description of the Foster equivalent circuits a quantization of the equivalent circuits is performed. The quantum mechanical interaction of the modes via nonlinear elements is discussed. As an example the DC-pumped Josephson parametric amplifier is treated quantum mechanically.

DOI
10.1109/ursigass.2011.6050401

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Network methods are applied to establish quantum mechanical models of distributed circuits. Based on the Hamiltonian description of the Foster equivalent circuits a quantization of the equivalent circuits is performed. The quantum mechanical interaction of the modes via nonlinear elements is discussed. As an example the DC-pumped Josephson parametric amplifier is treated quantum mechanically.
In RF nanoelectronics and mesoscopic electronics quantum effects play a role on the circuit and system level. In this work a quantum electrodynamics based circuit theory is introduced that allows to establish quantum mechanical models of distributed circuits. Based on the Hamiltonian description of the Foster equivalent circuits a quantization of the equivalent circuits is performed. The quantum mechanical interaction of the modes via nonlinear elements is discussed. A DC-pumped Josephson parametric amplifier is treated quantum mechanically.
Hamiltonian formulations for lossy nonlinear quantum circuits are investigated. As an example for a nonlinear quantum circuit a DC biased Josephson junction connected to a resonator is treated and a circuit quantum electrodynamic representation is introduced. The DC bias is chosen such that a strong interaction of the nonlinear Josephson junction with two resonant modes occurs. The DC-biased nonlinear Josephson junction acts as a frequency converter and power exchange occurs between the two considered resonator modes and also between the resonator modes and the DC power supply. The thermal losses in the resonator are described by a heat bath and treated by the Langevin approach.
Circuit quantization is an extraordinarily successful theory that describes the behavior of quantum circuits with high precision. The most widely used approach of circuit quantization relies on introducing a classical Lagrangian whose degrees of freedom are either magnetic fluxes or electric charges in the circuit. By combining nonlinear circuit elements (such as Josephson junctions or quantum phase slips), it is possible to build circuits where a standard Lagrangian description (and thus the standard quantization method) does not exist. Inspired by the mathematics of symplectic geometry and graph theory, we address this challenge, and present a Hamiltonian formulation of nondissipative electrodynamic circuits. The resulting procedure for circuit quantization is independent of whether circuit elements are linear or nonlinear, or if the circuit is driven by external biases. We explain how to rederive known results from our formalism, and provide an efficient algorithm for quantizing circuits, including those that cannot be quantized using existing methods. Published by the American Physical Society 2024
In recent years Josephson-junction-based parametric amplifiers have been developed for single-photon-level signals in the microwave spectral region, where they found applications in quantum information processing. Typically, Josephson traveling wave parametric amplifiers (JTWPA) have been studied based on classical circuit models. Although this approach shows good agreement with experimental results, it describes the system behavior in the framework of a classical theory, neglecting the quantum nature of the device. A quantum mechanical treatment of the JTWPA was only given in a few recent papers, where [1] derives a Hamiltonian for a chain of unit cells without resonant phase-matching (RPM), and [2] solves the dynamics of the system with and without RPM, also partly considering noise and squeezed state generation. However, our approach differs from [2] in that we give an explicit solution to the resulting nonlinear wave equation for narrow-band signals, starting from discrete chain Hamiltonians with and without RPM. We focus on describing the dynamics of the device in a noiseless quantum setting and present solutions for the governing nonlinear wave equation in Fig. 1. The starting point of this work is the amplifier design as given in [3]. The mathematical description relies on the Hamiltonian of a discrete chain of unit cells, which was derived in [1], and which we extended by a RPM circuit [2]. As the size Δl of a single chain element is relatively small compared to the wavelength, the chain of discrete Josephson elements can be considered in terms of a continuous transmission line.
In this paper, we are constructing a circuit quantum electrodynamic model describing dissipative effects in a Josephson traveling-wave parametric amplifier (JTWPA). Losses and noise are treated using the quantum Langevin method. The total Hamiltonian of our model describes the coupling of the modes in a parametric four-wave-mixing scheme, as well as the interaction with the environment. Based on the Heisenberg equations of motion, we obtain the temporal evolution of the signal photon number and analyze the signal gain of the JTWPA.
This short and opinionated review starts with the concept of quantum signals at microwave frequencies and focuses on the principle of linear parametric amplification. This process emerges from the dispersive nonlinearity of Josephson junctions driven with appropriate tones. We discuss two defining characteristics of the corresponding amplifying devices: i) the number of modes excited by the signal, idler and pump waves and ii) the number of independent ports through which these waves enter into the circuit.