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Acta Physica Sinica

Classical correspondence of quantum entanglement in mesoscopic circuit

Hong-Yi Fan, Ze Wu

Published Dec 22, 2021• 0 citations
Publisher

Abstract

Since the birth of quantum mechanics, its classical correspondence (or analogy) has been a hot topic for physicists. In this paper, we first discuss whether there is a classical correspondence of quantum entanglement. We give a positive answer through the following examples: in the framework of quantization of mesoscopic circuits, two mesoscopic capacitance inductance (LC) circuits with mutual inductance are proved to be the source of quantum entanglement by using the integration within an ordered product, and then the formula of their characteristic frequency is obtained, It is found that it is similar to the expression of the small oscillating frequency of a classical system described below. The classical system is shown in Fig. 1 . Two walls are connected with the same spring. And between the two springs a sliding trolley can move on a smooth table. The trolley is hung with a simple pendulum, The small oscillating frequency of the system is calculated by analytical mechanics. It is found that the swing of the simple pendulum will cause the trolley to oscillate back and forth. The mutual restraint effect of the pendulum, the trolley and the spring reflects the “entanglement” between them.

DOI
10.7498/aps.71.20210992

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Since the birth of quantum mechanics, its classical correspondence (or analogy) has been a hot topic for physicists. In this paper, we first discuss whether there is a classical correspondence of quantum entanglement. We give a positive answer through the following examples: in the framework of quantization of mesoscopic circuits, two mesoscopic capacitance inductance (LC) circuits with mutual inductance are proved to be the source of quantum entanglement by using the integration within an ordered product, and then the formula of their characteristic frequency is obtained, It is found that it is similar to the expression of the small oscillating frequency of a classical system described below. The classical system is shown in Fig. 1 . Two walls are connected with the same spring. And between the two springs a sliding trolley can move on a smooth table. The trolley is hung with a simple pendulum, The small oscillating frequency of the system is calculated by analytical mechanics. It is found that the swing of the simple pendulum will cause the trolley to oscillate back and forth. The mutual restraint effect of the pendulum, the trolley and the spring reflects the “entanglement” between them.
Mesoscopic electric circuits can exhibit a large number of effects relevant to quantum mechanics, quantum electrodynamics and quantum statistics. The topic presented here can be identified as an analog of conventional cavity quantum electrodynamics, where the quantum objects are multilevel atoms and quantized optical fields (photons), namely, a theoretical subject called “cavity-circuit quantum electrodynamics”, where the atoms are replaced with quantum mesoscopic circuits, will be developed. We will study the quantum characteristics of the interaction between the quantum mesoscopic circuits and the quantized electromagnetic field at microwave frequencies. These issues include quantum entangled eigenstates of two coupled mesoscopic circuits, time evolution of the circuit energy quanta (governed by the time-dependent Schrödinger equation) and entanglement transfer between external photons and mesoscopic circuit energy quanta. Since there is inevitable electromagnetic interaction such as mutual capacitance and inductance coupling between two neighboring circuits, the quantum effects resulting from the aforementioned quantized circuit coupling would unavoidably affect the relevant processes in quantum computing devices and hence they deserve consideration in some issues of quantum information.
Quantum disentanglement refers to the transformation of entangled quantum system into disentangled system via some physical processes. In this paper, we search for quantum disentangling operator for the mesoscopic two-loop [Formula: see text] circuit with mutual inductance [Formula: see text]. It is this mutual inductance that causes quantum entanglement. By virtue of the method of integration within ordered product (IWOP) of operators, we find the disentangling operator and deduce the energy level (characteristic frequency). The quantum noise expression of squeezed vacuum state is also derived based on which we see that the large number of quantum entanglement engendered by the mutual inductance is, the more quantum noise produces in the mesoscopic circuit.
A mesoscopic circuit, where field quantization (e.g., coherent states and squeezed states) can be exhibited, is one of the ideal quantum systems for manifesting diverse quantum field theoretical phenomena and thermally statistical characteristics. Because of inevitable electromagnetic coupling (e.g., mutual capacitor coupling and mutual inductor coupling) in two neighboring mesoscopic electric circuits, quantum effects relevant to mesoscopic circuit coupling may play a key role in some devices of quantum information processing. Such a circuit coupling can lead to circuit entangled states, where the Fock number states of energy quanta in different circuits are entangled with each other. Since the time dependence in parameters of coupled-circuit Hamiltonian would result from environmental perturbations (e.g., temperature fluctuations in devices) or from some controllable manipulation (e.g., an incident ultrasonic wave), the circuit entangled states would evolve due to these external conditions. Time evolution of entangled states in coupled mesoscopic circuits is considered within a framework of time-dependent quantum theoretical formalism. Note that an applied photon field can also be coupled to such quantum entangled circuits. Then it is possible that an effect of entanglement transfer (i.e., transportation of superposition coefficients in the entangled state) from the quantum mesoscopic coupled LC circuits to the applied external photon field would also occur under some proper parameter conditions. The present characteristics of both quantized coupled mesoscopic circuits and entanglement transfer can also be found in some alternative microscopic- and mesoscopic-scale devices such as the systems in cavity quantum electrodynamics (QED).
By employing the continuous parameter entangled state representations, we investigate the energy level and the wave function for a capacitively and mutual-inductively coupled LC mesoscopic circuit. It is found that investigating the mesoscopic circuit in such representations can bring us the following conveniences. Firstly, the dynamical equation is naturally transformed into a single-variable differential equation. Secondly, the center-of-mass kinetic energy is included in the energy level of the system. Thus it is instructive to introduce the entangled state representation into the investigation of mesoscopic circuits.
Superconducting circuits, photonic cavities, and tiny mechanical oscillators are versatile systems in which quantum dynamics can be observed and controlled. These various technologies can be combined, and the resulting “circuits” are often referred to as nano-electromechanical or optomechanical systems. In the context of quantum physics, such devices are referred to as “mesoscopic” because they contain a large number of atoms. This terminology distinguishes them from the “microscopic” systems of atom-optics, such as single ions [364] and atoms [545, 233, 26], and the spins of individual nuclei [412]. Nano-engineered systems provide an especially powerful arena for exploring quantum measurement and control because of their potential to realize complex circuits. In this chapter we explain how to treat these systems and describe in detail a common method used to measure them. Superconducting circuits The electrical circuits that we describe in this chapter are quite different from those that use normal conductors, such as copper wire at room temperature. Normal conductors are subject to strong dissipation and decoherence from the environment. As a result the dynamical degrees of freedom of the circuits, the currents and voltages, behave as classical variables. In superconducting circuits, the resistance, and thus the dissipation, is so small that the currents and voltages behave as quantum mechanical degrees of freedom. Of course, the current in a wire is actually composed of many elementary particles that each obey quantum mechanics. Why then should the current act like a single quantum degree of freedom? A mechanical analogy is useful here. Recall that even though a macroscopic pendulum is composed of many particles, when one talks about the motion of the pendulum one means the motion of the center-of-mass of its constituent particles. Since the center of mass is merely a linear combination of the coordinates of each particle, it is itself a valid quantum mechanical coordinate with a conjugate momentum. In a similar way the total current in a superconducting wire is a collective degree of freedom that obeys quantum mechanics.