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arXiv:1704.02735 · Physical Review A

Generation of the superposition of mesoscopic states of a nanomechanical resonator by a single two-level system

Manoj Das, Jitendra K. Verma, Pradyumna K. Pathak

Published Sep 21, 2017• 5 citations
arXivHTMLPDF

Abstract

We propose measurement-based conditional generation of the superposition of mesoscopic states of a nanomechanical resonator. We consider a two-level quantum mechanical system (qubit) coupled with a nanomechanical resonator through phonon exchange. An interaction, which produces shifts in the state of the nanomechanical resonator depending on the state of the qubit, is realized by driving the qubit through two resonant fields or a single field with controlled phase. The measurement of the state of the qubit produces superposition states of the nanomechanical resonator. We show that the quantum interference between the generated states in the superposition may lead to an arbitrary large displacement in the resonator. We also discuss decoherence of the generated states using the Wigner function.

DOI
10.1103/physreva.96.033837

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We propose measurement-based conditional generation of the superposition of mesoscopic states of a nanomechanical resonator. We consider a two-level quantum mechanical system (qubit) coupled with a nanomechanical resonator through phonon exchange. An interaction, which produces shifts in the state of the nanomechanical resonator depending on the state of the qubit, is realized by driving the qubit through two resonant fields or a single field with controlled phase. The measurement of the state of the qubit produces superposition states of the nanomechanical resonator. We show that the quantum interference between the generated states in the superposition may lead to an arbitrary large displacement in the resonator. We also discuss decoherence of the generated states using the Wigner function.
Macroscopic quantum superpositions, such as mechanical Schrödinger cat states, are central to emerging quantum technologies in sensing and bosonic error-correcting codes. We propose a scheme to generate such states by coupling a nanomechanical resonator to a coherently driven two-level system via both transverse and longitudinal interactions. Driving the qubit at twice the oscillator frequency activates resonant two-phonon exchange processes, enabling coherent conversion of drive energy into phonon pairs and their dissipative stabilization. Starting from the full time-dependent Hamiltonian, we derive an effective master equation for the mechanical mode by perturbative elimination of the lossy qubit. The reduced dynamics feature engineered two-phonon loss and a coherent squeezing term, which together drive the resonator into a deterministic Schrödinger-cat state. Our approach requires only a single driven qubit and no auxiliary cavity, offering a scalable and experimentally accessible route to macroscopic quantum superpositions in circuit-QED and related platforms.
We propose a scheme in which the quantum coherence of a nanomechanical resonator can be probed using a superconducting qubit. We consider a mechanical resonator coupled capacitively to a Cooper-pair box and assume that the superconducting qubit is tuned to the degeneracy point so that its coherence time is maximised and the electro-mechanical coupling can be approximated by a dispersive Hamiltonian. When the qubit is prepared in a superposition of states this drives the mechanical resonator progressively into a superposition which in turn leads to apparent decoherence of the qubit. Applying a suitable control pulse to the qubit allows its population to be inverted resulting in a reversal of the resonator dynamics. However, the resonator's interactions with its environment mean that the dynamics is not completely reversible. We show that this irreversibility is largely due to the decoherence of the mechanical resonator and can be inferred from appropriate measurements on the qubit alone. Using estimates for the parameters involved based on a specific realization of the system we show that it should be possible to carry out this scheme with existing device technology.
A scheme for generating the Schrödinger cat state based on geometric operations by a nanomechanical resonator coupled to a superconducting charge qubit is proposed. The charge qubit, driven by two strong classical fields, interacts with a high‐frequency phonon mode of the nanomechanical resonator. During the operation, the charge qubit undergoes no real transitions, while the phonon mode of the nanomechanical resonator is displaced along different paths in the phase space, dependent on the states of the charge qubit. This generates the entangled cat state between the NAMR and charge qubit, and the NAMR cat state can be achieved after some operations applied on this entangled cat state. The robustness of the scheme is justified by considering noise from environment, and the feasibility of the scheme is discussed.
There has recently been a surge of interest in the study of mechanical systems near the quantum limit. Such experiments are motivated by both fundamental interest in studying quantum mechanics with macroscopic engineered systems and potential applications as ultra‐sensitive transducers, or even in quantum information processing. A particularly promising system is a microwave cavity optomechanical system, in which a nanomechanical resonator is embedded within (and capacitively coupled to) a superconducting microwave cavity. Here we discuss two schemes for the generation and measurement of quantum states of the nanomechanical resonator. A quantum squeezed state may be generated via mechanical parametric amplification, while a number state may be conditionally generated via continuous measurement and feedback control mediated by a superconducting qubit.