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arXiv:quant-ph/0308122

Qubit Assisted Probing of Coherence Between States of a Macroscopic Apparatus

Sougato Bose

Published Aug 22, 2003• 0 citations
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Abstract

I present a general scheme through which the evidence of a superposition involving distinct classical-like states of a macroscopic system can be probed. The scheme relies on a qubit being coupled to a macroscopic harmonic oscillator in such a way that it can be used to both prepare and probe a macroscopic superposition. Two potentially realizable implementations, one with a flux qubit coupled to a LC circuit, and the other with an ion-trap qubit coupled to the collective motion of several ions, are proposed.

DOI
10.48550/arxiv.quant-ph/0308122

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I present a general scheme through which the evidence of a superposition involving distinct classical-like states of a macroscopic system can be probed. The scheme relies on a qubit being coupled to a macroscopic harmonic oscillator in such a way that it can be used to both prepare and probe a macroscopic superposition. Two potentially realizable implementations, one with a flux qubit coupled to a LC circuit, and the other with an ion-trap qubit coupled to the collective motion of several ions, are proposed.
I present a general scheme through which the evidence of a superposition involving distinct states of a mesoscopic system can be probed. The scheme relies on a single qubit being coupled to a mesoscopic harmonic oscillator in such a way that it can be used to both prepare and probe a superposition of states of the oscillator. Two potentially realizable implementations, one with a single flux qubit coupled to an LC circuit, and the other with a single ion-trap qubit coupled to the collective motion of several ions, are proposed.
In this paper I present a foundational application of a qubit in probing the domain of validity of the quantum superposition principle. I show how a qubit can be coupled to a mesoscopic harmonic oscillator to both create and probe a superposition of states which involves distinct coherent states of the oscillator. Many harmonic oscillator systems are being developed as apparatus for a qubit and I argue that simply setting these oscillators to work in an under‐damped regime is enough to implement the scheme I discuss. I present two realizations: one based on a flux qubit coupled to a LCR circuit and the other based on a ion trap qubit coupled to the vibration of N ions.
We propose a scheme to generate macroscopic Schr\"odinger-cat states in a quantum harmonic oscillator (electromagnetic field or mechanical resonator) coupled to a quantum bit (two-level system) via a conditional displacement mechanism. By driving the qubit monochromatically, the oscillation of the qubit state modifies the effective frequency of the driving force acting on the oscillator, and a resonant or near-resonant driving on the oscillator can be achieved. The displacement of the oscillator is then significantly enhanced due to the small detuning of the driving force and can exceed that of the zero-point fluctuation. This effect can be used to prepare quantum superpositions of macroscopically distinct coherent states in the oscillator. We present detailed studies on this state-generation scheme in both the closed- and open-system cases. This approach can be implemented in various experimental platforms, such as cavity- or circuit-QED systems, electromechanical systems, and spin-cantilever systems.
We have observed the coherent exchange of a single energy quantum between a flux qubit and a superconducting LC circuit acting as a quantum harmonic oscillator. The exchange of an energy quantum is known as the vacuum Rabi oscillation: the qubit is oscillating between the excited state and the ground state and the oscillator between the vacuum state and the first excited state. We also show that we can detect the state of the oscillator with the qubit and thereby obtained evidence of level quantization of the LC circuit. Our results support the idea of using oscillators as couplers of solid-state qubits.