Using a biased qubit to probe complex systems

TL;DR

This paper introduces a method using a biased qubit to directly probe the spectral properties of complex mesoscopic systems, enabling energy landscape analysis without detailed knowledge of system-probe coupling.

Contribution

It presents a novel approach employing a biased qubit and the CPTP map formalism to extract spectral information from complex systems without assumptions on coupling strength.

Findings

Successfully retrieves energy spectra of complex systems.

Can test for coherent superpositions of energy eigenstates.

Operates without prior knowledge of system-probe coupling.

Abstract

Complex mesoscopic systems play increasingly important roles in modern science -- from understanding biological functions at the molecular level, to designing solid-state information processing devices. The operation of these systems typically depends on their energetic structure, yet probing their energy-landscape can be extremely challenging; they have many degrees of freedom, which may be hard to isolate and measure independently. Here we show that a qubit (a two-level quantum system) with a biased energy-splitting can directly probe the spectral properties of a complex system, without knowledge of how they couple. Our work is based on the completely-positive and trace-preserving map formalism, which treats any unknown dynamics as a `black-box' process. This black box contains information about the system with which the probe interacts, which we access by measuring the survival probability of the initial state of the probe as function of the energy-splitting and the process time. Fourier transforming the results yields the energy spectrum of the complex system. Without making assumptions about the strength or form of its coupling, our probe could determine aspects of a complex molecule's energy landscape as well as, in many cases, test for coherent superposition of its energy eigenstates.