  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.

Quantum Physics