The interaction between matter and light represents one of the
most fundamental processes in physics. Whether a car that heats up
like an oven in the summer due to the absorption of light quanta or
solar cells that extract electricity from light or light-emitting
diodes that convert electricity into light, we encounter the
effects of these processes throughout our daily lives.
Understanding the interactions between individual light particles
– photons – and atoms is crucial for the development of
a quantum computer.
Physicists from the Technische Universitaet Muenchen (TUM), the
Walther-Meissner-Institute for Low Temperature Research of the
Bavarian Academy of Sciences (WMI) and the Augsburg University have
now, in collaboration with partners from Spain, realized an
ultrastrong interaction between microwave photons and the atoms of
a nano-structured circuit. The realized interaction is ten times
stronger than levels previously achieved for such systems.
The simplest system for investigating the interactions between
light and matter is a so-called cavity resonator with exactly one
light particle and one atom captured inside (cavity quantum
electrodynamics, cavity QED). Yet since the interaction is very
weak, these experiments are very elaborate. A much stronger
interaction can be obtained with nano-structured circuits in which
metals like aluminum become superconducting at temperatures just
above absolute zero (circuit QED). Properly configured, the
billions of atoms in the merely nanometer thick conductors behave
like a single artificial atom and obey the laws of quantum
mechanics. In the simplest case, one obtains a system with two
energy states, a so-called quantum bit or qubit.
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Coupling these kinds of systems with microwave resonators has
opened a rapidly growing new research domain in which the TUM
Physics, the WMI and the cluster of excellence Nanosystems
Initiative Munich (NIM) are leading the field. In contrast to
cavity QED systems, the researchers can custom tailor the circuitry
in many areas.
To facilitate the measurements, Professor Gross and his team
captured the photon in a special box, a resonator. This consists of
a superconducting niobium conducting path that is configured with
strongly reflective "mirrors" for microwaves at both ends. In this
resonator, the artificial atom made of an aluminum circuit is
positioned so that it can optimally interact with the photon. The
researchers achieved the ultrastrong interactions by adding another
superconducting component into their circuit, a so-called Josephson
junction.
The measured interaction strength was up to twelve percent of
the resonator frequency. This makes it ten times stronger than the
effects previously measureable in circuit QED systems and thousands
of times stronger than in a true cavity resonator. However, along
with their success the researchers also created a new problem: Up
to now, the Jaynes-Cummings theory developed in 1963 was able to
describe all observed effects very well. Yet, it does not seem to
apply to the domain of ultrastrong interactions. "The spectra look
like those of a completely new kind of object," says Professor
Gross. "The coupling is so strong that the atom-photon pairs must
be viewed as a new unit, a kind of molecule comprising one atom and
one photon.
Experimental and theoretical physicists will need some time to
examine this more closely. However, the new experimental inroads
into this domain are already providing researchers with a whole
array of new experimental options. The targeted manipulation of
such atom-photon pairs could hold the key to quanta-based
information processing, the so-called quantum computers that would
be vastly superior to today's computers.
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