Speaker
Rainer Kaltenbaek
Description
Already before physicists started to discuss
entanglement with all its consequences and
applications, they argued about an even more
fundamental concept: quantum superposition. One of
the clearest visualizations of this concept is the double-
slit experiment. As Feynman once stated, this simple
experiment contains the heart of quantum theory. In this
experiment a source emits a particle towards some
distant detection screen where the particle's position
can be measured. Between the screen and the source
there is an inpenetrable wall that has two open slits
through which particles can pass. When many runs of
this experiment are performed, one will get a distribution
of particle positions measured on the screen. According
to quantum physics, if it is impossible even in principle to
know which slit each particle went through, this
distribution will exhibit an interference pattern as if the
particles, in some way, behaved like interfering waves
(“matter waves”). The explanation of quantum physics
for the occurrence of the interference pattern is that
each particle is in a superposition of passing through
one or the other slit. Such interference has been shown
to occur experimentally for increasingly heavy particles.
While the concept may be easier to grasp (or to shrug
away) for microscopic particles, Schrödinger
demonstrated in a thought experiment that quantum
theory, in principle, allows for quantum superposition
states even of macroscopic objects like a cat in a
superposition of being dead or alive. Here, we will
discuss experimental efforts using matter-wave
interference and quantum optomechanics in order to
test quantum superposition for increasingly massive
objects. Such experiments probe the boundaries
between the macroscopic, classical world and the
microscopic, quantum world. We will also discuss recent
investigations that indicate that this quest may
eventually lead us to perform experiments in space.
