According to general relativity, time flows differently at different positions due to the distortion of space-time by a nearby massive object. A single clock being in a superposition of two locations allows probing quantum interference effects in combination with general relativity. Image: Quantum Optics, Quantum Nanophysics, Quantum Information; University of Vienna. |
The
unification of quantum mechanics and Einstein’s general relativity is
one of the most exciting and still open questions in modern physics.
General relativity, the joint theory of gravity, space, and time gives
predictions that become clearly evident on a cosmic scale of stars and
galaxies. Quantum effects, on the other hand, are fragile and are
typically observed on small scales, e.g. when considering single
particles and atoms. That is why it is very hard to test the interplay
between quantum mechanics and general relativity. Now theoretical
physicists led by Professor ?aslav Brukner at the University of Vienna
propose a novel experiment which can probe the overlap of the two
theories. The focus of the work is to measure the general relativistic
notion of time on a quantum scale. The findings will be published in Nature Communications.
Time in general relativity
One
of the counterintuitive predictions of Einstein’s general relativity is
that gravity distorts the flow of time. The theory predicts that clocks
tick slower near a massive body and tick faster the further they are
away from the mass. This effect results in a so-called “twin paradox”: If one twin moves out to live at a higher altitude, he will age faster
than the other twin who remains on the ground. This effect has been
precisely verified in classical experiments, but not in conjunction with
quantum effects, which is the aim of the newly proposed experiment.
Quantum interference and complementarity
The
Viennese group of researchers wants to exploit the extraordinary
possibility that a single quantum particle can lose the classical
property of having a well-defined position, or as phrased in quantum
mechanical terms: it can be in a “superposition”. This allows for
wave-like effects, called interference, with a single particle. However,
if the position of the particle is measured, or even if it can in
principle be known, this effect is lost. In other words, it is not
possible to observe interference and simultaneously know the position of
the particle. Such a connection between information and interference is
an example of quantum complementarity, a principle proposed by Niels
Bohr. The experimental proposal now published in Nature Communications
combines this principle with the “twin paradox” of general relativity.
Einstein’s “twin paradox” for a quantum “only child”
The
team at the University of Vienna considers a single clock (any particle
with evolving internal degrees of freedom such as spin) which is
brought in a superposition of two locations – one closer and one further
away from the surface of the Earth. According to general relativity,
the clock ticks at different rates in the two locations, in the same way
as the two twins would age differently. But since the time measured by
the clock reveals the information on where the clock was located, the
interference and the wave-nature of the clock is lost. “It is the twin
paradox for a quantum ‘only child’, and it requires general relativity
as well as quantum mechanics. Such an interplay between the two theories
has never been probed in experiments yet,” says Magdalena Zych, the
lead author of the paper and member of the Vienna Doctoral Program
CoQuS. It is therefore the first proposal for an experiment that allows
testing the genuine general relativistic notion of time in conjunction
with quantum complementarity.
This
work was supported by the Austrian Science Fund (FWF) projects: W1210,
P19570-N16 and SFB-FOQUS, the Foundational Questions Institute (FQXi)
and the European Commission Project Q-ESSENCE (No. 248095).
Quantum interferometric visibility as a witness of general relativistic proper time
