Experiments demonstrate that it is possible to put a photon in a superposition of processes going in opposite time directions.
Two different groups have tested a seemingly
counter-intuitive property of the quantum world: That it’s possible to put a
photon, a particle of light, in a superposition of states going forward and
backward in time. This is not time travel and won’t lead to communicating with
the past – but it is an intriguing demonstration of how time can be thought to
work at a quantum level.
Unless you have a TARDIS or a DeLorean, time only flows in
one direction (forward) for us. This annoying little fact that protects us from
all sorts of paradoxes is called the arrow of time. It is believed to be
related to the concept of entropy (which always increases in an isolated system
like the universe) but it doesn’t seem to be as fundamental at the quantum
level.
Instead, something that appears to be fundamental is the
so-called CPT symmetry (charge, parity, and time reversal symmetry). This holds
for all physical phenomena, and if a combination of two of them is violated
(such as famously the CP violations) there ought to be a violation in time
symmetry as well.
This has some fun theoretical applications, for example
where an electron moving forward in time can be thought of as a mirrored
positron (the antimatter equivalent) going back in time. Mathematically, it
works – but it’s not like the little antimatter we see in the universe is doing
a Back to the Future.
Recent research suggested that it was possible to have
photons in a superposition of states backward and forward in time. Now, that
idea has been demonstrated experimentally. Superposition is a quantum
phenomenon where a particle is in multiple states at once. Only when it is
measured does the particle settles into one of them.
In one of the experiments, a photon was placed on a
superposition of processes as it moved through a crystal. Photons have no
charge, so by changing parity (mirroring its motion in 3D) it is equivalent to
the behavior of a photon moving back in time. The process called quantum time
flip was demonstrated.
Again, the work doesn’t allow us to change the past.
However, it might have some interesting applications in quantum computers,
especially when it comes to logical operations, the crucial architecture of
computer processes. The ability to have quantum time flips could allow for
better performances and switching inputs and outputs.
The papers discussing these experiments are awaiting peer
review and are available on the ArXiv here and here.
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