The time-flipped photon can't be used to restage
"Back to the Future," but it could help us figure out some of the
universe's most mysterious phenomena.
For the first time, physicists have made light
appear to move simultaneously forward and backward in time. The new technique
could help scientists improve quantum computing and understand quantum gravity.
By splitting a photon, or packet of light, using a
special optical crystal, two independent teams of physicists have achieved what
they describe as a "quantum time flip," in which a photon exists in
both forward and backward time states.
The effect results from the convergence of two
strange principles of quantum mechanics(opens in new tab), the counterintuitive
rules that govern the behavior of the very small. The first principle, quantum
superposition, enables minuscule particles to exist in many different states,
or different versions of themselves, at once, until they are observed. The
second — charge, parity and time-reversal (CPT) symmetry — states that any
system containing particles will obey the same physical laws even if the
particles' charges, spatial coordinates and movements through time are flipped
as if through a mirror.
By combining these two principles, the physicists produced
a photon that appeared to simultaneously travel along and against the arrow of
time. They published the results of their twin experiments Oct. 31(opens in new
tab) and Nov. 2(opens in new tab) on the preprint server arXiv, meaning the
findings have yet to be peer-reviewed.
"The concept of the arrow of time is giving a
word to the apparent unidirectionality of time that we observe in the
macroscopic world we inhabit," Teodor Strömberg(opens in new tab), a
physicist at the University of Vienna who was first author on one of the
papers, told Live Science. "This is actually in tension with many of the
fundamental laws of physics, which by and large are time symmetric, and which
therefore do not have a preferred time direction."
The second law of thermodynamics(opens in new tab)
states that the entropy of a system, a rough analogue of its disorder, must
increase. Known as the "arrow of time," entropy is one of the few
quantities in physics that sets time to go in a particular direction.
This tendency for disorder to grow in the universe
explains why it's easier to mix ingredients than to separate them. It's also
through this growing disorder that entropy is wedded so intimately to our sense
of time. A famous scene in Kurt Vonnegut's novel "Slaughterhouse-Five"
demonstrates how differently entropy makes one direction of time look to the
other by playing World War II in reverse: Bullets are sucked from wounded men;
fires are shrunk, gathered into bombs, stacked in neat rows, and separated into
composite minerals; and the reversed arrow of time undoes the disorder and
devastation of war.
However, as entropy is primarily a statistical
concept, it doesn’t apply to single subatomic particles. In fact, in every
particle interaction scientists have observed so far — including the up to 1
billion interactions per second that take place inside the world's largest atom
smasher, the Large Hadron Collider — CPT symmetry is upheld. So particles
seeming to move forward in time are indistinguishable from those in a mirrored
system of antiparticles moving backward in time. (Antimatter was created with
matter during the Big Bang and doesn't actually move backward in time; it just
behaves as if it is following an opposite arrow of time to normal matter.)
The other factor at play in the new experiments is
superposition. The most famous demonstration of quantum superposition is
Schrödinger's cat, a thought experiment in which a cat is placed inside a
sealed box with a vial of poison whose release is triggered by the radioactive
decay of an alpha particle. Radioactive decay is a quantum mechanical process
that occurs at random, so it is initially impossible to know what happened to
the cat, which is in a superposition of states, simultaneously dead and alive,
until the box is opened and the outcome observed.
This superposition of states enables a particle to
exist in both forward and backward time states at the same time, but witnessing
this feat experimentally is tricky. To achieve it, both teams devised similar
experiments to split a photon along a superposition of two separate paths
through a crystal. The superposed photon moved on one path through the crystal
as normal, but another path was configured to change the photon's polarization,
or where it points in space, to move as if it were traveling backward in time.
After recombining the superposed photons by sending
them through another crystal, the team measured the photon polarization across
a number of repeated experiments. They found a quantum interference pattern, a
pattern of light and dark stripes that could exist only if the photon had been
split and was moving in both time directions.
"The superposition of processes we realized is
more akin to an object spinning clockwise and counter-clockwise at the same
time," Strömberg said. The researchers created their time-flipped photon
out of intellectual curiosity, but follow-up experiments showed that time flips
can be paired with reversible logic gates to enable simultaneous computation in
either direction, thus opening the way for quantum processors with greatly
enhanced processing power.
Theoretical possibilities also sprout from the work.
A future theory of quantum gravity, which would unite general relativity and
quantum mechanics, should include particles of mixed time orientations like the
one in this experiment, and could enable the researchers to peer into some of
the universe's most mysterious phenomena.
"A nice way to put it is to say that our
experiment is a simulation of exotic scenarios where a photon might evolve
forward and backward in time," Giulio Chiribella, a physicist at the
University of Oxford who was the lead author of the other paper, told Live
Science. "What we do is an analogue to some experiments that simulate
exotic physics, such as the physics of black holes or time travel."
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