Time:
it's constantly running out and we never have enough of it. Some say it’s an
illusion, some say it flies like an arrow. Well, this arrow of time is a big
headache in physics. Why does time have a particular direction? And can such a
direction be reversed?
A
new study, published in Scientific Reports,
is providing an important point of discussion on the subject. An international
team of researchers has constructed a time-reversal program on a quantum
computer, in an experiment that has huge implications for our understanding of
quantum computing. Their approach also revealed something rather important:
the time-reversal operation is so complex that it is extremely improbable,
maybe impossible, for it to happen spontaneously in nature.
As
far as laws of physics go, in many cases, there’s nothing to stop us going
forward and backward in time. In certain quantum systems it is possible to
create a time-reversal operation. Here, the team crafted a thought experiment
based on a realistic scenario.
The
evolution of a quantum system is governed by Schrödinger’s
Equation, which gives us the probability of a particle being in a certain
region. Another important law of quantum mechanics is the Heisenberg Uncertainty
Principle, which tells us that we cannot know the exact position and
momentum of a particle because everything in the universe behaves like both a particle
and a wave at the same time.
The
researchers wanted to see if they could get time to spontaneously reverse
itself for one particle for just the fraction of a second. They use the example
of a cue breaking a billiard ball triangle and the balls going in all
directions – a good analog for the second law of thermodynamics, an
isolated system will always go from order to chaos – and then having the
balls reverse back into order.
The
team set out to
test if this can happen, both spontaneously in nature and in the lab. Their
thought experiment started with a localized electron, which means they were
pretty sure of its position in a small region of space. The laws of quantum
mechanics make knowing this with precision difficult. The idea is to have the
highest probability that the electron is within a certain region. This
probability "smears" out as times goes on, making it more likely for
the particle to be in a wider region. The researchers then suggest a
time-reversal operation to bring the electron back to its localization. The
thought experiment was followed up by some real math.
The
researchers estimated the probability of this happening to a real-world
electron due to random fluctuations. If we were to observe 10 billion “freshly
localized” electrons every second over the entire lifetime of the universe
(13.7 billion years), we would only see it happen once. And it would merely
take the quantum state back one 10-billionth of a second into the past, roughly
the time it takes between a traffic light turning green and the person behind
you honking.
While
time reversal is unlikely to happen in nature, it is possible in the lab. The
team decided to simulate the localized electron idea in a quantum computer and
create a time-reversal operation that would bring it back to the original
state. One thing that was clear was this; the bigger the simulation got, the
more complex (and less accurate) it became. In a two quantum-bit (qubit) setup
simulating the localized electron, researchers were able to reverse time in 85
percent of the cases. In a three-qubit setup, only 50 percent of the cases were
successful, and more errors occurred.
While
time reversal programs in quantum computers are unlikely to lead to a time
machine (Deloreans are better suited for that), it might have some important
applications in making quantum computers more precise in the future.