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, 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.
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