Time: it is constantly lacking and we never have enough. Some say it’s an illusion, others 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 study, published in Scientific Reports, provides an important discussion point on the subject. An international team of researchers has built 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 quite important: the time reversal operation is so complex that it is extremely unlikely, if not impossible, for it to occur spontaneously in nature.
When it comes to the laws of physics, in many cases there is nothing stopping us from moving forward and backward in time. In some quantum systems, it is possible to create a time reversal operation. Here, the team designed a thought experiment based on a realistic scenario.
The evolution of a quantum system is governed by the Schrödinger equation, which gives us the probability that a particle is in a certain region. Another important law of quantum mechanics is Heisenberg’s uncertainty principle, which tells us that we cannot know the exact position and momentum of a particle because everything in the universe behaves both like a particle and like a wave.
The researchers wanted to see if they could find time for a particle to spontaneously invert for just a fraction of a second. They use the example of a cue breaking a triangle of billiard balls 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 – then doing return the balls in order.
The team set out to test whether this can happen, both spontaneously in nature and in the lab. Their thought experiment started with a localized electron, meaning they were pretty sure of its position in a small region of space. The laws of quantum mechanics make this knowledge difficult. The idea is to have the highest probability that the electron is in a certain region. This probability “fades” over time, making it more likely that the particle is in a larger region. The researchers then propose a time reversal operation to bring the electron back to its location. The thought experiment was followed by real math.
The researchers estimated the likelihood of this happening to a real-world electron due to random fluctuations. If we were to observe 10 billion “freshly located” electrons every second over the lifetime of the universe (13.7 billion years), we would only see it happen once. And that would just take the quantum state a 10 billionth of a second back in time, roughly the time it takes between a traffic light turning green and the person behind you honking their horn.
Although time reversal is unlikely to occur in nature, it is possible in the laboratory. The team decided to simulate the localized electron idea in a quantum computer and create a time reversal operation that would return it to its original state. One thing that was clear was this; the larger the simulation, the more complex (and less accurate) it became. In a two-quantum-bit (qubit) configuration simulating the localized electron, the researchers were able to reverse time 85% of the time. In a three-qubit configuration, only 50% of the cases succeeded and more errors occurred.
Although time reversal programs in quantum computers are unlikely to lead to a time machine (Deloreans are better suited for this), they could have important applications for making quantum computers more accurate in the future.
This article was originally published in March 2019.
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