The arrow of time works differently in the quantum world

If you watch a movie projected in the opposite direction, you will probably get confused. If a quantum computer sees it, it will probably be able to understand what is happening. This is what researchers from the Center for Quantum Technologies in Singapore have shown in collaboration with a number of researchers from around the world in the field

In a study published on July 18 in the journal Physical Review X, an international group showed that quantum computers are less "enslaved" by time pressure compared to classical computers, and in some cases quantum computers do not need to know what is the cause and what is the result.

The published article was inspired by a discovery made ten years ago by a number of scientists (James Crutchfield and John Mahoney from the University of California) who showed that there are processes that contain within them no arrow of time. If a viewer sees a chain of information, slide after slide in the correct order, he will be able to guess the next slide based on his past memory, but if the slides are shown in the opposite order it will be much more difficult for the viewer to guess the next slide because the chain of information has a clear time arrow.

This discovery has a name in the scientific literature - circumstantial asymmetry: the memory needed to determine what will happen in the future is different from the memory needed to reconstruct what happened in the past. The idea sounds intuitive, after all, running a video in reverse is like trying to guess cause from effect, a process that requires much more thinking from the opposite direction. In everyday life, guessing what will happen next is much easier if you have just seen what happened and what happened before it.

Researchers have always been curious about the arrow of time and the asymmetry that exists in nature. Why does it move in one direction if all the physical equations seem symmetric with respect to time? If the equations look the same even if time moved backwards, what is the source of asymmetry in nature? asked Gu - one of the authors of the article. The first study in causal asymmetry used models from classical physics to make predictions but now Crutchfield and Mahoney together with Gu, and other researchers from the Center for Quantum Technologies in Singapore tried to see if the laws of quantum mechanics could change the picture about the arrow of time.

If you guessed correctly, quantum mechanics changes the whole story. The researchers were able to show that there are models based on quantum mechanics that succeed in deciphering processes that lastly move in time with a much lower memory than classical computers, and even surpass in their ability to decipher these processes even against classical computers that receive the information in the normative direction. This work has several important implications for the world of computing, "The thing that excites us the most is the research connection with the arrow of time" said Thompson, the first author of the article. "If circumstantial asymmetry exists only in classical systems then it can be said that the concept of cause and effect, and as a consequence the arrow of time, arises from the nature of quantum systems to become classical."

The researchers continued with the question of how their findings can be connected to other ideas that require explanations about the nature of time. "Each scientific community has its own explanation for time pressure, but everyone is looking for an explanation of where it comes from?" Wedrel said. The most popular idea at the moment comes from the laws of thermodynamics, and is more accurate than the second law of thermodynamics which indicates an increase in entropy. In simple words, some attribute the idea of ​​entropy to the growth of disorder - an orderly system (with low entropy) that advances in time and tends to get messy (with greater entropy). Entropy will not necessarily increase everywhere and at any time, there can be fluctuations, and under physical processes in certain areas the entropy can actually decrease at the expense of the increase of the other, but on average over time it can be said that the entropy increases and therefore some attribute it to the breaking of the symmetry of time.

Because entropy must increase, time must move in the direction that increases it. At first thought circumstantial asymmetry (based on the idea of ​​memory) does not agree with the definition of the thermodynamic arrow of time (based on the principle of entropy), but on second thought you can see connections - systems that accumulate more information increase disorder in the system, because larger systems contain more disordered states And the likelihood that such a situation will occur increases. "This idea can lead us to new implications for the entropy principle," said Thompson.

The research results can also have practical implications. "Just as a video is projected in the opposite direction, sometimes researchers need to give explanations for the processes shown to them in the opposite direction and those that are difficult to model, in these cases researchers will be able to use models from the quantum world and use calculations that quantum computers will surpass classical computers", said Gu.

for the scientific article 

More of the topic in Hayadan:

• A pair of aluminum atomic clocks verify Einstein's twin paradox and time dilation
• The arrow of time and the Archimedes point are new directions for the physics of time
• The mysteries of reverse time