Quantum Physicists Superimpose Past and Future Events

The ability to process ‘A before B’ and ‘B before A’ at the same time may revolutionize quantum computing.

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Nov 9 2015, 11:30am

Image: leoplus/Flickr

In 1935, Austrian physicist Erwin Schrödinger placed a cat in an opaque box with a flask of poison and a device for measuring the decay of atoms (aka radioactivity). Being the twisted man he was, Schrödinger arranged the box so that as soon as the device recorded a single atom decaying, a lever would smash the flask, releasing the poison, and thereby killing the cat.

According to Schrödinger, after the cat had been in the box for a certain amount of time, it would be impossible to know whether the cat was dead or alive without looking. In this sense, the quantum physical sense, the cat was both dead and alive—it was only once an observer opened the box to check on the cat that this contradictory state, in which the cat is both dead and alive, would collapse into a single reality in which the cat was either dead or alive.

Of course, no actual cats were hurt in the making of Schrödinger's thought experiment. It was solely designed to demonstrate a fundamental principle of quantum mechanics known as superposition, in which it is possible for a quantum object to exist in two seemingly incompatible states simultaneously. Recently, a multinational team of physicists took the principle of superposition and applied it to ordered events—before and after, after and before—an advance that may prove to be of immense practical importance in the quest to build large-scale quantum computers.

We are accustomed to thinking about events happening in a definite causal order, in which event A causes event B, but, as the team of physicists details in a paper published in the New Journal of Physics, it is possible for events to not have a single definite order, meaning "A before B" and "B before A" occur at the same time.

"Now we find that not just physical properties, but also causal relations themselves can be undefined."

"Quantum theory ... has shaken our understanding of reality by telling us that physical systems may not have well-defined properties," the study's co-author, Cyril Branciard at CNRS and the Universite Grenoble Alpes, told Phys.org. "Now we find that not just physical properties, but also causal relations (or causal orders) themselves can be undefined, and can be put in some kind of superposition—a phenomenon that had not been observed experimentally until very recently."

The study introduced a test for this phenomenon, which is known as causal nonseparability, to demonstrate its presence in a quantum switch, which has been proposed as a way to improve the efficiency of quantum computers. (The best way to conceptualize a quantum switch is vis-à-vis a normal switch, which can be either on or off, whereas a quantum switch can be in a superposition of states where it is both on and off). They named their test the "causal witness," a mathematical algorithm that can be used to detect quantum nonseparability in a given computer program's process matrices.

It is important to note here that causal nonseparability is not the same as causal inequality. In the latter case, future events cause events in the past, which for the moment, remains impossible; in the former case, events merely don't follow a definite order—in other words, there is no definite future or past in the quantum switch, which allows for the superposition of the events.

By allowing events to be superimposed, this may blow open the door for significant improvements in the efficiency of quantum computers. Before the advent of the quantum switch, operations in a quantum computer would have to be performed in a definite order: either "A then B" or "B then A." But for certain tasks, changing the order of the operations might yield better results.

Without a quantum switch, in order to determine whether "A then B" or "B then A" is more efficient, a computer must perform both tasks and then compare the two. With a quantum switch, both tasks are performed simultaneously and the more efficient ordering is selected.

"More generally, I expect causally nonseparable processes to find applications in various other kinds of situations—just like entanglement proved to be useful for various applications in quantum information processing," said Branciard. "The full power of causal nonseparability is still to be discovered, and this makes this line of research particularly exciting!"