Quantum Physics News: 'Causal Witness' Provides First Experimental Evidence Of Indefinite Causal Order

These words you are reading right now had to be written before you could read them. This might seem like a fairly straightforward and safe assumption to make, given that what we know about the universe tells us that cause (in this case, the article being written) precedes effect (the article being read).

In recent years, though, as our knowledge of what goes on in the realm of subatomic particles has expanded, scientists have come to realize that what seems to be true, and, in some cases, even deeply intuitive, is not always so.

Read: In The Quantum Realm, Time Is Always Blurry

Take the concept called superposition, for instance. As exemplified by Schrödinger’s ill-fated feline, quantum superposition is the phenomenon wherein particles exist in two or more states simultaneously and can be at two different locations at the same time.

The weirdness doesn’t end there. In recent years, physicists have also shown that this superposition can violate causality — even the causal order of events can be in superposition. In other words, much like the location of particles, the order of events can also be “indefinite.”

Now, in a study published Friday in the journal Science, a team of researchers has provided the first experimental verification of a process with an indefinite causal order.

“In our experiment, we perform a measurement in a superposition of causal orders — without destroying the coherence — to acquire information both inside and outside of a ‘causally nonordered process,’” the researchers wrote in the study.

Quantum coherence — the fragile state during which particles are in superposition — lasts for only a fraction of a second before the whole system decoheres, which is a phenomenon that marks the transition from the realm of quantum to classical mechanics. The fact that a coherent quantum state is short-lived is what allows reality as we know it to exist, but, for researchers looking to actually study superposition, this presents a major roadblock.

In order to overcome this hurdle, the researchers implemented a new measurement technique called “causal witness” — which allowed them to witness the superposition of particles (in this case, photons placed in superposition of two different quantum operations).

“To implement the causal witness, the physicists needed to devise a scheme which allowed them to extract information from inside of a highly-fragile quantum process without destroying it,” the University of Vienna, whose researchers carried out the experiment, said in a statement. “To do so, they used another quantum system to essentially raise a flag when the photon passed by one of the quantum operations. Although this could have still collapsed the system, the physicists found a new trick to measure the additional quantum system while keeping the superposition intact.”

Upon doing so, the researchers were able to confirm that the photons really had passed through both quantum operations in two orders at the same time — both of them had won and lost simultaneously.

“The group's next goal is to exploit new technological advances to create superpositions of more complex processes,” the university said in the statement. “This will allow them to gain deeper insights into the interplay between causal relations and quantum mechanics.”