There’s a lot we don’t understand about quantum systems. But over the past century, physicists have found ways to exploit the many oddities of quantum mechanics to their liking—including a new protocol that essentially manipulates the arrow of time.
Quantum control protocols refer to techniques that leverage quantum systems for practical and experimental purposes. In a recent Physical Review X paper, physicists report a new protocol that appears to reshape how a so-called arrow of time emerges in quantum systems. Using this mechanism, the team designed a measurement engine to extract energy from quantum systems—finding that their tool made the quantum system’s evolution “more consistent with time flowing backward” in a statistical sense, Luis Pedro García-Pintos, study lead author and a physicist at Los Alamos National Laboratory, told Gizmodo.
“Our work is in no way related to time travel,” noted García-Pintos. “Foundationally, it shows concrete ways in which quantifiers for the arrow of time can be ‘fooled’ into signaling ‘the wrong flow of time.’”
Here’s something you don’t learn in school
Imagine a glass of water next to a cat. Then imagine that the cat, in typical cat fashion, knocks over the cup, causing the water to spill and the glass to break. Say someone recorded this event and held a screening of the video. If the video shows the glass putting itself back together and the water droplets collecting back into the cup, you’d know that the video was playing in reverse.
In this way, we have a general sense of the order of daily events and can identify when that order is reversed. That is, we know that it’s physically impossible for a broken glass to piece itself together or for spilled water to be unspilled; the arrow of time is set in one distinct direction.
But this perception of time’s arrow is also something we ascribe to systems. According to García-Pintos, many natural phenomena inherently lack a preference for the direction of time. In the strictest sense, the direction of this arrow of time makes no difference in the mathematical framework of physics. It’s only when a system is measured—and those measurements introduce randomness—that an arrow of time emerges in a stochastic (probabilistic) way.
Controlling the arrow of time
Still, the fundamental laws of physics maintain that time is symmetrical at the quantum level, and the latest work builds on this idea. Researchers typically quantify the emergence of time’s arrow “by comparing the likelihood, or probability, of the occurrence of a forward-in-time process to its time-reversed version,” García-Pintos explained.
The new protocol is a mathematical framework to describe how monitored quantum systems evolve. The researchers formulated an engine-like design, applying the protocol to the system via a feedback mechanism. Fascinatingly, that allowed them to “cancel, amplify, or overcompensate” for the disturbance caused by measuring the system, according to a statement on the findings. As a result, the researchers could freely “change how the system evolves” and “reshape the arrow of time,” García-Pintos explained.
An unreal engine?
It’s worth noting that this engine apparatus does not exist yet, although the paper presents some compelling ideas. That said, the team is very keen on experimentally demonstrating its idea using superconducting qubits. If realized, the researchers believe this mechanism could maximize the energy potential of technology like quantum batteries or specific algorithms in quantum devices.
Overall, García-Pintos said that the tool was “general enough” such that other researchers could find some unexpected uses for it. Next, they plan to test their protocol on superconducting qubits to evaluate how the findings could be applied, for now, to more academic setups.
“The way we’re re-expressing how a monitored quantum system evolves allows different ways of thinking about the stochastic dynamics of monitored quantum systems,” he said. “This, in turn, allows fun new ways to leverage quantum measurements to steer the dynamics of a quantum system.”