It sounds like the cosmetics industry dream: discovering a way to resist time’s arrow. For centuries, scientists and philosophers have pondered why we move relentlessly from past to future. What is the origin of this flow? Can it be reversed or halted? Now physicists in the burgeoning field of “quantum thermodynamics”—which examines heat and energy transfer at the smallest scales—are investigating a peculiar set of circumstances in which time’s inexorable march may be slowed in some tiny systems. Their findings won’t lead to a new anti-aging cream. But they may help revolutionize miniaturized quantum technologies, by making them more robust against the ravages of time, and quantum computers easier to build.

When physicists think about microscopic chemical reactions, for instance, they seem to all be reversible—just as likely to occur backwards as they are forwards. “There is nothing that seems to point towards a special direction of time,” says Marcus Huber, a quantum physicist at the Institute of Quantum Optics and Quantum Information, in Vienna, Austria.

When pondering why we get older and never younger, physicists instead tend to invoke the thermodynamic concept of entropy—crudely, a measure of the disorder of a system. Enshrined in the second law of thermodynamics is the idea that the entropy of an isolated system will never spontaneously decrease: your desk won’t get tidier, cracked eggshells won’t recombine, your milk won’t separate from your tea, and people will always age. You can’t turn back time, according to these “classical” laws of thermodynamics. 

But Nicole Yunger Halpern, a physicist at the University of Maryland (UMD) in College Park argues that there may be more to the tale of time than classical thermodynamics suggests. She and her colleagues are now investigating whether physicists can shed new light on time’s arrow—and maybe even exploit it—in tiny quantum systems. Doing so requires scrutinizing whether the classical laws of thermodynamics apply to the microscopic quantum world in exactly the same way they apply to the macroscopic everyday world around us. 

Shrinking Down

There is good reason to think that they do not and that the classical laws of thermodynamics will need modifying for small systems, says UMD physicist Chris Jarzynski. He notes that classical thermodynamics was developed during the nineteenth century to better understand the processes powering the industrial revolution. In particular, classical thermodynamics usually concerns macroscopic machines and its laws describe the average behavior of huge ensembles of molecules. But fast forward two centuries and technology is shrinking down so dramatically that physicists now talk of engines and refrigerators made from only a few particles. “Since about the 1990s, there’s been a lot of interest in trying to understand how the laws of thermodynamics apply to very very small systems on the size of individual molecules or even smaller,” says Jarzynski. 

At the level of individual particles, the weird quantum laws that govern the micro realm come into play. Quantum systems notoriously dance to their own, often counterintuitive, tune. At the quantum level, there is an inherent uncertainty about a particle’s properties. So, for instance, as Heisenberg famously noted, you can never know both where the particle is—its position—and where it is going—its momentum—with absolute certainty. In addition, before looking, a particle can possess contradictory characteristics simultaneously, for example, existing both here and there, at the same time. But this fuzzy quantumness is disturbed by the act of measuring its properties, which forces the quantum particle to behave classically, for instance, snapping into a single location. 

Over the past few decades, physicists have been attempting to exploit these bizarre quantum effects to enhance technologies. For instance, quantum computers should, in principle, be able to harness the ability of quantum bits, or qubits, to take on values of both 0 and 1 simultaneously to perform certain operations faster than standard machines. Teams around the globe—including at IBM and Google—have reported building rudimentary quantum computers in recent years. But none have yet successfully strung together much more than a thousand qubits, holding back any major breakthroughs. The big issue is that quantum features are very fragile and difficult to sustain in the lab for long periods, says Jarzynski. One solution to this obstacle may be to develop a way to slow down the destructive effects of time on quantum systems.

Quantum systems are so delicate to build, and so easy to break, Jarzynski explains, because the act of measuring the properties of the system irrevocably disturbs it. Unfortunately, “measurement” can be unintentional: a knock to your lab bench or even just heat in the room can set off a process called “decoherence” that destroys your system’s handy quantumness and its enhanced powers. Decoherence works one way, turning a quantum system into a classical one, and never the reverse.

says Jarzynski. Huber goes a step further stating that, in his view, “this irreversible measurement…in quantum mechanics is actually the second law of thermodynamics in disguise.”

Decoherence is thus the bane of many budding quantum-device makers’ lives. “If you can stave off the effects of decoherence that may help you reach the goal of actually building a practical quantum computer,” Jarzynski says. So the quest is now on to slow the march of time by holding back decoherence. 

Thermal Protection

With her colleague Amir Kalev, at the University of Southern California in Los Angeles, Yunger Halpern says she has been investigating whether some quantum systems have properties that can “help them resist, in a loose sense, the arrow of time.” She laughs that she always emphasizes the words “in a sense” to avoid wild speculation that such techniques might lead to anti-ageing tonics. Rather, her team is focused on delays to the natural process of “thermalization,” through which nearby objects reach the same temperature. “So the goal in this case is to look at one of these properties [of a quantum system], and ask, to what extent does it offer protection against thermalization?”

We are all familiar with thermalization at the macroscale, which happens whenever a small system, such as a cup of tea, interacts with a big environment, like the air in a room. As the hot tea exchanges energy and water molecules with the surrounding air, it will cool to the same temperature as the air, at which point the tea has thermalized. Something similar should happen in the quantum-thermodynamic realm, but the processes and energy exchanges involved are a little more complicated. “A small system can exchange all sorts of things with the big environment — maybe it exchanges heat, maybe it exchanges particles, maybe it exchanges electric charge — and [classical] thermodynamics predicts thermalization for these general cases,” says Yunger Halpern. 

But a few years ago, Yunger Halpern and her colleagues realized that these classical predictions relied on some basic assumptions that may not be valid when quantum processes are also in the mix. The issue that physicists had been overlooking, according to Yunger Halpern, relates to whether or not the order in which operations are carried out matters. For instance, in everyday life, it probably does not make a huge amount of difference to your tea-drinking pleasure if you choose to add sugar to your cup of tea and then add milk, or if you choose instead to add milk first and then sugar; these operations can be said to “commute.” On the other hand, your enjoyment will be greatly affected depending on whether you choose to first pour tea into your empty cup and then drink from the cup, or you instead decide to first drink from your empty cup and then pour tea into it; these operations do not commute. 

In the quantum world, whether or not two operations commute has especially profound consequences. When the operations of measuring two quantities, such as momentum and position, do not commute, then the quantities cannot both be measured simultaneously. This is the root of Heisenberg’s uncertainty principle and why we cannot know both the momentum and position of a quantum particle precisely. “Quantum theory is fascinating because…non-commutation leads to measurement disturbance and uncertainty relations,” says Yunger Halpern.

So Yunger Halpern and Kalev analyzed what happens if you take the possibility of non-commutation into account when considering thermalization and found that the textbook predictions need to be tweaked. The system “in some cases and in some senses, doesn’t quite thermalize as much as usual,” says Yunger Halpern. An experiment to test this is now planned in Maryland. 

Investigating time’s arrow and decoherence are just two of the myriad goals of quantum thermodynamics, which is growing in popularity around the world. Next year’s flagship quantum-thermodynamics conference will be hosted in Maryland, with at least 150 researchers from diverse branches of theoretical and experimental quantum physics and philosophy expected to attend. 

The time is ripe for this once niche area to explode with interest thanks to rapid advances in technology, says Huber. “All this money being poured into quantum devices, quantum computing, quantum communication, quantum sensing, basically leads us to a regime where we can plausibly explore quantum thermodynamics,” he explains.

“It’s a very young field,” adds Yunger Halpern, noting that there is much still to learn. “There are lots of problems and there’s lots to do, so come join us.”

Zeeya Merali is a London-based science journalist and author of the popular physics book, A Big Bang in a Little Room.