The arrow of time moves forward. Eggs don’t break. Milk won’t spill. But now, new research has discovered a way to reverse this arrow in quantum systems, causing events to flip-flop as if time were flowing backwards.
The finding is theoretical at this point, but could be tested experimentally, said Luis Pedro García-Pintos, a physicist at Los Alamos National Laboratory and lead author of the new study, published February 19 in the journal Science. Physical Review X.
Ultimately, reversing time at the quantum level could halt the information loss that hampers quantum computers, said Andrea Rocco, a physicist at the University of Surrey in the UK who was not involved in the study. “This will immediately be an incredible advantage when it comes to building these quantum technologies.”
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The idea of reversing time is not new. In the 19th century, physicist James Clerk Maxwell conceived a thought experiment that reversed the second law of thermodynamics, which states that the total entropy (a measure of disorder) of a system cannot decrease over time. According to this law, heat always flows from a hotter object to a cooler object, resulting in an increase in the entropy of that cooler object. Anyone who’s ever poured a mug of hot chocolate to warm their hands on a snowy day can attest to this rule. However, by random chance, there will always be molecules that move slowly in a hot object and molecules that move quickly in a cold object. This means that an external entity known as Maxwell’s Devil could theoretically funnel these molecules preferentially from one object to another, sorting faster-moving molecules into hot objects and cooler molecules into cold objects. Therefore, hot objects become hotter and cold objects become colder. To the observer, the normal order appears to be proceeding in reverse. A cup of hot chocolate takes away the heat from your hands.
No little devils messing around with hot chocolate mugs. However, very small quantum systems have elements that are controlled externally. Quantum systems include all small particles, such as atoms and electrons, that behave according to the rules of quantum mechanics. Based on these rules, when we measure a quantum system, it changes. Before observation, a system can exist in multiple states simultaneously. This is a concept called superposition. In other words, the particle’s spin, momentum, and other properties are not yet defined. However, measurements break this superposition and yield one definitive result.
García-Pintos and his colleagues discovered that by using computer simulations to know the original state of a quantum system and its measured results, it is possible to reverse the arrow of time. For the external controller, the researchers constructed a series of fields and pulses that instantly return the virtual system to its starting position and, in some cases, propel it toward the opposite outcome. This control sequence, called the Hamiltonian, operates like Maxwell’s devil, reversing an irreversible sequence of events from forward to backward.
“We’re emulating a universe where things flow backwards in time,” Garcia-Pintos says.
These Hamiltonian controls can be used to create continuous measurement engines. Energy put into a quantum system by a measurement can be immediately extracted by the Hamiltonian and stored in a battery to power other processes, Garcia-Pintos says. Another application could be reversing quantum decoherence. Quantum decoherence is a phenomenon in which a quantum system loses its special quantum behavior and transforms into a classical system due to interaction with the external environment. Decoherence is a major barrier to quantum computing, so the step toward making it reversible is important, Rocco said.
But there are challenges ahead, says Kater March, an experimental physicist at the University of California, Berkeley. Actually creating these Hamiltonians requires perfect measurements with no loss of information, said March, who was not involved in the research. However, perfect measurements are impossible. Currently, researchers measure the properties of quantum systems by shining light or microwave light onto them, collecting the light, and observing how its components change. But it’s only about 50% efficient at collecting the light that comes back to see how the system changes, he says, meaning some details are fuzzy. “Some of the measurement signals were lost, so we no longer knew exactly what the quantum system was doing,” he says. That means that before researchers can build the perfect Hamiltonian for reversing time in real quantum systems, they need to get better at how they measure it.
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