Would it be possible to control and even reverse the flow of time in a laboratory setting? While taking a trip back through time is a staple of science fiction, new breakthroughs in quantum physics and computing are making it possible to investigate this phenomenon not as a journey, but an operation.
What lies at the root of these advances is the “arrow of time” itself, this one-way flow from past to future. In ordinary experience, this arrow is imposed upon us by the second law of thermodynamics: Entropy, or disorder, always tends to increase. A broken glass never reassembles itself, and heat flows from hotter to cooler bodies. But in certain quantum systems, correlations between particles might reverse this flow of heat, in effect turning back the arrow of time itself. In one experiment, nuclei in chloroform molecules were arranged so that the hotter nucleus of hydrogen became hotter still and the cooler nucleus of carbon became cooler, an impossibility unless these nuclei had been correlated in such a way as to permit information to reverse this flow.
Such reversals are not spontaneous and naturally occurring. As has been demonstrated in the theory, the reversal of a quantum state is impossible without the intervention of an external ‘supersystem’ that has the capability of performing the complex operation of wave function conjugation, which reverses the velocities and the phases of the quantum state. Even for a single particle, the probability of such an event happening naturally within the lifetime of the universe is extremely low, and for an entangled multi-particle state, it is exponentially so.
However, it is possible to design quantum computers as supersystems for this task. In this regard, it would be possible to encode the state of a physical system into a qubit register and then perform a series of gate operations to reverse time with a high degree of fidelity. Experiments have been carried out on IBM’s accessible quantum computer to test this concept for two- and three-qubit models of particle scattering processes. The procedure consists of evolving the system forward in time, followed by a complex conjugation (and possibly a unitary rotation, depending on the specific problem), and then evolving forward again to determine if the original state is returned.
Google Quantum AI has pushed this concept even further and applied the time reversal protocol to estimate out-of-time-ordered correlators (OTOCs) on a 105-qubit superconducting processor. These correlators encode the behavior of quantum information in many-body systems in a way that standard measurements do not. By simulating processes in both the forward and reverse directions sometimes in rapid succession the algorithm can detect the interference patterns or “quantum echoes” that can be decoded to reveal things such as molecular structure. The quantum simulation was found to be as much as 13,000 times faster than the best classical simulations for second-order OTOCs.
But there are implications that extend far from the lab bench. OTOC-inspired simulation methods might improve nuclear magnetic resonance methods, offering information to distinguish long-range couplings in large molecules. OTOCs might provide insight into basic physics, ranging from exotic matter to the marriage of quantum mechanics and general relativity. As quantum computers become larger and error rates decrease, reversing the arrow of time might become common practice not to relive history, but to investigate basic reality with un precedented resolution.
As things stand, however, the possibility of humans traveling backwards in time is purely theoretical. Yet in the quantum world, where the conventions of cause and effect can be warped without necessarily breaking, scientists are finding a way to turn back the clock with potentially profound implications for technology and our current comprehension of time.
