Researchers demonstrate how to build “time-traveling” quantum sensors.

The idea of ​​time travel has dazzled science fiction enthusiasts for years. Science tells us that traveling to the future is technically feasible, at least if you’re willing to approach the speed of light, but going back in time is impossible. But what if scientists could take advantage of quantum physics to uncover data about complex systems that occurred in the past?

New research suggests that this assumption may not be so far-fetched. In an article published on June 27, 2024 in Physical inspection lettersKater Murch, the Charles M. Hohenberg Professor of Physics and director of the Center for Quantum Leaps at Washington University in St. Louis and colleagues Nicole Yunger Halpern of NIST and David Arvidsson-Shukur of the University of Cambridge demonstrate a new type of quantum sensor that uses quantum entanglement to make time-traveling detectors.

Murch describes the concept as analogous to being able to send a telescope back in time to catch a shooting star you saw out of the corner of your eye. In the everyday world, this idea is uninteresting. But in the mysterious and mysterious land of quantum physics, there may be a way around the rules. This is thanks to a property of entangled quantum sensors that Murch calls “backward looking”.

The process starts with the entanglement of two quantum particles in a quantum singlet state—in other words, two qubits with opposite spin—so that no matter which way you think, the spins point in opposite directions. From there, one of the qubits—a “probe,” as Murch calls it—is exposed to a magnetic field that causes it to spin.

The next step is where the proverbial magic happens. When the auxiliary qubit (the one not used as a probe in the experiment) is measured, the entanglement properties effectively send its quantum state (ie, spin) “back in time” to the other qubit in the pair. This brings us back to the second step in the process, where the magnetic field rotated the “probe qubit”, and this is where the real benefit of hindsight comes in.

Under normal circumstances, for this kind of experiment, where spin rotation is used to measure the magnitude of the magnetic field, there is a one in three chance that the measurement will fail. This is because when a magnetic field interacts with a qubit along the x, y, or z axis, if it is parallel or anti-parallel to the direction of rotation, the results will cancel out—no rotation can be measured.

Under normal conditions, when the magnetic field is unknown, scientists would have to guess which way to prime the spin, leading to a one-third chance of failure. The beauty of hindsight is that it allows experimenters to set the best spin direction—in hindsight—through time travel.

Einstein once referred to quantum entanglement as “spooky action at a distance.” Perhaps the creepiest part of entanglement is that we can think of entangled pairs of particles as the exact same particles going forward and backward in time.

This gives quantum scientists creative new ways to make better sensors – especially ones you can effectively send back in time. There are a number of potential applications for these kinds of sensors, from the detection of astronomical phenomena to the aforementioned advantage gained in studying magnetic fields, and more will surely come into focus as the concept continues to be developed.