Researchers demonstrate how to build ‘time travel’ quantum sensors

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

New research shows that this premise may not be so far-fetched. In a paper published on June 27, 2024, in Physical review papersKater Murch, Charles M. Hohenberg Professor of Physics and Director of the Center for Quantum Leaps at Washington University in St. Louis. Quantum sensor that uses quantum entanglement to make time-traveling detectors.

Murch describes this 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 a non-starter. But in the mysterious and enigmatic land of quantum physics, there may be a way around the rules. This is thanks to a property of entangled quantum sensors that Murch refers to as “hindsight”.

The process begins with the entanglement of two quantum particles into a single quantum state—in other words, two opposite-spin qubits—so that no matter which direction you consider, the spins point in opposite directions. From there, one of the qubits—the “probe,” as Murch calls it—is subjected 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 of the process, where the magnetic field rotated the “probe qubit”, and here comes the real advantage of hindsight.

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

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

Einstein once referred to quantum entanglement as “vibrant action at a distance.” Perhaps the most exciting part about entanglement is that we can consider pairs of entangled particles to be the same particle, going forward and backward in time.

This gives quantum scientists creative new ways to build better sensors — especially ones you can effectively send back in time. There are a number of potential applications for these types of sensors, from the detection of astronomical phenomena to the aforementioned advantages gained in the study of magnetic fields, and more will come into focus as the concept develops further.