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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 want to approach the speed of light, but going back in time is forbidden. But what if scientists could use the advantages of quantum physics to uncover data about complex systems that happened in the past?

New research suggests that this premise may not be so far-fetched. In an article published on June 27, 2024, c Physical examination lettersCather 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 Junger Halpern of NIST and David Arvidsson-Shukkur of the University of Cambridge demonstrate a new type of quantum sensor that uses the quantum entanglement to make time travel detectors.

Murch describes this concept as analogous to being able to send a telescope back in time to capture 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 side of quantum physics, there may be a way around the rules. This is due to a property of entangled quantum sensors that Murch calls “hindsight”.

The process begins by entangling two quantum particles into a quantum singlet state—in other words, two qubits with opposite spin—so that no matter what 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 properties of the entanglement 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 the usual circumstances for this kind of experiment, where spin 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, if it is parallel or anti-parallel 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 which direction 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 direction to spin – in hindsight – through time travel.

Einstein once called quantum entanglement “spooky action at a distance.” Perhaps the most sinister part of entanglement is that we can view entangled pairs of particles as the same particle moving both forward and backward in time.

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