Researchers show how to build ‘time-traveling’ quantum sensors

The idea of ​​time travel has baffled sci-fi fans for years. Science tells us that traveling into the future is technically feasible—at least if you’re willing to travel close to the speed of light—but going back in time is a no-go. But what if scientists could harness the power of quantum physics to reveal details about complex systems that occurred in the past?

New research suggests that this premise may not be so far-fetched. In a paper published on June 27, 2024, in Physical assessment lettersKater Murch, 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 create time-traveling 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 land of quantum physics, there may be a way to get around the rules. This is thanks to a property of entangled quantum sensors that Murch calls “hindsight.”

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

The next step is where the proverbial magic happens. When the auxiliary qubit (the one not being used as a probe in the experiment) is measured, the properties of entanglement effectively send its quantum state (i.e., 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 conditions for these kinds of experiments, where the rotation of a 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 antiparallel to the direction of the spin, the results will be nullified – there will be no rotation to measure.

Under normal circumstances, when the magnetic field is unknown, scientists would have to guess 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 the spin – after the fact – by time travel.

Einstein once called quantum entanglement “spooky action at a distance.” Perhaps the scariest thing about entanglement is that we can think of 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, especially sensors that can effectively be sent 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 view as the concept is further developed.