Researchers from the Austrian Academy of Sciences and the University of Vienna have demonstrated a precise method for reversing time in a quantum system, achieving an average fidelity of more than 95%. The study, published in Optica, shows that it is possible to make a quantum particle return to a previous state without knowing anything about its internal dynamics—a feat that could redefine error correction in quantum computing.
Reversing Time Without Knowing the System
In classical physics, time’s arrow points forward, enforced by the second law of thermodynamics. But in the quantum world, the laws of physics allow time evolution to be reversible—if you have the right control over the system.
The Austrian team, led by Miguel Navascués and Philip Walther, used the non-commutative nature of quantum operators to build a “time reversal” protocol. The method works through a quantum switch, a device that lets a photon’s evolution follow two different orders simultaneously.
Navascués explained the concept in simple terms to El País: in classical physics, watching a movie in a theater means it plays from start to end; in quantum physics, “we have a remote control to manipulate the movie. We can rewind to a previous scene or skip ahead.”
How the Experiment Was Done
The experiment encoded a qubit in the polarization of a single photon and guided it through a Sagnac interferometer. Inside, the photon experienced two possible evolutions—labelled U and V—in a superposition of sequences. By carefully orchestrating interference between these paths, the researchers effectively applied the mathematical inverse of the photon’s evolution, making it return to its initial state.
Crucially, this process didn’t require knowing the exact form of U or V, or the starting state. Tests ran across 50 different combinations of evolutions, four distinct initial states, and three time-step lengths (n = 1, 2, 3). Across 1,800 experimental runs conducted over three weeks, the fidelity stayed consistently above 93%, with some reaching 97%. The team notes that this far outperformed the best possible classical strategies for the same task.
More Than Just a Physics Curiosity
This is not time travel in the science-fiction sense—no human will be stepping into the past anytime soon. The researchers estimate it would take millions of years to reverse just a second of a human’s “quantum information.” Instead, the real potential lies in quantum error correction.
In a quantum processor, unwanted interactions can corrupt data. A reliable “rewind” could restore the system to a prior state without detailed knowledge of what went wrong. As Navascués put it, this is like having “a rewind button” for quantum machines, allowing them to correct mistakes before they cascade.
The study’s protocol also runs in real time: rewinding one second of quantum evolution takes exactly one second, unlike earlier approaches that needed triple that time and had much lower success rates.
What Comes Next
While the current work used photons, the protocol is not tied to light. In principle, it could be adapted to cold atoms, trapped ions, or other quantum platforms. The authors suggest that as integrated quantum photonics advances, the setup could be miniaturized and optimized for even higher fidelities.
Future research may also test the protocol on more complex systems or explore “fast-forwarding” quantum states—something the same team has theorized. The ultimate goal is to integrate these capabilities into practical quantum computers, turning the concept of quantum time control from a lab demonstration into a real-world tool.
This study, blending high-level theory with precise experimental execution, pushes the boundaries of what is technically possible in quantum manipulation—not by bending the rules of physics, but by using them to their full potential.
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