Physicists in Vienna rewound a single photon’s state with over 95 percent accuracy. The result “rewinds” the time of a quantum system to an earlier moment without learning what happened in between.
This happened inside a standard lab, not a sci-fi set. The team used light as the test case and focused on the smallest possible system that still carries information.
Quantum time rewind
The work was led by Philip Walther at the University of Vienna, whose group focuses on experimental quantum optics and quantum information processing.
The research targets a qubit, a two level quantum bit used for information. The qubit forms the fundamental unit of quantum computation and can exist in multiple states at once rather than just the binary 0 or 1 of classical bits.
Rewinding does not move clocks on the wall or reverse physical time itself. It maps the particle’s internal state back to a precise condition it held earlier in its quantum evolution.
The protocol succeeds without ever peeking at or measuring the system directly, which is crucial because in quantum physics, even the smallest observation can collapse a particle’s delicate state. Avoiding that disturbance is what makes this approach both elegant and powerful.
How to rewind quantum time
The experiment relies on a quantum switch, a device that makes two operations occur in a superposed order. That order dependence is the heart of quantum computing gains.
By choosing operations that do not simply commute, the team arranged controlled interference. These paths are orchestrated so their combined effect cancels forward evolution for a chosen interval.
Mathematically, the key ingredient is the commutator, a measure of how two operations fail to swap cleanly. When the commutator is nonzero, it can be harnessed to reverse a prior change.
Many of the building blocks are unitary, reversible operations that preserve total probability. Unitary control keeps errors from multiplying when steps are repeated.
Quantum advantage appears when steps are noncommuting, order sensitive in a way classical steps are not. This order sensitivity is what the switch exploits to pull evolution backward.
“It was one of the most difficult experiments we’ve ever built for a single photon,” said Walther. The setup uses precise timing and optics to keep the fragile state aligned.
Time’s arrow bends in tiny systems
In daily life, disorder tends to grow, which sets the apparent direction of time. For a single quantum system, that statistical pressure relaxes and reversals become feasible.
This topic touches thermodynamics, the statistical science of energy and disorder. The rule guides big collections of particles but need not bind one particle.
The rules still respect energy and information limits. They work because microscopic dynamics can be run backward if you control the needed ingredients.
Here the control avoids learning the hidden details. It steers the system through paths whose interference undoes the net change.
Quantum behavior depends on coherence, delicate phase relationships across possibilities. Keeping coherence long enough is the reason the hardware is so carefully built.
Fast forward by sharing time
The theory behind the protocol also allows fast forwarding under strict rules. A team showed you can relocate evolution time among identical systems, as explained in a paper.
If you spread nine systems to stand still, the tenth can age nine times faster. The price is paid by the frozen systems, not by creating time from nothing.
This tradeoff unlocks accelerated testing of processes that would otherwise take too long. It does not violate conservation laws or common sense about bookkeeping.
Fast forwarding cannot help every system at once. Time can be shuffled only among the group under shared control.
What happened inside the lab
Photons were routed through fiber loops and interferometers to build the timing needed. Electro-optical switches send each photon through chosen paths without smearing its quantum state.
The polarization, the orientation of a photon‘s electric field, carried the information. Careful wave plate settings created the menu of operations to combine and invert.
One key element is an interferometer, a device that recombines paths to make intensities add or cancel. That recombination is how the unwanted forward evolution is erased.
The team verified outcomes with fidelity, a score for how close states match. High fidelity across many cases shows the protocol generalizes beyond a single lucky try.
Why this matters for quantum tech
Rewinding can help diagnose or remove unwanted changes in a processor. It can reverse a stray rotation on a qubit without knowing how it arose.
Benchmarks across many cases show consistent, high quality outcomes. That performance suggests the method belongs in practical toolkits, not just demonstrations.
Their approach also runs in real time. Rewinding five minutes takes about five minutes of lab time, aside from modest overhead.
These controls support better sensors. They also point toward sturdier quantum memories that keep stored information consistent.
Reliability of the quantum time rewind
The underlying theory supports error correction, techniques that detect and fix changes. When a step fails, the protocol can be retried without starting from scratch.
Earlier work showed that when multiple rewind attempts are linked together, the probability of success can climb arbitrarily close to 100 percent, as demonstrated in a paper.
This predictable reliability, combined with precise timing control, makes the protocol a practical tool rather than a theoretical curiosity.
Engineers value operations with predictable costs and timing. A protocol that rewinds T seconds of evolution in roughly T seconds of real time is straightforward to plan, optimize, and integrate into larger quantum systems.
Limitations and next steps
The protocol does not scale to complex objects anytime soon. A human contains too much information to rewind in any realistic effort.
The protocol does extend beyond light in principle. Any platform with coherent control and a reliable Hamiltonian, the mathematical rule for time evolution, could adopt similar steps.
Follow-up work is pushing integrated photonics for higher stability. That direction could enable active error correction that raises success rates further.
Future tests will explore higher dimensional systems and different physical platforms. Those steps will probe how far universal time reversal can be taken without breaking practicality.
The study is published in the journal Optica.
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