Engineered "time mirrors" for electromagnetic waves
Researchers at CUNY ASRC reported an experimental demonstration of what physicists call a temporal reflection, or a "time mirror," for electromagnetic waves. In ordinary life a mirror flips spatial coordinates: a light pulse hits a mirror and the front of the pulse bounces back first. A time mirror does something distinct and counterintuitive — it flips the time direction of a portion of a wave, so that the end of the waveform is sent backward in time relative to the rest.
That behaviour requires creating a sudden, uniform change in the medium that carries the wave across the entire field — a temporal boundary. In practice, producing such a uniform, extremely fast change over an extended volume is energetically costly and technically difficult. The CUNY team avoided trying to change the host material wholesale. Instead they built a metamaterial strip: a metal transmission line populated with fast electronic switches and reservoir capacitors. When the switches are triggered simultaneously, the effective impedance of the strip doubles in a fraction of a microsecond, producing the abrupt temporal interface that yields a time-reflected copy of an incoming broadband electromagnetic pulse.
Because the reflected component originates from the end of the waveform rather than the front, a time-reflected optical pulse looks and sounds like a reversed recording — think of a tape played backwards. That reversal also shifts frequency content; in their experiments the team saw the expected changes in spectral makeup and timing. The demonstration — published in a peer-reviewed physics venue and discussed in recent press coverage — turns a decades-old theoretical prediction into an engineered, laboratory-scale reality.
Quantum rewinders: reversing a particle's history
Separately, teams led from Austria and the University of Vienna have shown that for quantum systems it is possible to implement a universal «rewind» that returns a qubit to an earlier state without knowing the unknown operations that acted on it. This is a different sense of "reversing time": there is no global temporal inversion of an environment, only a controlled manipulation of the quantum state of a tiny system so it returns to a prior configuration.
Crucially, the quantum rewinding experiment does not violate thermodynamics or provide a way to send macroscopic objects into the past. It works for carefully prepared microscopic quantum systems, and its power lies in quantum information control: the technique could become a tool in the error-management toolbox for quantum processors. If a qubit in a quantum computer is corrupted by an unknown process, a rewind protocol could, in principle, return it to a usable earlier state without destructive measurements.
Why the headlines sound like time travel — and why that’s misleading
Both experiments legitimately used the word "reverse" and both produced effects that mimic a tiny element of what we think of as backward time evolution. But they operate on different principles and domains. The CUNY time mirror is a classical-wave effect created by a fast change in material parameters; the Austrian quantum rewind is a manipulation of quantum information exploiting superposition and interference. Neither creates a causal loop that would let macroscopic objects or conscious observers travel into the past.
Scaling remains the central barrier. The metamaterial time mirror needs a spatially uniform, extremely fast switch across the whole field of the wave — that gets progressively more expensive as the wavelength shortens or the region grows. The quantum rewind protocols succeed for single qubits or small photonic systems under carefully isolated laboratory conditions; decoherence, environmental coupling, and the explosion of degrees of freedom make applying the same trick to large, thermally open systems effectively impossible with current knowledge and hardware.
Practical payoffs: communications and quantum processors
Neither result is scientific theatre alone. Time reflections for electromagnetic waves open new handles on wave control. Engineers who can make partial temporal reflections could design novel signal-processing elements: ways to clean, compress, or redirect waveforms that are fundamentally different from spatial mirrors or conventional filters. The CUNY authors and commentators have highlighted potential long-term applications in wireless communications, radar, and low-energy wave-based computing architectures where controlling spectral flow precisely matters.
On the quantum side, a rewind button for qubits addresses a concrete engineering problem — errors in fragile quantum processors. Error correction is already central to quantum-computing roadmaps, and a robust, low-overhead method to reverse unknown disturbances would reduce the need for repeated destructive diagnostics and heavy redundancy. Researchers envision integrating rewind primitives into quantum control stacks, or adapting the approach to trapped ions, cold atoms, or superconducting circuits as hardware matures.
What comes next in the lab
Expect both fields to press forward methodically. For metamaterials, engineers will explore more efficient switch designs, denser integration, and extending the effect across wider bandwidths and higher frequencies. For quantum rewinds, teams will test the protocols on different physical qubits, increase robustness to loss and noise, and investigate how rewind steps combine with conventional error-correction codes.
Importantly for scientists and the public, these developments illustrate how the language of "reversing time" can be precise and non-mystical. The experiments are powerful demonstrations of control — new dials on electromagnetic and quantum systems — not loopholes in the universe's bookkeeping. They expand the roster of techniques researchers can use to tame waves and qubits, and they will likely seed practical advances even while the prospect of macroscopic time travel remains solidly in science fiction.
Sources
- Nature Physics (paper and related materials on temporal reflections / CUNY ASRC metamaterial experiment)
- Optica (research paper on universal quantum rewinding protocol)
- CUNY Advanced Science Research Center (research team and press materials)
- Austrian Academy of Sciences / University of Vienna (quantum switch experiments and press materials)
