The idea is simple : A bit like a laser emits light particles or photons one after another in a neat & orderly row. An anti-laser sucks up photons one after another in reverse order. Researchers have long speculated that a tool like this might make wires & charging cables a thing of the past, allowing people to beam energy invisibly across a room to a laptop or phone & power it without plugging it in. But though basic anti-lasers are tested before, the real world isn’t as neat & orderly as a laser pointed at a constant or fixed receiver in a laboratory. Electronics move around, objects get in the way, walls reflect energy in an unexpected way. The new anti-laser demonstrated in this experiment accounts for all that & it receives scattered energy beamed around an area in unpredictable pattern, still receiving 99.996% of the sent power.
The formal term for the method they used is “Coherent Perfect Absorption (CPA)“. CPA use one machine to send power across the room & another (the “anti-laser”) to suck it backup. Previous CPA experiments, the researchers wrote in the journal Nature Communications, were exciting but had a fundamental limitation: the direction of time. The experiments worked only in situations where time could flow as easily backward as forward, which rarely exist in our day to day lives.
The simplest model of an anti-laser setup involving a laser pointer shooting photons one after another into a receiver that gobbles them-up would look basically the same whether you played a tape of its action forward or backward: Photon pops out of 1 device, travels across space & enters the opposite device. Setups like this in physics terms are said to possess “time reversal symmetry“. Time-reversal symmetry only pops up in systems without much entropy or the inherent tendency of systems to descend into disorder.
Until now, even the most complex CPA experiments have time reversal symmetry. Some were more complex than the laser pointer, aimed toward a receiver. But even complicated projects have that symmetry if they’re set-up such the process reversed.
(Here’s an example of how a sophisticated event can be time reversal symmetric: Imagine a videotape of a hobbyist picking Lego pieces from a neatly organized case & using them to create a model Eiffel Tower. The result would look complicated but the tape would record where each piece had gone, so playing the tape backward would just show the hobbyist taking apart the pieces & organizing them again.)
But for this work, the researchers used magnetic fields to jostle the photons so aggressively that point reversal symmetry was lost. The process of transferring power shooting the photons was like stirring soup: It doesn’t work backward. (Imagine trying to not stir soup.) But the device still received power.
This “proves that the concept of CPA goes far beyond its initial conception as a ‘time-reversed laser’ “, the researchers wrote in their paper, suggesting it’d in the future at some point have practical applications in the real world. That’s because the real world isn’t as neat as a time reversible laboratory experiment. It’s messy & unpredictable & never time-reversible over the long term. For CPA, to work in those challenging conditions, it’s to be ready to deal with it.
The researchers achieved this non-time reversed CPA in two experimental setups both using microwave energy. The first was a “labyrinth“ of wires that photons had to navigate to reach receiver. The second was small, irregular “brass cavity” with a receiver in the middle which the photons reached after scattering & traversing the open space in the cavity.
To pull-off this, the researchers emitted microwaves of different properties and tested which combination of frequencies, amplitudes & phases (three features of any electromagnetic wave) was presumably to land on the receiver & get absorbed even after passing through the magnetic fields and the labyrinth or irregular open space. In each case, they determined a perfect “tuning“ of the microwave emitter that caused most of the microwaves to urge absorbed (99.999% in the labyrinth, 99.996% in the open space). In real-world applications (like your living room), the emitter would test & retest the different frequencies, amplitudes & phases to transfer photons to its receiver.
There are three major potential-applications of this technology. The first is the wireless energy transfer at a distance, the researchers wrote. (Goodbye to plugging in your laptop.) Another is a a sensing device that would detect subtle changes in any room where the photons are scattered. (Imagine a security camera which will feel an intruder moving through a room.)
The third is a messaging system that would securely transfer information to a hidden receiver; signals sent over CPA could use the constantly changing tuning numbers as a kind of password to encrypt data. Only the receiver or someone who knew the receiver’s exact behavior from moment to moment could decrypt the message.
Any such real world uses are still an extended way off. But this experiment shows they’re at least possible, the researchers wrote.