We see the world around us because light is being absorbed by specialized cells in our
retina.
But can vision happen without any absorption at all – without even a single particle
of light?
Surprisingly, the answer is yes.
Imagine that you have a camera cartridge that might contain a roll of photographic film.
The roll is so sensitive that coming into contact with even a single photon would destroy
it.
With our everyday classical means there is no way to know whether there’s film in the
cartridge, but in the quantum world it can be done.
Anton Zeilinger, one of the winners of the 2022 Nobel Prize in Physics, was the first
to experimentally implement the idea of an interaction-free experiment using optics.
Now, in a study exploring the connection between the quantum and classical worlds, researchers
from Aalto University have discovered a new and much more effective way to carry out interaction-free
experiments.
The team used transmon devices –superconducting circuits that are relatively large but still
show quantum behaviour– to detect the presence of microwave pulses generated by classical
instruments.
Their research was published in Nature Communications.
Although the Aalto researches were fascinated by the work done by Zeilinger’s research
group, their lab is centred around microwaves and superconductors instead of lasers and
mirrors.They had to adapt the concept to the different experimental tools available for
superconducting devices.
Because of that, they also had to change the standard interaction-free protocol in a crucial
way: they added another layer of “quantumness” by using a higher energy level of the transmon.
Then, they used the quantum coherence of the resulting three-level system as a resource.
Quantum coherence refers to the possibility that an object can occupy two different states
at the same time – something that quantum physics allows for.
However, quantum coherence is delicate and easily collapses, so it wasn’t immediately
obvious that the new protocol would work.
To the team’s pleasant surprise, the first runs of the experiment showed a marked increase
in detection efficiency.
They went back to the drawing board several times, ran theoretical models confirming their
results, and double-checked everything.
The effect was definitely there.
The researcher says that they also demonstrated that even very low-power microwave pulses
can be detected efficiently using their protocol.
The experiment also showed a new way in which quantum devices can achieve results that are
impossible for classical devices – a phenomenon known as quantum advantage.
Researchers generally believe that achieving quantum advantage will require quantum computers
with many qubits, but this experiment demonstrated genuine quantum advantage using a relatively
simpler setup.
Interaction-free measurements based on the less effective older methodology have already
found applications in specialised processes such as optical imaging, noise-detection,
and cryptographic key distribution.
The new and improved method could increase the efficiency of these processes dramatically.
In quantum computing, this new method could be applied for diagnosing microwave-photon
states in certain memory elements.
This can be regarded as a highly efficient way of extracting information without disturbing
the functioning of the quantum processor.
This research group is also exploring other exotic forms of information processing using
their new approach, such as counterfactual communication (communication between two parties
without any physical particles being transferred) and counterfactual quantum computing (where
the result of a computation is obtained without in fact running the computer).