Ordinarily, to measure an object, we have to interact with it in some way. Whether by a sting or a nudge, an echo of sound waves or a shower of light, it is almost impossible to look without touching.
In the world of quantum physics, there are a few exceptions to this rule.
Researchers at Aalto University in Finland are proposing a way to “see” a microwave pulse without absorbing or re-emitting light waves. This is an example of a non-interacting special measurement, where something is observed without being jolted by a mediating particle.
The fundamental concept of “looking without touching” is not new. Physicists have shown that it is possible to use the wave nature of light to explore spaces without evoking its particle-like behavior by splitting perfectly aligned light waves into different paths and then comparing their paths.
In the place of lasers and mirrorsthe team used microwaves and semiconductors, making it a distinct achievement. The setup used what is called a transmon device to detect a pulsed electromagnetic wave in a room.
Although relatively large by quantum standards, these devices mimic the quantum behavior of individual particles on many levels using superconducting circuitry.
“Interactionless measurement is a fundamental quantum effect by which the presence of a photosensitive object is determined without irreversible absorption of photons,” the researchers write in their published paper.
“Here we propose the concept of non-interactive coherent detection and demonstrate it experimentally using a three-level superconducting transmon circuit.”
The team relied on the quantum coherence produced by their bespoke system – the ability of objects to occupy two different states at the same time, like Schrödinger’s cat – in order to pull off the complex setup.
“We had to adapt the concept to the different experimental tools available for superconducting devices,” says quantum physicist Gheorghe Sorin Paraoanu, from Aalto University in Finland.
“Because of this, we also had to modify the standard non-interacting protocol in a crucial way: we added another quantum layer using a higher energy level of the transmon. Then we used quantum coherence of the result at three levels: the system as a resource.
The experiments conducted by the team were supported by theoretical models confirming the results. It’s an example of what scientists call the quantum advantage, the ability of quantum devices to go beyond what’s possible with classical devices.
In the delicate landscape of quantum physics, touching things is like breaking them. Nothing ruins a nice wave of probability like the crunch of reality. For cases where detection requires a softer touch, other detection methods – like this one – might come in handy.
Areas where this protocol can be applied include quantum computing, optical imaging, noise detection, and cryptographic key distribution. In each case, the efficiency of the systems involved would be significantly improved.
“In quantum computing, our method could be applied to diagnose microwave-photon states in certain memory elements,” says Paraoanu. “This can be considered a very efficient way to extract information without disturbing the operation of the quantum processor.”
The research has been published in Nature Communication.