The quantum revolution mainly involve visible wavelengths of the electromagnetic spectrum, but everything changed when Shabir Barzanjeh and his team at the Institute of Science and Technology Austria used special properties of quantum mechanics to make measurements that were not previously possible. Together, they used entangled microwaves to create a quantum radar.
The radar works with only a few photons. In principle, the researchers used a device called the Josephson parametric converter to create pairs of entangled microwave photons. The first photon, called the “signal photon” is emitted towards the object of interest, and the researchers wait for a reflection. The second “idler photon” is emitted shortly after the first, so that when the signal photon reflects and travels back, it interferes with the idler photon to release a signal which can help determine the distance that was travelled by the signal photon.
Conventional radars are at a disadvantage when it comes to lower power levels. Lower power levels mean that fewer microwave photons are emitted, which in turn have to compete with objects in the vicinity that emit microwaves of their own. To put things into context, there are around 1000 microwave photons in the background at room temperature, which make it hard to detect the radars’ echoes.
Since greater power is needed to emit more microwave photons in conventional radars, quantum radars have the advantage of using entangled photons that are very similar to one another (yet different than the background photons) making them easily distinguishable even at lower power levels when the signal and idler photons interfere.
Reflection of the photons does destroy the delicate process of entanglement, but the signal produced is still strong enough to be differentiated from background noise. The property of entanglement helped Barzanjeh and his team detect objects one meter away at room temperature using just a handful of photons – outperforming conventional radars at such small distances.
These experiments showed the first application of microwave-based entanglement, and thanks to the low amount of electromagnetic radiation required, they also showed potential uses of quantum radars in imaging human tissues, and in security purposes.