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World’s First Quantum Metamaterial Unveiled

German researchers have designed, built, and tested the first metamaterial made out of superconducting quantum resonators.

In recent years, physicists have been excitedly exploring the potential of an entirely new class of materials known as metamaterials. This stuff is built from repeating patterns of sub-wavelength-sized structures that interact with photons, steering them in ways that are impossible with naturally occuring materials.

The first metamaterials were made from split-ring resonators (C-shaped pieces of metal) the size of dimes that were designed to interact with microwaves with a wavelength of a few centimetres. These metamaterials had exotic properties such as a negative refractive index that could bend light “the wrong way”.

But they were far from perfect, not least because the split-ring resonators introduced losses because of their internal resistance.

It doesn’t take much imagination to think of a solution to this problem: use superconducting resonators that have zero internal resistance.

And that’s a good idea in theory. In practice, however, it is hugely challenging. Apart from the obvious difficulty of operating at superconducting temperatures just above absolute zero, the main problem is that superconducting resonators are quantum devices with strange  quantum properties that are fragile and difficult to handle.

In particular, these properties are exponentially sensitive to the physical shape of the resonator. So tiny differences between one resonator and another can lead to huge differences in their resonant frequency.

And since metamaterials are periodic arrays of structures with identical properties,  that’s a problem. Indeed, nobody has ever made a quantum metamaterial for precisely this reason.

Today that changes thanks to the work of Pascal Macha at the Karlsruhe Institute of Technology in Germany and a few pals. These guys have built and tested the first quantum metamaterial, which they constructed as an array of 20 superconducting quantum circuits embedded in a microwave resonator.

This experiment is a significant challenge. These guys fabricated their quantum circuits out of aluminium in a niobium resonator, which they operated below 20 milliKelvin.

Their success comes from two factors. The first was in minimising the differences between each quantum circuit  so there was less than a 5 per cent difference in the current passing through each. 

The second was in clever design. A quantum circuit influences an incoming photon by interacting with it. To do this as a group, the quantum circuits must also interact with each other.

The problem in the past is that physicists had arranged the circuits in series so that the combined state must be a superposition of the states of all the circuits. So if a single circuit was out of kilter, the entire experiment failed.  

Macha and co got around this by embedding the quantum circuits inside a microwave resonator–a chamber about a wavelength long in which the microwaves become trapped.

To interact with a photon, each quantum circuit need only couple with the resonator itself and its nearest neighbours. That’s much easier to do with a large ensemble of quantum circuits.

And the results  show that it worked, at least in part.

The interaction with the quantum circuits changes the phase of the outgoing photons in subtle but measurable ways. So by studying this change, Macha and co were able to work out exactly what kind of interaction was occurring.

What they saw was that eight of the circuits formed a coherent group that influenced the photons. But over time, this dissociated into two separate groups of four quantum circuits.

That raises the tantalising question of why the large ensemble dissociated into two smaller ones, something that Macha and co will surely be investigating in future work.

It also raises the prospect of a new generation of devices. “Quantum circuits…based on this proof-of-principle experiment offer a wide range of prospects, from detecting single microwave photons to phase switching, quantum birefringence and superradiant phase transitions,” say Macha and co.

All in all, a significant first step for quantum metamaterials.

Ref: Implementation of a Quantum Metamaterial

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