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: http://arxiv.org/abs/1309.5268: Implementation of a Quantum Metamaterial
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