On the right, an
artificial atom generates sound waves consisting of ripples on the surface of a
solid. The sound, known as a surface acoustic wave (SAW) is picked up on the
left by a "microphone" composed of interlaced metal fingers.
According to theory, the sound consists of a stream of quantum particles, the
weakest whisper physically possible. The illustration is not to scale. Credit: Philip
Krantz, Krantz NanoArt.
The interaction between atoms and light is well known
and has been studied extensively in the field of quantum optics. However, to
achieve the same kind of interaction with sound waves has been a more
challenging undertaking. The Chalmers researchers have now succeeded in making
acoustic waves couple to an artificial atom. The study was done in
collaboration between experimental and theoretical physicists.
"We have opened a new door into the quantum world
by talking and listening to atoms", says Per Delsing, head of the
experimental research group. "Our long term goal is to harness quantum
physics so that we can benefit from its laws, for example in extremely fast
computers. We do this by making electrical circuits which obey quantum laws,
that we can control and study."
The researchers by the cryostat used for the
measurements. From left to right: Per Delsing, Thomas Aref, Göran
Johansson, Martin Gustafsson, Maria Ekström, Anton Frisk Kockum.
Photo: David Niepce
An artificial atom is an example of such a quantum
electrical circuit. Just like a regular atom, it can be charged up with energy
which it subsequently emits in the form of a particle. This is usually a
particle of light, but the atom in the Chalmers experiment is instead designed
to both emit and absorb energy in the form of sound.
"According to the theory, the sound from the atom
is divided into quantum particles", says Martin Gustafsson, the article's
first author. "Such a particle is the weakest sound that can be detected."
Since sound moves much slower than light, the acoustic
atom opens entire new possibilities for taking control over quantum phenomena.
"Due to the slow speed of sound, we will have
time to control the quantum particles while they travel" says Martin
Gustafsson. "This is difficult to achieve with light, which moves 100,000
times more quickly."
The low speed of sound also implies that it has a
short wavelength compared to light. An atom that interacts with light waves is
always much smaller than the wavelength. However, compared to the wavelength of
sound, the atom can be much larger, which means that its properties can be
better controlled. For example, one can design the atom to couple only to certain
acoustic frequencies or make the interaction with the sound extremely strong.
The frequency used in the experiment is 4.8 gigahertz,
close to the microwave frequencies common in modern wireless networks. In
musical terms, this corresponds approximately to a D28, about 20
octaves above the highest note on a grand piano.
At such high frequencies, the wavelength of the sound
becomes short enough that it can be guided along the surface of a microchip. On
the same chip, the researchers have placed an artificial atom, which is 0.01 millimeters
long and made of a superconducting material.
The paper Propagating phonons coupled to an
artificial atom is published online by the journal Science, at the Science Express web site.
Text: Johanna Wilde and Martin
details about the research:
The sample that the researchers use is made on a
substrate of gallium arsenide (GaAs) and contains two important parts. The
first one is the superconducting circuit that constitutes the artificial atom. Circuits
of this kind can also be used as qubits, the building blocks of a quantum
computer. The other essential component is known as an interdigital transducer
(IDT). The IDT converts electrical microwaves to sound and vice versa. The
sound used in the experiment has the form of surface acoustic waves (SAWs) which
appear as ripples on the surface of a solid. The experiments are performed at
very low temperatures, near absolute zero (20 millikelvin), so that energy in
the form of heat does not disturb the atom.
Microscope image: The artificial atom, in grey-blue
to the upper right, can emit and absorb sound that moves across the
surface of a microchip. The grey-blue structure to the lower left is the
combined loudspeaker/microphone used to communicate acoustically with
Credit: Martin Gustafsson and Maria Ekström
The theoretical research group, led by Göran
Johansson, recently published a paper on how the acoustic atom functions:
The research was funded by the Swedish Research
Council, the Knut and Alice Wallenberg Foundation, the European Research
Council and the Wenner-Gren Foundations.
Microscope image: Zoom-in of the artificial atom, with its integrated Superconducting Quantum Interference Device (SQUID) in violet. The SQUID gives the atom its quantum properties, and the fingers sticking up to the left provide the coupling to sound waves.
Credit: Martin Gustafsson and Maria Ekström