Qubits, or quantum bits, are the key building
blocks that lie at the heart of every quantum computer. In order to
perform a computation, signals need to be directed to and from qubits.
At the same time, these qubits are extremely sensitive to interference
from their environment, and need to be shielded from unwanted signals,
in particular from magnetic fields. It is thus a serious problem that
the devices built to shield qubits from unwanted signals, known as
nonreciprocal devices, are themselves producing magnetic fields.
Moreover, they are several centimeters in size, which is problematic,
given that a large number of such elements is required in each quantum
processor. Now, scientists at the Institute of Science and Technology
Austria (IST Austria), simultaneously with competing groups in
Switzerland and the United States, have decreased the size of
nonreciprocal devices by two orders of magnitude. Their device, whose
function they compare to that of a traffic roundabout for photons, is
only about a tenth of a millimeter in size, and -- maybe even more
importantly -- it is not magnetic. Their study was published in the open
access journal Nature Communications.
When researchers want to receive a signal, for instance a microwave
photon, from a qubit, but also prevent noise and other spurious signals
from traveling back the same way towards the qubit, they use
nonreciprocal devices, such as isolators or circulators. These devices
control the signal traffic, similar to the way traffic is regulated in
everyday life. But in the case of a quantum computer, it is not cars
that cause the traffic but photons in transmission lines.
"Imagine a
roundabout in which you can only drive counterclockwise," explains first
author Dr. Shabir Barzanjeh, who is a postdoc in Professor Johannes
Fink's group at IST Austria. "At exit number one, at the bottom, there
is our qubit. Its faint signal can go to exit number two at the top. But
a signal coming in from exit number two cannot travel the same path
back to the qubit. It is forced to travel in a counterclockwise manner,
and before it reaches exit one, it encounters exit three. There, we
block it and keep it from harming the qubit."
The 'roundabouts' the group has designed consist of aluminum circuits
on a silicon chip and they are the first to be based on micromechanical
oscillators: Two small silicon beams oscillate on the chip like the
strings of a guitar and interact with the electrical circuit. These
devices are tiny in size -- only about a tenth of a millimeter in
diameter -- , one of the major advantages the new component has over its
traditional predecessors, which were a few centimeters wide.
Currently, only a few qubits have been used to test the principles of
quantum computers, but in the future, thousands or even millions of
qubits will be connected together, and many of these qubits will require
their own circulator. "Imagine building a processor that has millions
of such centimeter-size components. It would be enormous and
impractical," says Shabir Barzanjeh.
"Using our nonmagnetic and very
compact on-chip circulators instead makes life a lot easier."
Yet some
hurdles need to be overcome before the devices will be used for this
specific application. For example, the available signal bandwidth is
currently still quite small, and the required drive powers might harm
the qubits. However, the researchers are confident that these problems
will turn out to be solvable.
Professor Johannes Fink joined IST Austria in the beginning of 2016.
He and his group study quantum physics in electrical, mechanical and
optical chip-based devices with the main objective of advancing and
integrating quantum technology. Earlier this year, he received a
prestigious ERC Starting Grant for his project to develop a fiber optic
transceiver for superconducting qubits, as well as a research grant from
the Swiss NOMIS foundation. Dr. Shabir Barzanjeh was awarded a Marie
Sk?odowsa-Curie fellowship to work at IST Austria. His main interests
are in circuit quantum electrodynamics and optomechanics. From February
12 to 14, 2018, Johannes Fink und Shabir Barzanjeh will host the
international conference "Frontiers of Circuit QED and Optomechanics"
(FCQO 2018) in Klosterneuburg with the aim to bring together leading
scientists in the field.
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