Take a fine strand of silica fiber, attach it at each end
to a slow-turning motor, gently torture it over an unflickering flame until it
just about reaches its melting point and then pull it apart. The middle will
thin out like a piece of taffy until it is less than half a micron across --
about 200 times thinner than a human hair.
That,
according to researchers at the Joint Quantum Institute at the University of
Maryland, is how you fabricate ultrahigh transmission optical nanofibers, a
potential component for future quantum information devices, which they describe
in AIP Publishing's journal AIP
Advances.
Quantum
computers promise enormous power, but are notoriously tricky to build. To
encode information in qubits, the fundamental units of a quantum computer, the
bits must be held in a precarious position called a superposition of states. In
this fragile condition the bits exist in all of their possible configurations
at the same time, meaning they can perform multiple parallel calculations.
The tendency of
qubits to lose their superposition state too quickly, a phenomenon known as
decoherence, is a major obstacle to the further development of quantum
computers and any device dependent on superpositions. To address this
challenge, researchers at the Joint Quantum Institute proposed a hybrid quantum
processor that uses trapped atoms as the memory and superconducting qubits as
the processor, as atoms demonstrate relatively long superposition survival
times and superconducting qubits perform operations quickly.
“The idea
is that we can get the best of both worlds,” said Jonathan Hoffman, a graduate
student in the Joint Quantum Institute who works in the lab of principal
investigators Steven Rolston and Luis Orozco. However, a problem is that
superconductors don’t like high optical power or magnetic fields and most
atomic traps use both, Hoffman said.
This is where the optical nanofibers come in: The Joint
Quantum Institute team realized that nanofibers could create optics-based,
low-power atom traps that would “play nice” with superconductors. Because the
diameter of the fibers is so minute -- 530 nanometers, less than the wavelength
of light used to trap atoms -- some of the light leaks outside of the fiber as
a so-called evanescent wave, which can be used to trap atoms a few hundred
nanometers from the fiber surface.
Hoffman
and his colleagues have worked on optical nanofiber atom traps for the past few
years. Their AIP Advances paper describes a new procedure they
developed that maximizes the efficiency of the traps through careful and
precise fabrication methods.
The group’s procedure, which yields an improvement of two
orders of magnitude less transmission loss than previous work, focuses on
intensive preparation and cleaning of the pre-pulling environment the
nanofibers are created in.
In the fabrication process, the fiber is brushed through
the flame to prevent the formation of air currents, which can cause
inconsistencies in diameter to arise, as it is pulled apart and tapered down.
The flame source is a mixture of hydrogen and oxygen gas in a precise
two-to-one ratio, to ensure that water vapor is the only byproduct. The motors
are controlled by an algorithm based on the existing work of a group in Vienna,
which calculates the trajectories of the motors to produce a fiber of the
desired length and profile.
Previous
pulling methods, such as carbon dioxide lasing and chemical etching, were
limited by the laser’s insufficient diameter and by a lesser degree of control
over tapering length, respectively.
Future
work includes interfacing the trapped atoms with the superconducting circuits
held at 10 mKelvin in a dilution refrigerator, as well as guiding more
complicated optical field patterns through the fiber (higher-order modes) and
using these to trap atoms.
The article, “Ultrahigh transmission optical nanofibers,”
is authored by J.E. Hoffman, S. Ravets, J.A. Grover, P. Solano, P.R. Kordell,
J.D. Wong-Campos, L.A. Orozco and S.L. Rolston. It will be published in AIP Advances on June 17, 2014 (DOI: After that
date, it may be accessed at:http://scitation.aip.org/content/aip/journal/adva/4/6/10.1063/1.4879799
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