Nanophotonic
integration for simultaneously controlling a large number of quantum mechanical
spins in nanodiamonds. Courtesy: P. Schrinner/AG Schuck.
Using
modern nanotechnology, it is possible nowadays to produce structures which have
a feature sizes of just a few nanometres. This world of the most minute
particles—also known as quantum systems—makes possible a wide range of
technological applications, in fields which include magnetic field sensing,
information processing, secure communication or ultra-precise time keeping. The
production of these microscopically small structures has progressed so far that
they reach dimensions below the wavelength of light. In this way, it is
possible to break down hitherto existent boundaries in optics and utilize the
quantum properties of light. In other words, nanophotonics represent a novel
approach to quantum technologies.
As
individual photons move in the quantum regime, scientists describe the relevant
light sources as quantum emitters that can be embedded in nanodiamonds, among
others. These special diamonds are characterized by their very small particle
size, which can range from just a few to several hundred nanometres.
Researchers at the University of Münster have now succeeded for the first time
in fully integrating nanodiamonds into nanophotonic circuits and at the same
time addressing several of these nanodiamonds optically. In the process, green
laser light is directed onto color centers in the nanodiamonds, and the
individual red photons generated there are emitted into a network of nano-scale
optical components. As a result, the researchers can now control these quantum
systems in a fully integrated state. The results have been published in the
journal Nano Letters.
Background
and methodology
Previously,
it was necessary to set up bulky microscopes in order to control such quantum
systems. With fabrication technologies similar to those for producing chips for
computer processors, light can be directed in a comparable way using waveguides
(nanofibres) on a silicon chip. These optical waveguides, measuring less than a
micrometer, were produced with the electron-beam lithography and reactive ion
etching equipment at the Münster Nanofabrication Facility (MNF).
"Here,
the size of a typical experimental set-up was shrunk to a few hundred square
micrometers," explains Assistant Professor Carsten Schuck from the
Institute of Physics at the University of Münster, who led the study in collaboration
with Assistant Professor Doris Reiter from the Institute of Solid State Theory.
"This downsizing not only means that we can save space with a view to
future applications involving quantum systems in large numbers," he adds,
"but it also enables us, for the first time, to control several such
quantum systems simultaneously."
In
preliminary work prior to the current study, the Münster scientists developed
suitable interfaces between the nanodiamonds and nanophotonic circuits. These
interfaces were used in the new experiments, implementing the coupling of
quantum emitters with waveguides in an especially effective way. In their
experiments, the physicists utilized the so-called Purcell effect, which causes
the nanodiamond to emit the individual photons with a higher probability into
the waveguide, instead of in some random direction.
The
researchers also succeeded in running two magnetic field sensors, based on the
integrated nanodiamonds, in parallel on one chip. Previously, this had only
been possible individually or successively. To make this possible, the
researchers exposed the integrated nanodiamonds to microwaves, thus inducing
changes of the quantum (spin) state of the color centers. The orientation of
the spin influences the brightness of the nanodiamonds, which was subsequently
read out using the on-chip optical access. The frequency of the microwave field
and therewith the observable brightness variations depend on the magnetic field
at the location of the nanodiamond. "The high sensitivity to a local
magnetic field makes it possible to construct sensors with which individual
bacteria and even individual atoms can be detected," explains Philip
Schrinner, lead author of the study.
First of
all, the researchers calculated the nanophotonic interface designs using
elaborate 3-D simulations, thus determining optimal geometries. They then
assembled and fabricated these components into a nanophotonic circuit. After
the nanodiamonds were integrated and characterized using adapted technology,
the team of physicists carried out the quantum mechanical measurements by means
of a set-up customized for the purpose.
"Working
with diamond-based quantum systems in nanophotonic circuits allows a new kind
of accessibility, as we are no longer restricted by microscope set-ups,"
says Doris Reiter. "Using the method we have presented, it will be
possible in the future to simultaneously monitor and read out a large number of
these quantum systems on one chip," she adds. The researchers' work
creates the conditions for enabling further studies to be carried out in the
field of quantum optics—studies in which nanophotonics can be used to change
the photo-physical properties of the diamond emitters. In addition to this
there are new application possibilities in the field of quantum technologies,
which will benefit from the properties of integrated nanodiamonds—in the field
of quantum sensing or quantum information processing, for example.
The next
steps will include implementing quantum sensors in the field of magnetometry,
as used for example in materials analysis for semi-conductor components or
brain scans. "To this end", say Carsten Schuck, "we want to
integrate a large number of sensors on one chip which can then all be read out
simultaneously, and thus not only register the magnetic field at one place, but
also visualize magnetic field gradients in space."