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NIST Team Shrinks Photonic Chip For Precision Spectroscopy

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A prototype photonic chip developed by a
research team at the US National Institute of Standards and Technology
(NIST) could provide the basis for a future generation of extremely
compact quantum sensors.

The researchers, working under the “NIST-on-a-chip" initiative,
say they have created a chip upon which near-infrared laser light
interacts with a tiny cloud of atoms, the basis for measuring quantities
such as length with quantum precision.

They also believe that current weaknesses in the design can be
overcome with some minor tweaks, and that tiny precision spectrometers
based on their approach could be mass-produced using existing
semiconductor wafer manufacturing technology.

Length reference

Describing the work in the journal Optica, Matt Hummon and colleagues report that the NIST prototype generates 780 nm light with sufficient precision (10-11 instability) to act as a length reference for calibrating other instruments through interaction with rubidium atoms.

“This device harnesses the benefits of both photonic integration and
precision spectroscopy for the next generation of quantum sensors and
devices based on atomic vapors," wrote the team in their paper.

And although the stability of the current prototype is limited by
thermal drifts of etalons created by back-reflections in the chip’s
waveguide, that could be overcome by using a larger probe beam, or a
narrower optical transition.

“Thus, we expect that future photonic chip designs that reduce the
etalons generated by the back-reflections in the waveguide will lead
directly to improved frequency stability," concludes the team.
“Wafer-level mass fabrication of both the micro-machined vapor cells and
the photonic chips may lead to inexpensive devices based on precision
spectroscopy of warm vapors."

While a quantum sensor based around an atomic vapor typically requires fridge-sized equipment, recent developments in the UK
have shown that it is possible to shrink that volume down by an order
of magnitude, suitable for integration within a “CubeSat" format.

The NIST prototype takes that much further, packing the atom cloud
and structures for guiding light into an area of less than a square
centimeter: a remarkable four orders of magnitude smaller than current
macro-sized sensors offering similar measurement precision.

The reported chip measures just 14 mm long by 9 mm wide, with its
silicon nitride waveguides able to handle a wide range of light
frequencies.

“Compared to other devices that use chips for guiding light waves to
probe atoms, our chip increases the measurement precision a
hundred-fold," added Hummon. “Our chip currently relies on a small
external laser and optics table, but in future designs we hope to put
everything on the chip."

Novel waveguide and grating

Applications of such a device could include measuring quantities such as
time, length and magnetic field strength, with potential for deployment
in navigation, communications, and medical equipment.

Critical to the tiny structure are a novel waveguide and grating,
which expand the beam diameter to excite approximately 100 million atoms
into a higher energy level.

In the current design, the NIST team used rubidium atoms, but in
principle the approach could work with a wide range of atomic and
molecular vapors to generate specific frequencies across the visible and
near-infrared spectrum.

Perhaps most significantly, the chip is unlike current high-precision
sensors in that it could be mass-produced in a similar manner to a
semiconductor wafer. Conventional systems instead require manual
assembly of bulky optics and blown-glass vapor cells, according to NIST
group leader John Kitching.

The NIST paper indicates frequency metrology with a precision error
of 1 part error in 10 billion at 100 seconds – with that performance
verified by comparison with a separate NIST frequency comb.

“This performance level is very good for something so small, although full-scale lab instruments are more precise," Kitching said.

Getting out of the lab

The “NIST-on-a-chip" program is intended to move new metrology
technologies out of the preserve of the specialist laboratory, and into
commercial markets where they can be manufactured by the private sector,
and deployed directly by users.

“To
that end, we are developing a suite of intrinsically accurate,
quantum-based measurement technologies intended to be deployed nearly
anywhere and anytime, performing uninterrupted without the need for
NIST’s traditional measurement services," states NIST.

“They will enable users to make precision measurements referenced to
the International System of Units (SI) on factory floors, in hospital
diagnostic centers, in commercial and military aircraft, in research
labs, and ultimately in homes, automobiles, and personal electronic
devices, among others."

With projects across 12 different technology areas planned, photonics
technology is a key element in many. Among the novel ideas are
“photonic thermometers", which would sense changes in the size and
thermal profile of an object to measure temperature, in place of a
traditional mercury instrument.


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