What are chip-scale QUANTUM SENSORS? How do we realize the physical-core of these sensors? Why small matters? How do we interact with the atoms? What are FREQUENCY COMBS and micro-COMBS? How are these realized? And how are these related to atomic clocks and sensing? RECENT RESEARCH! Some highlights of our recent research, and some read recommendations.
Quantum Sensors Top Frequency Comb Top Research Words TOP
What are chip-scale quantum sensors?

Chip-scale Quantum Sensors

In the heart of atomic vapor-based sensors are atomic vapor cells. The requirement to isolate the atoms from the environment as much as possible makes necessary the use of evacuated cells, containing only the required atoms. Traditionally, this has been achieved using glass blowing techniques. Recent years have introduced the ability to fabricate micromachined vapor cells. Using advanced lithography and bonding techniques allow mass-production, scalability, integration with photonics and electronics and arbitrary geometrical control. Anodic Bonded Cells

(small) size matters!
Heavy Large Device We can replace large systems with reduced SWAP(*) systems and enable new applications

Examples? Telecommunication, medical, space
(*SWAP: Size, Weight and Power)
Reduced SWAP
Reduced SWAP But that is not all! We effectively have a new “material” or building block. It may have new properties, and can be hybridized with other building blocks, perhaps from other disciplines (biology, chemistry etc.) to achieve new (and often unpredictable) applications Hybridization
Last and not least – We gain a basic understanding regarding the physics of light-vapor interactions in these dimensions. This will enable us to understand the very fundamental limits of chip-scale quantum sensors and enable new types of sensors. Equations

In the heart of these sensors are atomic vapors:

Quantum mechanics provides us a very comprehensive picture of the inner electronic dynamics of atomic systems. This understanding has evolved over the years to the point where we have an excellent understating of how external fields (light, radio-frequency and magnetic fields) interact with electronic levels in an atomic medium. As a result we can:
Probe the atomic energy states: energy difference, populations and superpositions Control lights degree of freedom: Amplitude, polarization, phase, timing Detect: use the above mentioned techniques to measure electric fields, magnetic fields and the atoms transition frequency
probe atomic state Control light with atoms probe fields

Frequency combs and the emergence of micro-combs:

The emergence of Optical Frequency Combs (OFCs) has cast a huge impact on the field of metrology. In fact, half the Nobel prize in physics was awarded in 2005 (Hall & Hansch) for their work on laser-based precision spectroscopy and the optical frequency comb technique. A frequency comb, compromising of series of equidistant, discrete frequency lines can phase coherently link between microwave or radio-frequency and optical frequencies. This can be viewed as a gear-box, that has a fast wheel ticking at the optical frequency (often a few hundred of THz) steering a slower turning wheel ticking at microwave frequency (often a few hundred of MHz).

Microresonator based optical frequency combs (microcombs) have recently initiated a new era of chip-scale frequency combs providing a compact platform for low-noise, microwave-rate, and low-power frequency combs. Pumped with a continuous wave laser, stable soliton pulses (in time domain; a frequency comb is created in frequency domain) can be generated in a microresonator, exploiting the Kerr non-linearity in the hosting material. As a result, a surge of applications and demonstrations have been reported including optical-frequency synthesis and division, optical clocks and ranging.

Pictures, and examples about the difference berween radio-frequency and optical atomic clocks are COMING SOON!

Recent Research
Here, we explore direct microcomb atomic spectroscopy. Fine and simultaneous repetition rate and carrier-envelope-offset frequency control of the soliton enables direct hyperfine spectroscopy. Moreover, the entire set of microcomb modes are stabilized to this atomic transition with kHz-level stability. Direct Kerr-comb atomic spectroscopy
Read more: L. Stern, J. Stone, S. Kang, D. C. Cole, M. Suh, C. Fredrick, Z. Newman, K. Vahala, John Kitching, S. Diddams, and S. Papp. 2020. “Direct Kerr frequency comb atomic spectroscopy and stabilization.” Science Advances, 6, 9, Pp. eaax6230
Here, we fabricate and demonstrate chip-scale, diffractive optical elements which map atomic states to the spatial distribution of diffracted light. Such SMART CELLS can be used as highly compact offset-locked frequency references and magnetic gradient sensors. Atomic Diffractive Gratings
L. Stern, D. Bopp, S. Schima, V. Maurice, and J. Kitching. 2019. “Chip-scale atomic diffractive optical elements.” Nature Communications, 10, 1, Pp. 3156.
Additional reading:

A few great reviews related to the fields of chip-scale metrology, micro-combs, atomic clocks and magnetometery in general. This section will be continuously updated.

John Kitching "Chip-scale atomic devices" Applied Physics Reviews 5, 031302 (2018);

Budker, D., Romalis, M. "Optical magnetometry." Nature Phys 3, 227–234 (2007).

To get a feel about optical-clocks (all-though not chip-scale):

Optical atomic clocks A. Ludlow, M. M. Boyd, J. Ye, E. Peik, and P. O. Schmidt Rev. Mod. Phys. 87, 637 – Published 26 June 2015

Micro combs!
Photonic-chip-based frequency combs, Alexander L. Gaeta, Michal Lipson & Tobias J. Kippenberg Nature Photonics volume 13, pages158–169(2019)

First demonstration of micro-comb based optical atomic clocks:

Z. Newman, V. Maurice, ,..., L. Hollberg, K. J. Vahala, K. Srinivasan, S. Diddams, J. Kitching, S. Papp, and M. Hummon, "Architecture for the photonic integration of an optical atomic clock" Optica Vol. 6, Issue 5, pp. 680-685 (2019)

S. Papp, K. Beha, P. Del’Haye, F. Quinlan, H. Lee, K. Vahala, and S. Diddams, "Microresonator frequency comb optical clock" Optica Vol. 1, Issue 1, pp. 10-14 (2014) COMBS CLOCKS