Chip-scale atomic clock

Last updated
The physics package of the NIST chip-scale atomic clock ChipScaleClock2 HR.jpg
The physics package of the NIST chip-scale atomic clock

A chip scale atomic clock (CSAC) is a compact, low-power atomic clock fabricated using techniques of microelectromechanical systems (MEMS) and incorporating a low-power semiconductor laser as the light source. The first CSAC physics package was demonstrated at the National Institute of Standards and Technology (NIST) in 2003, [1] based on an invention made in 2001. [2] The work was funded by the US Department of Defense's Defense Advanced Research Projects Agency (DARPA) with the goal of developing a microchip-sized atomic clock for use in portable equipment. In military equipment it is expected to provide improved location and battlespace situational awareness for dismounted soldiers when the global positioning system is not available, [3] but many civilian applications are also envisioned. Commercial manufacturing of these atomic clocks began in 2011. [4] The CSAC, the world's smallest atomic clock, is 4 x 3.5 x 1 cm (1.5 x 1.4 x 0.4 inches) in size, weighs 35 grams, consumes only 115 mW of power, and can keep time to within 100 microseconds per day after several years of operation. A more stable design based on the vibration of rubidium atoms was demonstrated by NIST in 2019. [5]

Contents

How it works

Like other caesium atomic clocks, the clock keeps time by a precise 9.192631770 GHz microwave signal emitted by electron spin transitions between two hyperfine energy levels in atoms of caesium-133. A feedback mechanism keeps a quartz crystal oscillator on the chip locked to this frequency, which is divided down by digital counters to give 10 MHz and 1 Hz clock signals provided to output pins. On the chip, liquid metal caesium in a tiny 2 mm capsule, fabricated using silicon micromachining techniques, is heated to vaporize the alkali metal. A semiconductor laser shines a beam of infrared light modulated by the microwave oscillator through the capsule onto a photodetector. When the oscillator is at the precise frequency of the transition, the optical absorption of the caesium atoms is reduced, increasing the output of the photodetector. The output of the photodetector is used as feedback in a frequency locked loop circuit to keep the oscillator at the correct frequency.

Development

The heart of NIST's next-generation miniature atomic clock -- ticking at high "optical" frequencies-- is this vapor cell on a chip, shown next to a coffee bean for scale. 18pml015 clock-chip-with-coffee-bean 2mb.jpg
The heart of NIST's next-generation miniature atomic clock -- ticking at high "optical" frequencies-- is this vapor cell on a chip, shown next to a coffee bean for scale.

Conventional vapor cell atomic clocks are about the size of a deck of cards, consume about 10 W of electrical power and cost about $3,000. Shrinking these to the size of a semiconductor chip required extensive development and several breakthroughs. [6] An important part of development was designing the device so it could be manufactured using standard semiconductor fabrication techniques where possible, to keep its cost low enough that it could become a mass market device. Conventional caesium clocks use a glass tube containing caesium, which are challenging to make smaller than 1 cm. In the CSAC, MEMS techniques were used to create a caesium capsule only 2 cubic millimeters in size. The light source in conventional atomic clocks is a rubidium atomic-vapor discharge lamp, which was bulky and consumed large amounts of power. In the CSAC this was replaced by an infrared vertical cavity surface emitting laser (VCSEL) fabricated on the chip, with its beam radiating upward into the caesium capsule above it. Another advance was the elimination of the microwave cavity used in conventional clocks, whose size, equal to a wavelength of the microwave frequency, about 3 cm, formed the fundamental lower limit to the size of the clock. [6] The cavity was made unnecessary by the use of a quantum technique, coherent population trapping.

Commercialization

The CSAC program achieved a hundredfold size reduction while using 50 times less power than traditional atomic clocks, which led to extensive CSAC use in military and commercial applications. [7] [8] According to an October 2023 report, the CSAC market is expected to grow at a "remarkable" compound annual growth rate (CAGR) from 2023 to 2030. [9] Major commercial players include Microsemi (Microchip Technology), Teledyne, Chengdu Spaceon Electronics, and AccuBeat. [9] [10]

NIST on a chip

Related Research Articles

<span class="mw-page-title-main">Maser</span> Device for producing coherent EM waves in the sub-visible spectrum

A maser is a device that produces coherent electromagnetic waves (microwaves), through amplification by stimulated emission. The term is an acronym for microwave amplification by stimulated emission of radiation. First suggested by Joseph Weber, the first maser was built by Charles H. Townes, James P. Gordon, and Herbert J. Zeiger at Columbia University in 1953. Townes, Nikolay Basov and Alexander Prokhorov were awarded the 1964 Nobel Prize in Physics for theoretical work leading to the maser. Masers are used as the timekeeping device in atomic clocks, and as extremely low-noise microwave amplifiers in radio telescopes and deep-space spacecraft communication ground stations.

<span class="mw-page-title-main">Microwave</span> Electromagnetic radiation with wavelengths from 1 m to 1 mm

Microwave is a form of electromagnetic radiation with wavelengths shorter than other radio waves but longer than infrared waves. Its wavelength ranges from about one meter to one millimeter, corresponding to frequencies between 300 MHz and 300 GHz, broadly construed. A more common definition in radio-frequency engineering is the range between 1 and 100 GHz, or between 1 and 3000 GHz . The prefix micro- in microwave is not meant to suggest a wavelength in the micrometer range; rather, it indicates that microwaves are small, compared to the radio waves used in prior radio technology.

<span class="mw-page-title-main">Second</span> SI unit of time

The second is the unit of time in the International System of Units (SI), historically defined as 186400 of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each. "Minute" comes from the Latin pars minuta prima, meaning "first small part", and "second" comes from the pars minuta secunda, "second small part".

<span class="mw-page-title-main">Optical spectrometer</span> Instrument to measure the properties of visible light

An optical spectrometer is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials. The variable measured is most often the irradiance of the light but could also, for instance, be the polarization state. The independent variable is usually the wavelength of the light or a closely derived physical quantity, such as the corresponding wavenumber or the photon energy, in units of measurement such as centimeters, reciprocal centimeters, or electron volts, respectively.

<span class="mw-page-title-main">Frequency standard</span> Stable oscillator used for frequency calibration or reference

A frequency standard is a stable oscillator used for frequency calibration or reference. A frequency standard generates a fundamental frequency with a high degree of accuracy and precision. Harmonics of this fundamental frequency are used to provide reference points.

<span class="mw-page-title-main">Rubidium standard</span> Frequency standard

A rubidium standard or rubidium atomic clock is a frequency standard in which a specified hyperfine transition of electrons in rubidium-87 atoms is used to control the output frequency.

<span class="mw-page-title-main">Klystron</span> Vacuum tube used for amplifying radio waves

A klystron is a specialized linear-beam vacuum tube, invented in 1937 by American electrical engineers Russell and Sigurd Varian, which is used as an amplifier for high radio frequencies, from UHF up into the microwave range. Low-power klystrons are used as oscillators in terrestrial microwave relay communications links, while high-power klystrons are used as output tubes in UHF television transmitters, satellite communication, radar transmitters, and to generate the drive power for modern particle accelerators.

The Primary Atomic Reference Clock in Space or PARCS was an atomic-clock mission scheduled to fly on the International Space Station (ISS) in 2008, but cancelled to make way for the Vision for Space Exploration. The mission, to have been funded by NASA, involved a laser-cooled caesium atomic clock, and a time-transfer system using Global Positioning System (GPS) satellites. PARCS was to fly concurrently with the Superconducting Microwave Oscillator (SUMO) a different type of clock that was to be compared against the PARCS clock to test certain theories. The objectives of the mission were to have been:

Louis Essen OBE FRS(6 September 1908 – 24 August 1997) was an English physicist whose most notable achievements were in the precise measurement of time and the determination of the speed of light. He was a critic of Albert Einstein's theory of relativity, particularly as it related to time dilation.

<span class="mw-page-title-main">Real-time clock</span> Circuit in a computer that maintains accurate time

A real-time clock (RTC) is an electronic device that measures the passage of time.

<span class="mw-page-title-main">Gunn diode</span> Form of diode

A Gunn diode, also known as a transferred electron device (TED), is a form of diode, a two-terminal semiconductor electronic component, with negative differential resistance, used in high-frequency electronics. It is based on the "Gunn effect" discovered in 1962 by physicist J. B. Gunn. Its main uses are in electronic oscillators to generate microwaves, in applications such as radar speed guns, microwave relay data link transmitters, and automatic door openers.

<span class="mw-page-title-main">History of watches</span>

The history of watches began in 16th-century Europe, where watches evolved from portable spring-driven clocks, which first appeared in the 15th century.

<span class="mw-page-title-main">NIST-F1</span> Atomic clock

NIST-F1 is a cesium fountain clock, a type of atomic clock, in the National Institute of Standards and Technology (NIST) in Boulder, Colorado, and serves as the United States' primary time and frequency standard. The clock took less than four years to test and build, and was developed by Steve Jefferts and Dawn Meekhof of the Time and Frequency Division of NIST's Physical Measurement Laboratory.

Nanophotonics or nano-optics is the study of the behavior of light on the nanometer scale, and of the interaction of nanometer-scale objects with light. It is a branch of optics, optical engineering, electrical engineering, and nanotechnology. It often involves dielectric structures such as nanoantennas, or metallic components, which can transport and focus light via surface plasmon polaritons.

<span class="mw-page-title-main">Hydrogen maser</span> Device used as a frequency standard

A hydrogen maser, also known as hydrogen frequency standard, is a specific type of maser that uses the intrinsic properties of the hydrogen atom to serve as a precision frequency reference.

An atomic fountain measures an atomic hyperfine transition by letting a cloud of laser-cooled atoms fall through an interaction region under the influence of gravity. The atomic cloud is cooled and pushed upwards by a counter-propagating lasers in an optical molasses configuration. The atomic transition is measured precisely with coherent microwaves while the atoms pass through the interaction region. The measured transition can be used in an atomic clock measurement to high precision.

<span class="mw-page-title-main">Opto-electronic oscillator</span> Circuit which produces an optical or electronic sine wave signal

In optoelectronics, an opto-electronic oscillator (OEO) is a circuit that produces a repetitive electronic sine wave and/or modulated optical continuous wave signals.

<span class="mw-page-title-main">Atomic clock</span> Clock that monitors the resonant frequency of atoms

An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions between such states they interact with a very specific frequency of electromagnetic radiation. This phenomenon serves as the basis for the International System of Units' (SI) definition of a second:

The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency, , the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s−1.

The Dick effect is an important limitation to frequency stability for modern atomic clocks such as atomic fountains and optical lattice clocks. It is an aliasing effect: High frequency noise in a required local oscillator (LO) is aliased (heterodyned) to near zero frequency by a periodic interrogation process that locks the frequency of the LO to that of the atoms. The noise mimics and adds to the clock's inherent statistical instability, which is determined by the number of atoms or photons available. In so doing, the effect degrades the stability of the atomic clock and places new and stringent demands on LO performance.

John Kitching is a British–Canadian–American physicist and inventor, and a fellow and group leader at the National Institute of Standards and Technology. His research focuses on the development of compact "chip-scale" devices such as atomic clocks and magnetometers.

References

  1. Knappe, Svenja; Shah, Vishal; Schwindt, Peter D. D.; Hollberg, Leo; Kitching, John; Liew, Li-Anne; Moreland, John (2004-08-30). "A microfabricated atomic clock". Applied Physics Letters. 85 (9): 1460–1462. Bibcode:2004ApPhL..85.1460K. doi:10.1063/1.1787942. ISSN   0003-6951. S2CID   119968560.
  2. Leo Hollberg and John Kitching, Miniature frequency standard based on all-optical excitation and a micro-machined containment vessel, US Patent 6,806,784 B2. , retrieved 2018-10-10
  3. "Miniaturized Atomic Clock to Support Soldiers In Absence of GPS". Defense-Aerospace.com. Archived from the original on 2018-10-11. Retrieved 2020-04-19.
  4. Jones, Willie D. (March 16, 2011). "Chip-Scale Atomic Clock". IEEE Spectrum. Inst. of Electrical and Electronic Engineers. Retrieved February 2, 2017.
  5. "NIST Team Demonstrates Heart of Next-Generation Chip-Scale Atomic Clock".
  6. 1 2 Kitching, John (2018). "Chip-scale atomic devices". Applied Physics Reviews. 5 (3): 031302. Bibcode:2018ApPRv...5c1302K. doi: 10.1063/1.5026238 . ISSN   1931-9401.
  7. "Success Story: Chip-Scale Atomic Clock". NIST. 2020-12-02.
  8. "An Assessment of Four Divisions of the Physical Measurement Laboratory at the National Institute of Standards and Technology". National Research Council. 2018. Retrieved June 7, 2024.
  9. 1 2 "Global Chip-Scale Atomic Clock (CSAC) Market By Type (Size : below 4.2 cm, Size : 4.2 cm-4.5 cm), By Application (Military, Commercial), By Geographic Scope And Forecast". verifiedmarketreports.com. October 2023. Retrieved June 5, 2024.
  10. "Chip Scale Atomic Clock (CSAC) | Microsemi". www.microsemi.com. Retrieved 2018-10-08.