Physics of the Atomic Clock - University of Yorkjke1/Atomic_Clocks/Papers/Atomic_Cl… · The atomic basis of the cesium atomic clock is the hyperfine splitting of the 6 S 1/2 state

04/24/071

Physics of the Atomic ClockPhysics of the Atomic ClockMike Cohler

04/24/07 Mike Cohler2

Introduction Introduction The atomic clock is based on atomic

physicsWe will explore the physics firstThen we will see how this is implemented

to result in a working deviceWe will look at the latest high accuracy

time standards


Basic ideas – magnetic resonanceBasic ideas – magnetic resonance First progress occurred in the 1930’s at

Columbia where I.I. Rabi invented nuclear magnetic resonance. He won the 1944 Nobel Prize for this work.http://nobel.se/physics/laureates/1944/press.html

He suggested to his students that the precision of the frequency of these resonances was so great that they could be used as a basis for a clock of extraordinary accuracy but did not pursue this idea


Basic ideas – magnetic resonanceBasic ideas – magnetic resonance Hyperfine structure

is the splitting in atomic energy levels when the coupling of the spin of the nucleus is included.

In a classical picture the nuclear spin can precess about the magnetic field at a specific frequency


Basic ideas – magnetic resonanceBasic ideas – magnetic resonance Magnetic resonance

used a radio frequency field to cause electrons in an atomic beam to absorb energy if the frequency matched that of the precession of spins aligned in a magnetic field.


Basic ideas – nonclassical viewBasic ideas – nonclassical view In a quantum picture the atomic energy levels

split. The splitting for a spin of ½ is given by ∆E and

this depends on the magnetic field B, the spin I and the magnetic moment µ

If the energy of photons in a beam passing the atoms matches ∆E then the photons can be absorbed if the atom is in the lower energy state


Using NMR to make a clock Using NMR to make a clock A beam of atoms in one hyperfine state passes

an oscillating magnetic field. A feedback loop adjusts the frequency until the number of atoms transitioning is maximum

Rabi’s student Norman Ramsey suggested in 1949 that atoms pass twice through the field to get more accuracy. This ultimately earned him the 1989 Nobel Prizehttp://nobel.se/physics/laureates/1989/press.html


Using NMR to make a clock Using NMR to make a clock The first atomic clock was established at the

National Physical Laboratory in 1955 by Louis Essen and John V.L. Parry, but this clock required a roomful of equipment

The electronics in 1955 that was required to extract the timing signals from the radio frequency drive and synchronize to a clock was large


Using NMR to make a clock Using NMR to make a clock Originally Ramsey based his clock on a hyperfine

transition in hydrogen – see:http://www.leapsecond.com/history/1965Metrologiav1n1Ramsey.pdf The atomic basis of the cesium atomic clock is the

hyperfine splitting of the 6 S 1/2 state in Cesium leading to a transition frequency of 9193 MHz (or 9.193GHz).

http://www.leapsecond.com/history/1965-Metrologia-v1-n1-Ramsey.pdf


Using NMR to make a clock Using NMR to make a clock There is more detail in the paper at:http://www.iop.org/EJ/abstract/09570233/14/8/301The fuller diagram of the energy levels is shown here.

The system is arranged to populate the F=4 component of the 2S1/2 statebefore the atoms enter a microwave cavity where they transition down to the F=3 state.

http://www.iop.org/EJ/abstract/0957-0233/14/8/301


Using NMR to make a clock Using NMR to make a clock There is detail about the laser cooling and other technical

information in the Canadian clock paper at:http://inmsienm.nrc

cnrc.gc.ca/research/cesium_clock_e.html

The clock signal is obtained electronically according to the simplified schematic to the right


The NIST Cesium fountain clock The NIST Cesium fountain clock The NISTF1 Cesium fountain clock uses

cooled stabilised cesium atomshttp://tf.nist.gov/cesium/fountain.htm

6 IR laser beams push the injected atoms into a ball and then vertical lasers push them upwards about a metre into a microwave cavity. The trip up and back down lasts about 1s. The frequency at resonance is 9,192,631,770 Hz which defines the second.

http://tf.nist.gov/cesium/fountain.htm


NISTF1 detail NISTF1 detail The atoms that absorb the microwave radiation and go into the

upper state, then fluoresce and this fluorescence is detected. When this is a maximum then the system is tuned.

Since the atoms are cooled and observed over a long time (1s) the accuracy of the frequency of the microwave signal is excellent and gave an ultimate stability of 1 part in 1016 in the summer of 2005. This is 1 second in 300 million years.

There is a final year project report on laser cooling and trapping at:

http://webphysics.davidson.edu/alumni/jocowan/Honors.HTM


High accuracy and small size High accuracy and small size As technology progresses one wonders about

the limits of accuracy that can be achieved for time measurement, but also how small can high accuracy clocks be made. Could you have an atomic clock as a wristwatch?


Optical Atomic clocks Optical Atomic clocks NIST has made cesium clocks based on a small cesium

vapour cell with a volume of 1 cm3, and uses a diode laser.http://tf.nist.gov/timefreq/ofm/smallclock/OverallDesign.htm

The diode is modulated at a frequency of half of the groundstate hyperfine splitting frequency. This results in the optical sidebands exciting the ground state hyperfine transition and absorbs the light. (Coherent Population Trapping)


Optical Atomic clocks 2 Optical Atomic clocks 2 The device is shown at right

and the accuracy from measurements such as below is 2.4 x 1010 times τ1/2

τ is the measurement time


Chip scale Atomic clock (CSAS)Chip scale Atomic clock (CSAS) NIST are currently making atomic clocks the size of a

grain of rice about 1 mm3

They can be powered by an AA battery! These are based on Rubidium rather than Cesium atoms

http://www.boulder.nist.gov/timefreq/general/pdf/2011.pdf


SummarySummary Atomic Clocks are based on transitions in the atomic electrons

which are due to hyperfine interactions caused by spin flips in the nucleus

Quantum transitions are initiated by radio, microwave or optical photons interacting with sets of atoms with their atomic states having a suitable population to accept transitions and absorb energy

The energy absorption is used to determine when the frequency is matched to the transition, and then the reference frequency is synchronized electronically to a clock.

Modern atomic clocks are based on specific transitions in Cesium or Rubidium


Where to Get More InformationWhere to Get More InformationSee the web references in the slides above

for this lectureUse a web search to look for “atomic

clock”, “nist” and similar keywordsUse the library and the electronic search

facilities

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Physics of the Atomic Clock - University of Yorkjke1/Atomic_Clocks/Papers/Atomic_Cl… · The atomic basis of the cesium atomic clock is the hyperfine splitting of the 6 S 1/2 state