School of Physics

Frequency and Quantum Metrology Research Group

Dr McFerran and the Yb ClockWe aim to build instruments with world-class precision and performance that we can use to make measurements of high value and interest in both fundamental physics and more practical applications.

Many modern developments in today's society are based on high-quality clocks and oscillators: the Global Positioning System (GPS) satellite system, radar, optical fibre communications, even mobile phones. The group's goal is to develop new frequency standards and technologies with two endpoints in mind: to improve systems that are based on high-quality clocks and oscillators (such as those listed above), and to use these as precision tools to test the foundations of physics.

We are dedicated to commercialising our inventions and thus hold patents in conjunction with industry. Our research programs include strong international and industrial collaborations.

Our group undertakes research projects that cover a broad spectrum of interests, ranging from engineering to fundamental physics. Some of our projects include:

  • Quantum Metrology within the ARC Centre of Excellence for Engineered Quantum Systems (EQuS).
  • Advancing the Cryogenic Sapphire Oscillator – one of the world’s most stable frequency sources.
  • The Ytterbium Lattice Clock.
  • Space applications: Ground Station for the European Space Agency's ACES mission.
  • Low noise frequency and phase synthesis and measurement techniques.
  • Testing Lorentz invariance by measuring speed of light isotropy, in collaboration with Humboldt University of Berlin.
  • Measurement of electronic and magnetic properties of materials.
  • Novel high-Q microwave and millimetre wave resonators.
  • Laboratory based searches for Weakly Interacting Slim Particles.

 

Main contact Professor Michael Tobar.

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Yb Lattice Clock

A ground based Yb lattice clock for participation in future space-clock missions.

Photo of the UWA Yb Clock 

 

Industry, defence and commerce all depend on accurate time keeping, and as clock technologies have improved they have unveiled capabilities such as the global positioning system (GPS) and very long baseline interferometry (VLBI). Atomic clocks have been at the forefront of physics for several decades and now they probe nature’s behaviour at the most fundamental level — in ways comparable in significance to high energy collider experiments, by searching for temporal changes in fundamental constants. Rather than rely on extraordinarily high energies, atomic spectroscopy relies on extraordinary levels of precision. The improvement in accuracy of clocks is advancing at approximately a factor of 500 per decade (compared to ~13 for Moore’s law). This rapid scientific advance suggests a certain inevitability with regard to finding new phenomena.

Recent measurements in various laboratories have shown astonishingly high accuracy for a number of different clock transition frequencies and ratios. These clocks are isolated in separate laboratories around the globe, thus the means to compare the clocks is heavily sought. With future space-clock missions such as the Atomic Clock Ensemble in Space  (ACES) mission and possible future missions, such as Space Optical Clock (SOC) and STE-QuEST (Space-Time Explorer and Quantum Equivalence Principle Space Test), a much greater opportunity will be granted for frequency comparisons between clocks distributed around the earth.

At UWA we are developing an optical lattice clock based on a particular electronic transition in 171Yb, with the aim of participating in future space-clock missions. Such clocks  have already demonstrated outstanding performance (e.g. at NIST, NMIJ, RIKEN & KRISS). The 171Yb ‘clock’ transition is included in the CIPM’s  (Comité International des Poids et Mesures) list of secondary representations of the second. 

Progress on the Yb lattice clock

The UWA atomic clock was first put into operation after 2.5 years of development (the first of its kind in the southern hemisphere). By comparison with a hydrogen-maser, its accuracy is in the parts per trillion range.

The experiment has also accomplished the following.
  1. A dual-wavelength magneto-optical trap (MOT) has been installed that laser cools and traps neutral ytterbium atoms. The device uses unconventional laser wavelengths at 399nm and 556nm. Both require moderately complex schemes for their generation. The temperatures achieved in the dual-wavelength MOT are 20 μK for (171)Yb (fermionic) and 40 μK  for (172)Yb (bosonic). We use a new, cost effective, means of generating the 556nm radiation (Applied Physics B publication).
  2. The absolute frequency of the 1S0-3P1(F’=3/2) inter-combination line in 171Yb has been measured to be 539390406833 ±310 kHz. The frequency separation between this line and the clock transition is found to be 21094570280 ± 36 (stat.) ±310 (syst.) kHz  (J. Phys. B article).
  3. An inverted crossover resonance has been observed in saturated absorption spectroscopy of 171Yb for the first time. This may also be the first occasion where the effect has been seen in a group II atom. We have used the signal to stablize the frequency of 556nm light needed for laser cooling of 171Yb and demonstrated a temperature of 20 μK (JOSA B article).
  4. A frequency comb has been generated to cover the relevant wavelengths of the (lattice) clock; for example: 1156nm, 1112nm and 759nm. The mode-locked laser light from a master oscillator (centred at 1550nm) has been amplified and coupled into a section of highly nonlinear fibre (high step index) to extend the wavelength range to below 1100nm. The frequency comb mode spacing is steered by a hydrogen maser.
  5. An ultra-stable laser at 1156nm has been set up, including a 4×105 finesse optical cavity inside a vacuum chamber with surrounding thermal shields.  This laser, when frequency doubled, probes the clock transition in the Yb atoms (at 578.4nm).  The drift rate of the laser is presently 60mHz/s  (4×10-16 s-1), and steadily falling.  We perform the clock transition spectroscopy with a Rabi frequency of ~6kHz.

Publications:

  1. Sub-Doppler cooling of ytterbium
  2. Further sub-Doppler cooling of Yb
  3. Injection locking for the yellow-green spectrum
  4. Absolute frequency measurements of the intercombination line in 171Yb
  5. An inverted crossover resonance in 171Yb aiding cooling to 20 μK

 

Links to interviews with Dr McFerran about atomic clocks:

http://www.radionz.co.nz/national/programmes/nights/audio/201767369/even-more-atomically-accurate
http://www.abc.net.au/radionational/programs/scienceshow/pushing-time-accuracy-to-the-limit/5926460
http://www.abc.net.au/science/audio/2013/10/16/3869638.htm?topic=space 
(The interview is about 10min in).
A candid discussion about atomic clocks

Contact: ARC Future Fellow John McFerran  (08 6488 3481)

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Laboratory Searches for Weakly Interacting Slim Particles

Many attempts to expand the Standard Model of particle physics via string theories or supersymmetry theories inevitably predict the existence of new particles that we have yet to observe. One such family of these hypothetical particles are the Weakly Interacting Slim Particles, or WISPs. As the name suggests, not only do WISPs interact very weakly with standard matter but they also have extremely small masses of less than 1 electronvolt. For comparison, a proton has a mass on the order of one billion electronvolts. What this means is that on an intrinsic level WISPs are remarkably difficult to detect and measure. It is these same properties that also allow WISPs to be formulated as compelling dark matter candidates.

The primary focus of our work is designing and performing experiments to search for a type of WISP known as the Hidden Sector Photon, or Paraphoton. This particle is coupled very weakly to the standard photon and does not interact with standard matter. We aim to constrain the strength of this photon / paraphoton coupling as a function of paraphoton mass.

One of our main experiments is known as a “Light Shining Through a Wall” experiment. Photons are generated on one side of an impenetrable barrier; in order to cross this partition the photon would need to convert to a paraphoton and pass through. By searching for photons on the other side of the barrier we can attempt to detect photon to paraphoton to photon conversion events. In order to reduce the level of background noise present in the detector the experiment is performed at cryogenic temperatures (approximately -270 C). As usual, we employ a variety of experimental tricks to enhance the sensitivity of our experiment and we are always applying the advances in measurement techniques we develop to other areas of research.

In addition to our work with Hidden Sector Photons we also collaborate with researchers at the University of Washington to try and improve the range and reach of their remarkable Axion Dark Matter eXperiment.

Contact: Stephen Parker

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