Aaron Chou's Homepage

 Scientist, Fermilab Analysis Chair, ADMX-G2 Co-spokesperson, Holometer former co-spokesperson, GammeV Curriculum vitae

Research on Dark Matter Axions

I currently serve as Analysis Chair for the ADMX-G2 experiment, responsible for coordinating all science data analyses within the collaboration.  ADMX (Axion Dark Matter Experiment) is the first experiment to date to reach sensitivity to the dark matter axion -- a particle predicted by the Peccei-Quinn model to simultaneously explain the apparent vanishing of the neutron electric dipole moment and explain the composition of cosmologically-produced dark matter.  ADMX is essentially the world's lowest noise radio, able to see radiofrequency signal power as low as 10-23 W.  It uses a microwave cavity as a high quality factor, tunable antenna and reads out the tiny signals using quantum-limited amplifiers.  Over the next couple of years, ADMX will slowly tune the radio from 600 MHz - 2 GHz (corresponding to 2.4 - 8 micro-eV axion mass) to search for the broadcast frequency of the galactic dark matter.

(Left) A day in the life of the ADMX collaboration, working on the apparatus at the University of Washington site.  (Right) Extracting the ADMX cavity from the cryostat.  The cavity and preamplifiers must be cooled to 100 mK temperature in order to reduce the thermal blackbody contribution to the background noise.

(Left) Ultra-cold, low mass, dark matter axions in our galactic halo has a high number density (1014/cc) and behave collectively as a single mode oscillator with milli-second coherence times comparable to a modern solid state laser.  Semi-classical techniques can be used to detect this classical dark matter wave.  (Right)  In contrast, the WIMP dark matter is heavier and has a much lower number density (1/liter).  It therefore scatters as individual particles.

The physics of detecting the classical axion wave is exactly the same as that of two coupled pendula.  In this demonstration, I have attached refrigerator magnets to two of the balls of a Newton's cradle.  This creates a weak repulsive interaction between these two oscillators.  One pendulum represents the dark matter axion wave, and the other pendulum represents the photon mode inside the microwave cavity.  Because both oscillators have the same frequency, determined only by the strength of gravity and the length of the pendula, the same weak repulsive force always comes back at exactly the right moment in the oscillation period to coherently add to the momentum that had been previously transferred.  For infinite coherence time, all of the energy stored in one oscillator would be eventually transferred to the other oscillator (and back again).   Note that this is also exactly the same physics as Rabi oscillations or neutrino oscillations.  In real life, the coherence time is not infinite and in an axion experiment attempting to detect an ultra-weak coupling between the two oscillators, one has to be able to determine whether a single quantum of oscillation has been transferred from the axion field to the photon field.  This requires quantum-limited amplifiers.

(Left) In this polar plot of amplitude and phase, the small cyan disk indicates the quantum of phase space ΔX×ΔP ≥ 1/2 given by the Heisenberg uncertainty principle for an arbitrary quantized harmonic oscillator, in our case the photon mode inside the microwave cavity.  X is the potential energy degree of freedom and P is the kinetic energy degree of freedom of the oscillator.  The corresponding zero point energy along with the quantum back reaction caused by amplification during the measurement process creates the standard quantum limit (SQL) readout noise which limits the sensitivity of contemporary dark matter axion searches.  To avoid the SQL noise, we are applying quantum non-demolition (QND) measurement technology from Professor David Schuster's quantum computing lab to create ultra-low noise single microwave photon detectors.  These detectors measure only amplitude and put the back reaction completely into the conjugate operator -- the unobserved phase of the oscillator.  After a sequence of QND measurements, the oscillator state is described by the blue annulus in phase space in which the photon occupation number is now very well determined with much lower uncertainty in the radial (amplitude) direction than that of the SQL noise represented by the cyan disk.  This large reduction in readout noise vastly improves the sensitivity for detecting rare axion-to-photon conversions.
(Right) Graduate student Akash Dixit installs a QND detector prototype onto a milliKelvin dilution refrigerator in the Schuster Lab at UC.

Research on Space-time Information Capacity

I serve as co-spokesperson of the Holometer experiment (FNAL E-990), which attempts to determine whether there is any fundamental limit to the number of significant digits that one can extract from an empirical measurement. In principle, if one can make a measurement of any physical observable N times while incurring negligible quantum back reaction or other accumulated systematic error, then the knowledge of the mean value improves by a factor of $\sqrt{N}$ from averaging down the individual measurement errors.

(Left) A Google Maps view of the Holometer  -- a pair of co-located 40m long Michelson interferometers. Each interferometer is a ultra-sensitive microphone able to detect seismic and acoustic motions at the level of 10-18 m/rtHz.  At extremely high frequencies above 1 MHz, vibrations are highly suppressed by the inertial mass of the apparatus and so there should be no detectable motion.  The Holometer is designed to measure this predicted motion amplitude of zero to a large number of significant digits.  The noise in individual measurements can be conveniently averaged away by cross-correlating the signals from the two interferometers and averaging the resulting cross-spectrum over a large number of measurements.  (Right) Graduate student Lee McCuller operating the two interferometers at kW power levels of stored laser light.  This gives 1022 measurements per second using the 1064 nm laser photons.