Haloscopes
Haloscopes are the most promising mechanism for detecting dark matter axions. Because the mass of the axion is so small, it’s signal in the detector would be very weak. This means that haloscopes must operate at cryogenic temperatures to reduce background noise which would otherwise drown out the axion signal.
Haloscopes are composed of a cavity with a certain known resonant frequency, permeated by a magnetic field. If an axion produces a signal with the same frequency as the cavity, then the signal will be easier to detect. The mass of the axion determines the frequency of its signal. The HAYSTAC experiment has a tunable cavity that can be adjusted to many resonant frequencies, allowing us to scan for axions in a certain frequency range.
Beating the Quantum Limit
Because the axion signal would be so weak, it would be easily drowned out by other sources of noise. The HAYSTAC experiment must combat three sources of noise in order to detect an axion signal: thermal noise, noise inherent to the aparatus’s elecronics and other hardware, and the quantum noise. Thermal noise has been largely suppressed by operating at cryogenic temperatures. Unfortunately, quantum noise is inherent to nature and simply cannot be reduced. However, the Yale HAYSTAC team has devised a way to conquer the quantum limit using a method called “quantum squeezing.”
Quantum noise can be shown to originate from our inherent uncertainties on measurable values, dictated by the Heisenberg Uncertainty Principal. Our cavity’s signal can be broken down into two components: the X-quadrature and the Y-quadrature. The Uncertainty Principal prevents us from simultaneously knowing both of these quadratures to maximum precision, giving rise to noise in our signal measurements. However, by amplifying one of these quadratures and “squeezing” the other, we maximize the noise in the amplified quadrature and minimize the noise in the squeezed quadrature. This makes the squeezed quadrature nearly noiseles while sacrificing our measurement of the amplified quadrature. By measuring only the squeezed quadrature, we can beat the quantum limit.
Quantum squeezing improves the signal’s visibility over a larger range, which also improves the rate at which we can search frequency space for an axion signal. Without squeezing, we would need to spend more time taking data at each point in frequency space to be able to “pick up” the axion signal amongst the noise. This method can save the HAYSTAC experiment years of data taking.