In the quantum world, uncertainty is everywhere. No particle is ever completely still, no vacuum completely empty, no darkness completely devoid of light, no measurement perfectly precise.
But quantum uncertainty can often be shifted between pairs of variables. The Heisenberg uncertainty principle places a lower bound not on the uncertainty of a particle’s position nor its momentum, but on the product of the two. It’s possible, therefore, to reduce one of those uncertainties at the expense of increasing the other.
Likewise, electromagnetic waves are subject to quantum uncertainty, but the uncertainty needn’t be evenly distributed over the wave period. One can shift the uncertainty from the part of the wave cycle one wants to measure to a part that’s irrelevant to the measurement. Those so-called squeezed states of light can be realized by nonlinear optical devices.
Squeezed light has been used to heighten LIGO’s (Laser Interferometer Gravitational-Wave Observatory’s) sensitivity to gravitational waves (see Physics Today, November 2011, page 11, and the Quick Study by Sheila Dwyer, Physics Today, November 2014, page 72)—but beyond that, its portfolio of practical uses is thin. Now researchers at the HAYSTAC experiment (the Haloscope at Yale sensitive to axion cold dark matter) are using squeezed light to speed their search for axions, one of the hypothetical particles that could make up dark matter.
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