Quantum systems are uncertain by nature. By 'squeezing' this uncertainty, physicists can make better measurements of quantities such as distance. But overdoing it makes things burst out all over the place. At the leading edge of experimental science, the latest measurement techniques are promising to provide breakthroughs in our understanding of the Universe. The ever-improving ability to sense small displacements, for example, is at the heart of projects such as the Laser Interferometer Gravitational Wave Observatory (LIGO), which seeks to observe the faint space-time ripples of distant supernovae. When technical noise is strongly suppressed, the ultimate limit to the precision of any measurement is set by the quantum uncertainty in the measuring system. But even this quantum uncertainty can be reduced - a technique known as 'squeezing'. On page 67 of this issue, Shalm et al. show that squeezing down this quantum uncertainty is not as simple as might be expected - too much squeezing actually worsens measurement precision. Fortunately, they also show that it is possible to recover the best precision allowed by the laws of physics by looking at the 'over-squeezed' system in a different way. That a fundamental limit to measurement precision exists at all is a purely quantum phenomenon. Consider light, the basis of a suite of sensitive interferometric measurement techniques. In the classical picture, light is a wave whose amplitude and phase - where the wave's peaks and troughs lie - can be specified with infinitesimal precision. But in reality, light has much more character. It is made up of indivisible photons that exhibit probabilistic behaviour when forced to decide which quantum state, out of a range of options presented, to be in.
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