University of Toronto physicists recently discovered a unique property of quantum physics—the ability to squeeze light to the farthest limit possible. This new research could make way for more precise measurements, advance computer technology, and change how information is processed.
In quantum experiments, light is used to measure variables. However, light also has a defined quantum boundary known as the shot-noise limit. Below this shot-noise level, there is another attainable frontier known as the limit set by the uncertainty principle. The act of squeezing involves passing through the shot-noise limit, reaching the uncertainty principle limit.
At the forefront of experimental science lies a measuring device employed in different research fields. The precision of measuring small changes is defined by its quantum uncertainty. U of T PhD graduate student Krister Shalm notes: “We can ‘squeeze’ this quantum uncertainty to a smaller value for one measurement with the sacrifice of another variable such as speed.” Shalm, Rob Adamson, and Professor Aephraim Steinberg of U of T’s Department of Physics and Centre for Quantum Information and Quantum Control published their work in a recent issue of Nature.
Light, just like matter, is quantized and comes in the form of photons. Frequently, investigators use sources that produce single photons, attempting to squeeze these states. Shalm’s paper posits that, for the first time, triphoton states have been used instead of single photon states.
“If we place three photons that are identical in every aspect except their polarization (or orientation) in an optical fibre, then it is impossible to distinguish between the three and we now call the system a triphoton,” explains Shalm. The system now has its own grouped polarization properties. This type of uniformity is the same property used in modern MRI machines.
The authors then squeezed this unique state to more precisely measure the triphoton’s polarization. As previously thought, these actions passed the shot-noise limit and eventually reached the Heisenberg limit, where squeezing has no effect on the uncertainty of the system. “What’s interesting is that if we squeeze past this limit the uncertainty actually begins to increase,” says Shalm.
This observation is unexplainable if the polarization surface is thought of as the classical flat disk. Instead, the physicists described the surface as spherical. “The uncertainty of these triphotons can be thought of as a balloon on a sphere,” explains Shalm. “As we squeeze the balloon, it initially compresses and the uncertainty is reduced.” He goes on to say that although the uncertainty is reduced in one direction, the balloon starts to wrap around the sphere in the other direction. “The balloon inevitably reaches all the way around increasing the uncertainty of the polarization.”
Furthermore, since the state is triphotonic, “over-squeezing” the system leads to a symmetrical spread of the uncertainty into three equally spaced balloons around the sphere. “These complex quantum states of light can then be represented with a simple spherical map,” notes Shalm.
This research will not only advance the field of quantum physics but other scientific disciplines that concern precise measurements. Specifically, production of super-powered processors and circuit boards found inside computers will be much more efficient.
“Eventually, instead of increasing the power of normal single photon states, these squeezed triphoton states of relatively low intensity can be used much more effectively in circuit boards, cryptography and quantum computing,” says Shalm.
