A new experiment suggests that we may still need to rely on classical light behavior in order to get its quantum behavior under control. A group of researchers has found that, by generating interference between streams of quantum light and classical light, they can produce a high population of entangled photons, all of which occupy only one of two possible states. This technique could come in handy for producing light that can be used to make precise measurements and images on a very small scale.
Recently, researchers have been experimenting with light in "Schrodinger cat states," where a set of photons are all entangled and occupy a single state. A subset of the cat states has been termed "high-NOON states." These involve measuring one of the entangled photons, which ensures that all the other photons in the signal should be in the same state.
One of the challenges of producing a high-NOON state is that photons don't always entangle perfectly, and there will be some that won't cooperate. Scientists have now shown that generating interference between the quantum light and classical light helps eliminate the offending photons and produces a purer signal.
The researchers produced entangled photons using spontaneous parametric down-conversion, a process that splits one photon into a variable number of entangled ones at longer wavelengths. They then interfered that light with a classical beam. Detectors picked up the filtered quantum signals for 2- and 3-photon entanglements 80 to 95 percent of the time. Five-photon entanglement was less frequent at 42 percent, but all of the entanglements had fidelity rates of at least 92 percent, which is quite high.
While the beams' behavior is still not fully predictable, the authors note that larger successful entanglements bring us closer to Schrodinger's vision of macroscopic entanglement, or entanglement of large items. In the shorter term, having better control over light will allow researchers to make more precise measurements that reveal finer details in quantum imaging, lithography, and high-sensitivity interferomertry.