A laser pointer shines a neat spot: all its light waves march in step, like soldiers on parade. Such light is called coherent. But nature and technology also feature another kind of light—'messy' light, where waves are out of sync, like a crowd at a market. It's called partially coherent. For a long time, it was considered just a nuisance, but recently it turned out that in some communication lines it works even better than neat light. For instance, it gets less distorted in optical fibers.
The problem is that until now, such light could only be produced in bulky lab setups. The engineers behind this work have for the first time assembled it on a tiny optical chip — a plate where light runs along transparent tracks, like water through channels. The chip consists of many miniature beam splitters and phase shifters — elements that can split a beam and shift its waves. First, they took two fully independent, 'messy' beams. Then, like traffic controllers at an intersection, they varied how strongly these beams mixed. This way, they obtained light with any desired degree of 'messiness' on the chip — from total chaos to almost perfect order.
To verify the result, the scientists measured parameters akin to the 'positions of hands' of light waves. These measurements showed that the chip exactly reproduces the specified 'messiness'. Previously, such control required entire optical tables with dozens of mirrors and lenses. Now everything fits on a plate the size of a fingernail. This means that in the future, such chips could be embedded directly into transmitters and receivers. Communication would become more reliable, and encryption harder to intercept, because 'messy' light is more difficult to eavesdrop on unnoticed.
Thus, light once considered defective becomes a tool. And if a coherent laser is a military band playing from sheet music, then partially coherent light is a jazz ensemble: it has freedom, but even that can be conducted.
In optical fibers, partially coherent light suffers less from nonlinear distortion — an effect where an overly powerful signal interferes with itself. So a 'messy' beam can travel farther without amplifiers.
In Greg Egan's novel 'Permutation City', characters manipulate quantum states to create copies invisible to an observer. Controlling the 'messiness' of light is a step toward such fine-tuning of reality.