Lasers are synonymous with precision, but generally speaking there's room for improvement. The "perfect" laser would emit light at one specific wavelength, but current technology can't achieve that yet, shining across the spectrum in a measure that scientists call "linewidth." Narrowing that linewidth down as far as possible is one of the aims of laser research, and now an international team has developed the world's sharpest laser, with a linewidth of just 10 mHz (0.01 Hz).
Typically, the best lasers can have a linewidth as narrow as a few kHz, but for particularly precise instruments, like optical atomic clocks, that needs to be squeezed down further. The other measure of a laser beam's quality is the stability of the light's frequency: after a certain amount of time, the light wave oscillations will wobble out of sync, so the longer a laser can maintain its "perfect wave train", the better.
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The new laser, developed by German scientists from PTB and a US research group known as JILA, excels in both of those areas. Along with its minuscule 10 mHz linewidth, its light waves remain stable for 11 seconds, by which time the beam has extended about 3.3 million km (2 million miles) – or about 10 times the distance between the Earth and the Moon.
In fact, the new laser is so precise that it was difficult to compare to existing ones, so to prove its worth, the team built two of them and compared them to each other. The devices are made with a Fabry-Pérot silicon resonator, containing two fixed mirrors lined up opposite each other. Since the length of the resonator determines the frequency of the light waves, the desired laser beam was achieved by making the resonator 21 cm (8.3 in) long. With such delicate measurements involved, special care was taken to keep the instruments free of other interference, like pressure, vibrations and temperature.
The sharp new lasers are being used to make more accurate atomic clocks, and take more precise measurements on ultracold atoms. The researchers believe that by tweaking the composition of the mirrors and finding ways to lower the temperature inside the resonator, the linewidth could be further reduced, even under 1 mHz.
The study was published in the journal Physical Review Letters.