Phys. Rev. X 5, 041036 (2015)

Optical isolators offering excellent performance are commercially available, but tend to be bulky devices on the scale of centimetres. Miniaturized isolators have been demonstrated but achieving high levels of isolation at nanoscale dimensions with very low light levels remains challenging. Now, Clément Sayrin and colleagues from the Vienna Center for Quantum Science and Technology in Austria have realized a nanoscale optical isolator using optical waveguides and cold atoms. The team used low-loss silica waveguides and demonstrated non-reciprocal transmission controlled via the internal state of spin-polarized atoms. And, importantly, their devices showed good performance in the single-photon regime. In the first part of the experimental work, cesium atoms were kept a distance of 230 nm from a nanofibre surface by an optical trapping scheme and 852-nm wavelength light was sent through the fibre to overlap with an atomic transition of the cesium atoms. Although interaction with a single atom is expected to result in only a weak, 0.3 dB, value of isolation, the team showed that the use of a few tens of atoms enabled a much higher level of isolation of around 8 dB. While increasing the number of atoms is one approach to increase isolation, improving the atom–light interaction strength is another. To do this the authors coupled a whispering-gallery-mode bottle microresonator to the fibre. This resulted in an experimentally demonstrated 13 dB of isolation using a single Rb atom coupled to the resonator mode.