Chipping in on quantum’s miniaturisation
A compact photonic circuit that uses sound waves to rein in light could help miniaturise quantum devices.
Researchers from the Illinois Quantum Information Science and Technology Center (IQUIST), USA, have designed a way to isolate, or control the directionality of light.
In a study published in Nature Photonics, the team claims that this approach to isolation currently outperforms all previous on-chip alternatives and is optimised for compatibility with atom-based sensors. This could make the new design useful for applications like quantum computing, where stray, uncontrolled magnetic fields, as well as unwanted light, can erode overall device performance.
The researchers explain that, in typical experiments, the best tool for achieving unidirectionality uses an isolator made of magnets. However, this presents two issues. First, in compact devices, magnetic fields negatively affect nearby atoms. Second, even if this issue is overcome, the materials that are inside the isolator do not work as well on the smaller length scales on a chip.
Gaurav Bahl, Professor at the University, and his team have developed a non-magnetic isolator that uses common optical materials and, they claim, is easily adaptable for different wavelengths of light.
Their paper, Electrically driven optical isolation through phonon-mediated photonic Autler–Townes splitting, reads, ‘Our concept is implemented using a lithium niobate racetrack resonator in which phonon-mediated photonic Autler–Townes splitting breaks the chiral symmetry of the resonant modes.’
‘We wanted to design a device that naturally avoids loss, and the best way to do that is to have light propagate through nothing. The simplest bit of ‘nothing’ that can still guide photons along a controlled path is a waveguide, which is a very basic component in photonic circuits,’ adds Bahl.
In a complete atom-based system, the waveguide would steer laser light through a series of elements to a small chamber containing atoms. With this in mind, the team has optimised their chip for use with 780nm light, which is the wavelength needed to configure common rubidium-based sensors.
For isolation, however, the light must also be simultaneously blocked in the opposite direction. Previously, the team has shown that they could launch sound waves into a photonic circuit to break the symmetric flow of light. In the new study, they have turned this idea into a functional chip element.
The complete photonic isolator contains a waveguide and an adjacent ring resonator, which looks like an oblong racetrack. Normally, incoming light would just pass from the waveguide into the resonator, irrespective of its direction, thus blocking all light flow. But when the researchers apply sound waves to the ring, the resonator only captures light that moves backwards through the waveguide. In the forward direction, light passes through the waveguide unimpeded, as if the resonator is simply not there.
This means that the design reduces losses, or undesirable light absorption, to nearly zero, which has been a long-standing problem with previous on-chip isolators.
The team affirms that the data shows the new devices exhibit record-breaking performance for on-chip isolation and operate as well as the larger magnet-based devices. In addition, the approach is flexible and can be used for multiple wavelengths without changing the starting material.
