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Magneto-optic modulator promises energy efficient cryogenic devices

Technology News |
By Rich Pell

Scientists say they have designed a device to help cryogenic computers talk with their “fair-weather” counterparts. The device is designed to help address the challenge of interfacing superconducting microprocessors and quantum computers – which operate at ultra-cold temperatures – with room temperature systems.

The mechanism uses a magnetic field to convert data from electrical current to pulses of light. The light can then travel via fiber-optic cables, which can transmit more information than regular electrical cables while minimizing the heat that leaks into the cryogenic system.

“A device like this could enable seamless integration with cutting-edge technologies based on superconductors, for example,” says Paolo Pintus, a project scientist in UC Santa Barbara’s Optoelectronics Research Group who led the resaearch.

Superconductors can carry electrical current without any energy loss, but typically require temperatures below -450° Fahrenheit to work properly. Currently, cryogenic systems use standard metal wires to connect with room-temperature electronics. Unfortunately, these wires transfer heat into the cold circuits and can only transmit a small amount of data at a time.

The researchers wanted to address both these issues at once.

“The solution, ” says Pintus, “is using light in an optical fiber to transfer information instead of using electrons in a metal cable.”

Standard in modern telecommunications, fiber optics carry information as pulses of light far faster than metal wires can carry electrical charges. As a result, fiber-optic cables can relay 1,000 times more data than conventional wires over the same time span. And glass is a good insulator, meaning it will transfer far less heat to the cryogenic components than a metal wire.

However, using fiber optics requires an extra step: converting data from electrical signals into optical signals using a modulator. This is a routine process at ambient conditions, but becomes a bit tricky at cryogenic temperatures, say the researchers.

Ro address this, the researchers built a device that translates electrical input into pulses of light. An electric current creates a magnetic field that changes the optical properties of a synthetic garnet – called the “magneto-optic effect.”

The magnetic field changes the garnet’s refractive index, essentially its “density” to light. By changing this property, the researchers can tune the amplitude of the light that circulates in a micro-ring resonator and interacts with the garnet. This creates bright and dark pulses that carry information through the fiberoptic cable like Morse code in a telegraph wire.

“This is the first high-speed modulator ever fabricated using the magneto-optic effect,” says Pintus.

While other researchers have created modulators using capacitor-like devices and electric fields, these modulators usually have high electrical impedance – they resist the flow of alternating current – making them a poor match for superconductors, which have essentially zero electrical impedance. Since the magneto-optic modulator has low impedance, the researchers hope it will be able to better interface with superconductor circuits.

The researchers also took steps to make their modulator as practical as possible. It operates at wavelengths of 1,550 nanometers, the same wavelength of light used in internet telecommunications. It was produced using standard methods, which simplifies its manufacturing.

The device’s bandwidth is around 2 gigabits per second – not a lot compared to data links at room temperature, but, say the researchers, it’s promising for a first demonstration. The researchers say they also need to make the device more efficient for it to become useful in practical applications. However, they believe they can achieve this by replacing the garnet with a better material.

“We would like to investigate other materials,” says Pintus, “and we think we can achieve a higher bitrate. For instance, europium-based materials show a magneto-optic effect 300 times larger than the garnet.”

While there are plenty of materials to choose from, the researchers say there is not a lot of information to help them make that choice. Scientists have studied the magneto-optic properties of only a few materials at low temperatures.

“The promising results demonstrated in this work could pave the way for a new class of energy efficient cryogenic devices,” says Pintus, “leading the research toward high-performing (unexplored) magneto-optic materials that can operate at low temperatures.”

For more, see “An integrated magneto-optic modulator for cryogenic applications.”


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