Researchers create device to streamline interactions between ultra-cold and room-temperature computers

Nature Electronics (2022). DOI: 10.1038/s41928-022-00823-w” width=”800″ height=”413″/>

Integrated MO modulator. a, Perspective view of the device (not to scale). The top gold coil is used to apply a radial magnetic field to Ce:YIG below, making it non-reciprocal. The silicon micro-ring and silicon waveguide, in the all-pass filter configuration, are visible through the transparent upper sheath. b, Cross section of microring and electromagnet (not to scale) where direction of electric current and magnetic field are highlighted. c, optical micrograph of fabricated sample (top view). Credit: Natural electronics (2022). DOI: 10.1038/s41928-022-00823-w

Many advanced technologies operate at incredibly low temperatures. Superconducting microprocessors and quantum computers promise to revolutionize computing, but scientists must keep them just above absolute zero (-459.67° Fahrenheit) to protect their delicate states. Yet ultra-cold components must interface with room-temperature systems, presenting both a challenge and an opportunity for engineers.

An international team of scientists, led by Paolo Pintus of UC Santa Barbara, has designed a device to help cryogenic computers communicate with their counterparts in good weather. The mechanism uses a magnetic field to convert electrical current data into light pulses. The light can then travel via fiber optic cables, which can transmit more information than ordinary electrical cables while minimizing the heat that escapes into the cryogenic system. The results of the team appear in the newspaper Natural electronics.

“A device like this could allow seamless integration with cutting-edge superconductor-based technologies, for example,” said Pintus, a project scientist with UC Santa Barbara’s Optoelectronics Research Group. Superconductors can carry electrical current without any loss of energy, but generally require temperatures below -450° Fahrenheit to function properly.

Currently, cryogenic systems use standard metal wires to connect to electronics at room temperature. Unfortunately, these wires transfer heat into cold circuits and can only transmit a small amount of data at a time.

Pintus and his collaborators wanted to address these two questions at the same time. “The solution is to use light in an optical fiber to transfer information instead of using electrons in a metal cable,” he said.

Optical fiber is the standard in modern telecommunications. These thin glass cables carry information in the form of light pulses much faster than metal wires can carry electrical charges. As a result, fiber optic cables can relay 1,000 times more data than conventional cables over the same period. And glass is a good insulator, which means it will transfer far less heat to cryogenic components than a metal wire.

However, using fiber optics requires an additional step: converting data from electrical signals to optical signals using a modulator. This is a routine process under ambient conditions, but gets a bit tricky at cryogenic temperatures.

Pintus and his collaborators have built a device that translates electrical input into light pulses. An electric current creates a magnetic field which changes the optical properties of a synthetic garnet. Scientists call this the “magneto-optic effect”.

The magnetic field changes the refractive index of garnet, essentially its “density” in light. By modifying this property, Pintus can tune the amplitude of the light that travels through a micro-ring resonator and interacts with the garnet. This creates light and dark pulses that carry information through the fiber optic cable like Morse code in a telegraph wire.

“This is the first high-speed modulator ever made using the magneto-optical effect,” Pintus remarked.

Other researchers have created modulators using capacitor-like devices and electric fields. However, these modulators typically have high electrical impedance – they resist alternating current flow – making them a poor match for superconductors, which essentially have zero electrical impedance. Since the magneto-optical modulator has a low impedance, the scientists hope that it will be able to interface better with superconducting circuits.

The team also took steps to make their modulator as convenient 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 manufacture.

The project was a collaborative effort. Pintus and group director John Bowers of UC Santa Barbara led the project from concept, modeling and design to manufacturing and testing. The synthetic garnet was grown and characterized by a group of researchers from the Tokyo Institute of Technology who have collaborated with the team from the Department of Electrical and Computer Engineering at UCSB on several research projects in the past.

Another partner, BBN Raytheon’s Quantum Computing and Engineering group, is developing the kinds of superconducting circuits that could benefit from the new technology. Their collaboration with UCSB is longstanding. BBN scientists performed the low-temperature tests of the device to verify its performance in a realistic superconducting computing environment.

The bandwidth of the device is around 2 gigabits per second. That’s not much compared to room-temperature datalinks, but Pintus said it’s promising for a first demonstration. The team also needs to make the device more efficient so that it becomes useful in practical applications. However, they believe they can achieve this by replacing the garnet with a better material. “We would like to study other materials,” he added, “and we think we can achieve a higher bit rate. important than garnet.”

There are many materials to choose from, but little information to help Pintus and his colleagues make that choice. Scientists have studied the magneto-optical 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,” Pintus said, “leading the search toward (unexplored) high-performance magneto-optical materials that can operate at low temperatures.”

The hand of light holds the key to better optical control

More information:
Paolo Pintus et al, An integrated magneto-optical modulator for cryogenic applications, Natural electronics (2022). DOI: 10.1038/s41928-022-00823-w

Provided by University of California – Santa Barbara

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