Many modern technologies operate at incredibly low temperatures. Superconducting microprocessors and quantum computers promise to revolutionize computing, but scientists need to keep them above absolute zero (-459.67 degrees Fahrenheit) to protect their sensitive states. However, extremely cold components must interact with room temperature regimes, presenting both a challenge and an opportunity for engineers.
An international team of scientists, led by Paulo Bentos of the University of California, Santa Barbara, has designed a device to help cooler computers talk to their counterparts in fair weather. The mechanism uses a magnetic field to convert data from an electric current into pulses of light. The light can then travel through fiber-optic cables, which can transmit more information than regular electrical cables while reducing heat that escapes into the cooling system. The team’s results appear in the journal nature electronics.
“A device like this could enable seamless integration with cutting-edge technologies based on superconductors, for example,” said Bentos, a project scientist in the Optoelectronics Research Group at the University of California, Santa Barbara. Superconductors can carry electric current without any loss of power, but usually require temperatures below -450 degrees Fahrenheit to function properly.
Currently, cryogenic systems use standard metal wires to communicate with electronic devices at room temperature. Unfortunately, these wires transfer heat to cold circuits and can only transmit a small amount of data at a time.
Bentos and his collaborators wanted to address these two issues simultaneously. “The solution is to use light in optical fibers to transmit information rather than using electrons in a metal cable,” he said.
Optical fibers are the standard in modern telecommunications. These thin glass cables carry information as light pulses much faster than metal wires can carry electrical charges. As a result, fiber-optic cables can transmit 1,000 times more data than conventional wires over the same period of time. And glass is a good insulator, which means it will transfer much less heat to the cooled components than metal wire.
However, the use of optical fibers requires an additional step: converting data from electrical signals into optical signals using a modulator. This is a routine process in ambient conditions, but gets a bit tricky in very cold temperatures.
Bentos and his collaborators built a device that translates electrical inputs into light pulses. The electric current creates a magnetic field that changes the optical properties of synthetic garnet. Scientists refer to this as the “magneto-optical effect”.
The magnetic field changes the refractive index of garnet, which is essentially its “density” into light. By changing this property, Pintus can adjust the amplitude of the light that circulates in the micro-ring resonator and interacts with the garnet. This results in bright, dark pulses that transmit information over a fiber-optic cable like Morse code in a telegraph wire.
“This is the first high-speed modifier ever manufactured using the magneto-optical effect,” Bentos commented.
Other researchers have created modifiers using capacitor-like devices and electric fields. However, these modifiers usually have a high electrical resistance – they resist the flow of alternating current – which makes them a poor match for superconductors, which have essentially no electrical resistance. Because the magneto-optical modulator has a low resistance, the scientists hope that it will be able to better interact with superconductor circuits.
The team also took steps to make their modulator as practical as possible. It operates at wavelengths of 1,550 nanometers, which is the same wavelength as light used in Internet communications. It was produced using standard methods, which simplifies its manufacture.
The project, funded by the Air Force Office of Scientific Research, was a collaborative effort. Bentos and Group Director John Powers at the University of California, Santa Barbara, led the project, from concept, modeling and design to manufacturing and testing. Synthetic opal was grown and characterized by a group of researchers from the Tokyo Institute of Technology who have collaborated with the team in UCSD’s Department of Electrical and Computer Engineering 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 cooperation with UCSB is a long-term one. Scientists at BBN conducted a low-temperature test of the device to verify its performance in a realistic superconducting computing environment.
The bandwidth of the device is about 2 Gbps. It’s not much compared to the data links at room temperature, but Bentos said it’s promising for the first illustration. The team also needs to make the device more efficient so that it can be useful in practical applications. However, they believe they can achieve this by replacing garnet with a better material. He added, “We’d like to investigate other materials, and we think we can achieve a higher bit rate. For example, europium-based materials show a magneto-optical effect 300 times greater than garnet.”
There is plenty of material to choose from, but not a lot of information to help Pintus and his colleagues make that choice. Scientists have studied the magneto-optical properties of 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,” Bentos said, “leading research toward high-performance (unexplored) optoelectromagnetic materials that can operate at lower temperatures.”