Researchers create a device to streamline interactions between ultra-cold and room-temperature computers. Up to now, metal cables have been used to connect deep-frozen quantum processors (QPU) with the traditional evaluation electronics that work at room temperature. The downside: you heat up the QPU, which is highly undesirable because heat destroys the quantum states. 

Electricity flowing through a metal coil generates electric (purple) and magnetic (faint green) fields. This changes the properties of the substrate, which tunes the resonance ring (red) to different frequencies. The whole setup enables the scientists to convert a continuous beam of light (red on left) into pulses that can carry data through a fiber-optic cable. Photo Credit: BRIAN LONG

Superconducting QPUs operate at temperatures just a few thousandths of a degree above absolute zero. Today, in the cryogenic systems that cool the QPUs to the desired temperatures, metal lines are used to get the signals in and out. Not too many signals must be transmitted at a time, because that would heat up the wires too much, which would then destroy the quantum states in the QPU.  

The magneto-optical modulator circumvents the problem: it uses a magnetic field to convert electrical current into light pulses that are coupled into optical cables. On the one hand, this minimizes the heat input into the QPU because, unlike metal, glass is a poor conductor of heat. On the other hand, fiber optic cables allow a transmission rate that is 1000 times higher, which is used as standard in telecommunications and for high-speed data transmission in data centers, for example. 

"With this we have produced the first high-speed modulator that works on the basis of the electro-optical effect. It makes it possible to connect deep-frozen systems such as QPUs based on superconductors with control and evaluation electronics that work at room temperature - in such a way that on the one hand not too much heat is transferred to the QPU and on the other hand the data transmission rate can be significantly increased. That's how we kill two birds with one stone," says Paolo Pintus of UC Santa Barbara's Optoelectronics Research Group, who led the development of the new magneto-optical modulator. 

However, the electrical signals must be converted into light signals. The modulators required for this have been available for a long time - but are not suitable for temperatures close to absolute zero. So the researchers at UC Santa Barbara had to look for a new approach. In their modulator, they use the "magneto-optical effect" to convert electric current into light pulses: the electric current creates a magnetic field that changes the refractive index of an artificial garnet. Based on this, the scientists can regulate the amplitude of the light that circulates in a micro-ring oscillator and interacts with the garnet. In this way, dark and light pulses can be generated so that information is transmitted in a similar way to Morse code. 

The magneto-optic modular: Gold coil (top), synthetic garnet ( green in middle), silicon micro-ring resonator and waveguide (bottom). Port 1 and 2 are the input and output for the optical transmission. Photo Credit: PAOLO PINTUS ET AL.

During development, the researchers made sure to use components that are already being manufactured in large quantities and to manufacture the modulator using standard processes. They work with a light wavelength of 1,550 nm, which is common in data transmission. 

Other scientists had already developed modulators, designed like a capacitor, that use electric fields to convert electricity into light. But they have high impedance, which ill-matches superconducting QPUs, which have zero impedance. Because the modulators that take advantage of the electro-optic effect have low impedance, they should be far better suited to interface with the superconducting QPUs.

The data transmission rate of the new modulator is 2 Gb/s, which is low compared to the 200 or even 400 Gb/s used in today's data centers. According to Pintus, this is a very good value for a first demonstration. However, the researchers are not yet where they want to be. In order to be able to advance into real applications, the modules would have to become significantly more efficient. But here, too, a way is emerging: The garnet could be replaced by a better material. According to Pintus, the electro-optical effect in europium-based materials is 300 times higher than in garnet. 

"Our research on previously unknown magneto-optical materials that work at very low temperatures could pave the way to a new class of energy-efficient cryosystems," says Pintus.