Using light to successfully connect quantum circuits for the first time Science Express

Quantum computers promise to solve challenging tasks in materials science and cryptography that will be out of reach for even the most powerful traditional supercomputers in the future. However, this will likely require millions of high-quality quantum bits due to the need for error correction.

Superconducting processors are advancing rapidly, and the largest number of quantum bits that have been implemented is a few hundred. The advantages of this technology are fast computational speed and compatibility with microchip fabrication, but the need for ultra-low temperatures ultimately limits the size of the processor and, once cooled down, prevents any physical access.

A modular quantum computer with multiple individually cooled processor nodes could solve this problem. However, individual microwave photons (microwave photons, particles of light that serve as the original information carriers between superconducting quantum bits within the processor) are not suitable for sending between processors through a room-temperature environment. The world at room temperature is full of heat and can easily disturb microwave photons and their fragile quantum properties, such as entanglement.

Researchers in the group of Johannes M. Fink at the Austrian Institute of Science and Technology (ISTA), together with collaborators at the Vienna University of Technology and the Technical University of Munich, have demonstrated an important technical step to overcome these challenges: for the first time, they have entangled low-energy microwaves with high-energy optical photons.

This entangled quantum state of two photons is the basis for connecting superconducting quantum computers via room temperature links. This has implications not only for scaling up existing quantum hardware, but is also necessary to enable interconnection with other quantum computing platforms and new quantum-enhanced remote sensing applications.

Rishabh Sahu, a postdoctoral fellow in the group and one of the first authors of this new study, explains, "One of the major problems with any quantum bit is noise. Noise can be thought of as any interference with a quantum bit. A major source of noise is the heat of the material on which the quantum orbit is based."

Heat causes the atoms in the material to move rapidly. This is destructive to quantum properties like entanglement, and as a result, it would make quantum bits unsuitable for computation. To remain functional, quantum computers must isolate quantum bits from their environment, cool them to extremely low temperatures, and place them in a vacuum to maintain their quantum properties.

They have a unique variety of properties, such as entanglement. Entanglement is important for quantum computers because it allows quantum computers to perform calculations in a way that is not possible for non-quantum computers.

For superconducting quantum bits, this happens in a special cylindrical device suspended from the ceiling - a dilution chiller - where the "quantum" part of the computation takes place. The quantum bits at the bottom are cooled to just a few thousandths of a degree above absolute zero (about -273 degrees C.) Sahu adds excitedly, "This makes these coolers in our lab the coldest place in the entire universe, even colder than space itself."

The coolers must continuously cool the quantum bits, but the more quantum bits and associated control lines there are and the more heat they generate, the harder it is to keep a quantum computer cool. The scientific community has predicted that we reach the cooling limit at about 1,000 superconducting quantum bits in a quantum computer; in response, Sahu cautions, "Simply scaling up is not a sustainable solution for building more powerful quantum computers."

Fink adds, "Larger machines are in development. But every assembly and cooling becomes comparable to a rocket launch, and you only find problems after the processor has cooled down and there is no ability to intervene and correct such problems."

"If a dilute cooler cannot adequately cool more than a thousand superconducting quantum bits at once, we will need to connect several smaller quantum computers to work together," explains Liu Qiu, a postdoctoral fellow in Fink's group and the other first author of this new research, "We will need a quantum network."

Connecting two superconducting quantum computers, each with its own dilute cooler, is not as simple as connecting them with wires: such connections require special considerations to preserve the quantum nature of the quantum bits.

Superconducting quantum bits work with tiny currents that move back and forth through the circuit at a frequency of about 10 billion times per second. They use microwave photons - particles of light - to interact; their frequencies are similar to those used in cell phones.

Experimental devices with dilute coolers, superconducting cavities and electro-optical crystals are splitting and entangling photons.

The problem is that even small amounts of heat can easily interfere with individual microwave photons and their quantum properties needed to connect the quantum bits in two separate quantum computers. The heat from the environment can render them useless when passing through the cable outside the cooler.

"Instead of the noise-susceptible microwave photons that perform calculations inside a quantum computer, we want to network quantum computers with much higher frequency photons similar to visible light." Qiu explained. These are the same photons that are sent through the optical fibers that deliver high-speed Internet to our homes; and the technology is well understood and much less susceptible to thermal noise.

"The existing challenge is how to get microwave photons to interact with optical photons, and, how to get them entangled."

In their new study, the researchers used a special electro-optical device: an optical resonator made of a nonlinear crystal that changes its optical properties in the presence of an electric field; a superconducting cavity houses this crystal and enhances the associated interaction.

Beam photons (red) enter and leave the electro-optic crystal and resonate within its circular section, as well as producing microwave photons (blue) that leave the device.

Physical and conceptual mode configuration.

Sahu and Qiu used a laser to send billions of photons into the electro-optical crystal for a fraction of a second. In this way, an optical photon splits into a new pair of entangled photons: an optical photon with only slightly lower energy than the original photon and a microwave photon with even lower energy.

"The challenge with this experiment is that optical photons are 20,000 times more energetic than microwave photons," Sahu explained, "and they bring a lot of energy, and therefore heat, to the device, which could then, potentially, destroy the quantum properties of the microwave photons. We've been working for months to tweak the experiment and get the measurements right."

To solve this problem, the researchers built a superconducting device that is more massive than previous attempts. This not only avoided the destruction of superconductivity, but also helped to cool the device more efficiently and keep it cold during the short timescale of the optical laser pulse.

"The breakthrough lies in the fact that the two photons (optical and microwave photons) leaving the device are entangled," Qiu explained, "This has been verified by measuring the correlation between the quantum fluctuations in the electromagnetic fields of the two photons, a correlation that is stronger than what can be explained by classical physics. "

"We are now the first team to entangle such a huge number of photons at different energy scales." Fink said, "This is a key step in creating quantum networks; it's also useful for other quantum technologies, such as quantum-enhanced sensing."

Reference links:

[1] https://www.science.org/doi/10.1126/science.adg3812

[2] https://phys.org/news/2023-05-wiring-quantum-circuits.html