Entangling microwave and optical photons for quantum networks
The number of superconducting quantum computers has increased rapidly in recent years, but further growth is limited by the need for ultra-cold operating temperatures. Connecting several smaller processors could create larger, more computationally powerful quantum computers; however, doing so poses challenges. Researchers led by Rishabh Sahu, Liu Qiu and Johannes Fink from the Institute of Science and Technology Austria (ISTA) have now demonstrated quantum entanglement between optical and microwave photons that could lay the foundations for such a future quantum network.
Quantum computers have the potential to solve challenging tasks in material science and cryptography; however, this will likely require millions of high-quality qubits due to the required error correction. Progress in superconducting processors advances quickly with a current qubit count in the few hundreds. The advantages of this technology are the fast computing speed and its compatibility with microchip fabrication, but the need for ultra-cold temperatures ultimately confines the processor in size and prevents any physical access once it is cooled down.
A modular quantum computer with multiple separately cooled processor nodes could solve this. However, single microwave photons — the particles of light that are the native information carriers between superconducting qubits within the processors — are not suitable to be sent through a room temperature environment between the processors. To address this challenge, the researchers entangled low-energy microwave with high-energy optical photons. Such an entangled quantum state of two photons is the foundation to wire up superconducting quantum computers via room temperature links. This has implications not only for scaling up existing quantum hardware but it is also needed to realise interconnects to other quantum computing platforms as well as for novel quantum-enhanced remote sensing applications. The research findings were published in the journal Science.
Sahu said that a major problem for any qubit is noise, which can cause disturbance to the qubit. A major source of noise is the heat of the material the qubit is based on. Heat causes atoms in a material to jostle around rapidly — this is disruptive to quantum properties like entanglement and, as a result, it would make qubits unsuitable for computation. Therefore, to remain functional, a quantum computer must have its qubits isolated from the environment, cooled to extremely low temperatures and kept within a vacuum to preserve their quantum properties.
For superconducting qubits, this happens in a cylindrical device that hangs from the ceiling, called a “dilution refrigerator”, in which the “quantum” part of the computation takes place. The qubits at its very bottom are cooled down to a few thousandths of a degree above absolute zero temperature (-273°C). The refrigerator also continuously cools the qubits, but the more qubits and associated control wiring are added, the more heat is generated and the harder it is to keep the quantum computer cool.
“The scientific community predicts that at around 1000 superconducting qubits in a single quantum computer, we reach the limits of cooling. Just scaling up is not a sustainable solution to construct more powerful quantum computers,” Sahu said.
According to Qiu, if a dilution fridge cannot sufficiently cool more than 1000 superconducting qubits at once, several smaller quantum computers must be linked to work together, necessitating a quantum network. Linking together two superconducting quantum computers, each with its own dilution refrigerator, requires special consideration to preserve the quantum nature of the qubits.
Superconducting qubits work with tiny electrical currents that move back and forth in a circuit at frequencies about ten billion times per second. They interact using microwave photons — particles of light. Their frequencies are similar to the ones used by mobile phones. Even a small amount of heat would easily disturb single microwave photons and their quantum properties needed to connect the qubits in two separate quantum computers. When passing through a cable outside the refrigerator, the heat of the environment would render them useless.
“Instead of the noise-prone microwave photons that we need to do the computations within the quantum computer, we want to use optical photons with much higher frequencies similar to visible light to network quantum computers together. The challenge was how to have the microwave photons interact with the optical photons and how to entangle them,” Qiu said.
The researchers used a special electro-optic device: an optical resonator made from a nonlinear crystal, which changes its optical properties in the presence of an electric field. A superconducting cavity houses this crystal and enhances this interaction. The researchers used a laser to send billions of optical photons into the electro-optic crystal for a fraction of a microsecond. In this way, one optical photon splits into a pair of new entangled photons: an optical one with less energy than the original one and a microwave photon with lower energy.
“The challenge of this experiment was that the optical photons have about 20,000 times more energy than the microwave photons, and they bring a lot of energy and therefore heat into the device that can then destroy the quantum properties of the microwave photons. We have worked for months tweaking the experiment and getting the right measurements,” Sahu said.
To address this, the researchers built a bulkier superconducting device compared to previous attempts. This avoided a breakdown of superconductivity while helping to cool the device more effectively, keeping it cold during the short timescales of the optical laser pulses. The two photons leaving the device — the optical and the microwave photon — are entangled.
“This has been verified by measuring correlations between the quantum fluctuations of the electromagnetic fields of the two photons that are stronger than can be explained by classical physics,” Qiu said.
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