Quantum Milestone: Initial Entanglement of Microwave and Optical Photons Achieved

The novel experimental device is shown with the beam of optical photons (depicted in red) entering and exiting the electro-optic crystal, resonating within its circular section, and the resultant microwave photons (shown in blue) leaving the apparatus. Image credits go to Eli Krantz, Krantz NanoArt.

In a landmark achievement, ISTA scientists have successfully entangled microwave and optical photons for the first time.

Quantum computing possesses the potential to solve intricate problems in sectors such as material science and cryptography, issues that are likely to remain beyond the grasp of even the most advanced traditional supercomputers. However, realizing this potential will likely require millions of high-quality qubits, considering the necessary error correction.

Rapid advancements are being made in superconducting processors, currently boasting a qubit count in the hundreds. The attraction of this technology is its high computational speed and compatibility with microchip manufacturing. However, the prerequisite for incredibly low temperatures restricts the processor’s size and bars any physical access post-cooling.

A modular quantum computer with various independently cooled processor nodes could be the solution. However, individual microwave photons—the native information carriers among superconducting qubits within the processors—are unfit for transmission through a room temperature environment between the processors. The heat prevalent in a room temperature environment easily interferes with the microwave photons and their delicate quantum attributes, such as entanglement.

Scientists from the Fink group at the Institute of Science and Technology Austria (ISTA), in collaboration with partners from TU Wien and the Technical University of Munich, have showcased a critical technological leap to surmount these obstacles. They’ve achieved the first-ever entanglement of low-energy microwaves with high-energy optical photons.

This entangled quantum state of two photons forms the bedrock to connect superconducting quantum computers via room-temperature links. This carries implications for not only scaling up current quantum hardware but also for establishing interconnects with other quantum computing platforms and for innovative quantum-enhanced remote sensing applications. Their findings have been published in the Science journal.

Qubits, the fundamental informational components of quantum computers, have unique attributes like entanglement. Entanglement is crucial for quantum computers because it enables calculations that are impossible for non-quantum computers. Credit: Mark Belan/ISTA

Suppressing the Noise through Cooling

Rishabh Sahu, a Fink group postdoc and one of the leading authors of the study, elucidates, “Noise is a significant problem for any qubit, essentially any disruption to the qubit. The material’s heat in which the qubit is embedded is a primary source of noise.”

Heat induces rapid movement of atoms in a material, which disrupts quantum attributes like entanglement, rendering qubits unfit for computation. Hence, for a quantum computer to function, qubits must be isolated, cooled to incredibly low temperatures, and kept in a vacuum to maintain their quantum properties.

Superconducting qubits are cooled in a specialized cylindrical device suspended from the ceiling, known as a “dilution refrigerator”, where the “quantum” aspect of computation takes place. The qubits at the refrigerator’s base are chilled to mere thousandths of a degree above absolute zero—at around -273 degrees Celsius. Sahu notes, “This makes these refrigerators the coldest places in the entire universe, colder even than space.”

The more qubits and related control wiring are incorporated, the more heat is generated, making it harder to maintain the quantum computer’s cool state. Sahu cautions, “The scientific community predicts that around 1,000 superconducting qubits in a single quantum computer will reach the cooling limits. Simply scaling up is not a sustainable strategy for building more powerful quantum computers.”

Adding to this, Fink notes, “Larger machines are being developed, but each assembly and cooldown then becomes akin to a rocket launch, where problems are only detected once the processor is cold, with no ability to rectify such issues.”

Quantum Waves

Liu Qiu, a Fink group postdoc and another primary author of the study, explains that if a dilution fridge can’t adequately cool over a thousand superconducting qubits simultaneously, several smaller quantum computers need to be linked to collaborate. “We need a quantum network.”

Connecting two superconducting quantum computers, each with its dilution refrigerator, isn’t as simple as linking them with an electrical cable. The connection needs to be specifically designed to preserve the qubits’ quantum nature.

Superconducting qubits operate using minuscule electrical currents that oscillate in a circuit at frequencies about ten billion times per second, interacting through microwave photons—light particles. Their frequencies are akin to those used by mobile phones.

However, even a modest amount of heat can easily interfere with single microwave photons and the necessary quantum properties to link the qubits in two separate quantum computers. The environmental heat while traversing a cable outside the refrigerator would render them ineffective.

Qiu explains, “Instead of using the noise-sensitive microwave photons for computations within the quantum computer, we want to utilize optical photons with much higher frequencies akin to visible light to network quantum computers.” These optical photons are identical to those transmitted through optical fibers for high-speed internet, a technology that is well-understood and far less noise-susceptible. Qiu adds, “The challenge was figuring out the interaction and entanglement of the microwave and optical photons.”

Light Division

In their study, the researchers utilized a unique electro-optic device—an optical resonator made from a nonlinear crystal that alters its optical properties in an electric field’s presence. A superconducting cavity holds this crystal and enhances the interaction.

Sahu and Qiu used a laser to inject billions of optical photons into the electro-optic crystal for a fraction of a microsecond. As a result, one optical photon divides into a pair of newly entangled photons: an optical photon with marginally less energy than the original and a lower-energy microwave photon.

Sahu explains, “The experiment’s challenge was that the optical photons have about 20,000 times more energy than the microwave photons, which brings a lot of energy and therefore heat into the device, potentially destroying the microwave photons’ quantum properties. We’ve spent months fine-tuning the experiment and making accurate measurements.”

To solve this, the scientists constructed a larger superconducting device than previous attempts. This not only prevents a superconductivity breakdown but also helps cool the device more efficiently and keep it cool during the optical laser pulses’ short durations.

“The breakthrough is the entanglement of the two photons—the optical and the microwave photon—exiting the device,” Qiu states. “We’ve verified this by measuring correlations between the quantum fluctuations of the two photons’ electromagnetic fields, stronger than can be explained by classical physics.”

Fink says, “We are now the first to entangle photons of such vastly different energy scales. This is a pivotal step towards creating a quantum network and is also beneficial for other quantum technologies, such as quantum-enhanced sensing.”

Reference: “Entangling microwaves with light” by R. Sahu, L. Qiu, W. Hease, G. Arnold, Y. Minoguchi, P. Rabl, and J. M. Fink, 18 May 2023, Science.
DOI: 10.1126/science.adg3812

The European Research Council, the Horizon 2020 Framework Programme, and the Austrian Science Fund financed the study.

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• Institute of Science and Technology Austria
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