Researchers from the Fink group at the Institute of Science and Technology Austria (ISTA), in collaboration with TU Wien and the Technical University of Munich, have achieved a significant breakthrough in quantum computing technology. They have successfully entangled low-energy microwave photons with high-energy optical photons, marking the first instance of such an achievement.
This development is crucial in addressing the challenges faced in quantum computing, particularly in establishing connections between separate processors that are cooled independently. Currently, the use of single microwave photons as information carriers between superconducting qubits within processors is hindered by the disruptive effects of heat in a room temperature environment. These heat-induced disturbances compromise the delicate quantum properties, such as entanglement, of the microwave photons.
By entangling low-energy microwave photons with high-energy optical photons, the researchers have paved the way for establishing connections between superconducting quantum computers through room temperature links. This breakthrough has far-reaching implications, not only for scaling up existing quantum hardware, but also for establishing interconnects with other quantum computing platforms and enabling novel quantum-enhanced remote sensing applications.
The groundbreaking results of this research have been published in the esteemed journal Science, signifying a significant advancement in quantum computing technology and its potential applications.
Cooling away the noise
According to Rishabh Sahu, a postdoc in the Fink group and one of the lead authors of the recent study, noise poses a significant challenge for qubits. Noise refers to any disturbance that affects the qubit’s stability and performance. One of the primary sources of noise is the heat generated by the material on which the qubit is based.
Heat causes the atoms within a material to exhibit rapid movement or jostling. This disruptive motion interferes with crucial quantum properties like entanglement, rendering the qubits unsuitable for reliable computation. Consequently, in order to maintain their functionality, quantum computers must ensure that the qubits are isolated from the surrounding environment, cooled to extremely low temperatures, and kept within a vacuum to preserve their delicate quantum characteristics.
To maintain the functionality of superconducting qubits, a specialized device called a “dilution refrigerator” is used. This cylindrical apparatus is suspended from the ceiling and serves as the location for the “quantum” part of the computation. The qubits positioned at the bottom of the refrigerator are cooled to a temperature just a few thousandths of a degree above absolute zero, approximately -273 degrees Celsius. Rishabh Sahu enthusiastically points out that the fridges in their labs are colder than any other place in the entire universe, including outer space.
The dilution refrigerator’s primary task is to continuously cool the qubits. However, as more qubits and associated control wiring are added, the heat generated becomes increasingly challenging to manage. Sahu emphasizes that the scientific community predicts that cooling becomes exceptionally difficult once a single quantum computer contains around 1,000 superconducting qubits. Simply scaling up the system is not a sustainable solution for constructing more powerful quantum computers.
Andreas Fink adds to this point, highlighting that while larger quantum machines are currently under development, each assembly and cooldown process becomes comparable to a rocket launch. Similar to a rocket launch, any problems or issues with the processor can only be identified once it has been cooled down. Unfortunately, at that stage, there is no way to intervene or rectify such problems.
Liu Qiu, another lead author of the study and a postdoc in the Fink group, explains that if a single dilution refrigerator is unable to adequately cool more than a thousand superconducting qubits simultaneously, the solution lies in establishing a connection between multiple smaller quantum computers. This necessitates the creation of a quantum network.
However, connecting two superconducting quantum computers, each equipped with its own dilution refrigerator, is not as simple as linking them with a standard electrical cable. The connection requires special considerations to ensure the preservation of the delicate quantum nature of the qubits.
Superconducting qubits operate based on tiny electrical currents that oscillate back and forth in a circuit at frequencies that are approximately ten billion times per second. These qubits interact with one another through the use of microwave photons, which are particles of light. Interestingly, the frequencies at which these microwave photons operate are similar to those used by cellphones.
The issue lies in the fact that even a small amount of heat can easily disrupt the delicate quantum properties of single microwave photons, which are necessary to establish connections between qubits in separate quantum computers. When these photons pass through a cable outside the refrigerator, the ambient heat renders them ineffective for their intended purpose.
Liu Qiu explains that instead of relying on these noise-prone microwave photons for quantum computations within the quantum computer, the goal is to utilize optical photons with higher frequencies similar to visible light in order to network quantum computers together. These optical photons are the same type that are transmitted through optical fibers to deliver high-speed internet to our homes. This technology is well understood and much less susceptible to noise caused by heat.
The challenge lies in finding a way for the microwave photons and the optical photons to interact and become entangled. This process of interaction and entanglement is crucial for establishing the desired quantum connections between the different components of the quantum computers.
In their recent study, the researchers utilized a specialized electro-optic device—a nonlinear crystal optical resonator—to facilitate their breakthrough. This crystal alters its optical properties in the presence of an electric field. Placed within a superconducting cavity, the crystal’s interaction is enhanced.
To achieve their goal, Sahu and Qiu directed a laser to introduce billions of optical photons into the electro-optic crystal for a brief period. During this process, one optical photon splits into a pair of newly entangled photons: an optical photon with only slightly less energy than the original and a microwave photon with significantly lower energy.
The experiment faced a significant challenge due to the stark energy difference between the optical and microwave photons. Sahu elaborates on the difficulty, explaining that optical photons carry about 20,000 times more energy than microwave photons. As a result, the influx of optical photons could introduce excessive energy and heat into the device, potentially disrupting the quantum properties of the microwave photons. Months of meticulous adjustments were required to overcome this hurdle and achieve accurate measurements. To mitigate the problem, the researchers developed a bulkier superconducting device compared to previous attempts. This design not only prevented superconductivity breakdown but also facilitated more effective cooling and maintained the device’s low temperature during the short intervals of the optical laser pulses.
The breakthrough lies in the fact that the two photons emitted from the device—the optical and microwave photons—are now entangled. Qiu emphasizes that this has been confirmed by measuring the correlations between the quantum fluctuations of the electromagnetic fields of the two photons, which exhibit stronger correlations than can be explained by classical physics.
As Fink notes, this achievement marks the first entanglement of photons with such vastly different energy scales. This milestone is crucial for advancing the development of a quantum network and holds implications for various other quantum technologies, including quantum-enhanced sensing.