Finnish researchers have engineered a circuit which is capable of generating high-quality microwave signals necessary for controlling quantum computers whilst functioning at temperatures close to absolute zero.
Artistic impression of an on-chip microwave source controlling qubits (Image Credit: Aleksandr Kakinen).
This is a major step toward enabling the control system to be closer to the quantum processor, which could render it possible to significantly increase the number of qubits in the processor.
One of the aspects restricting the size of quantum computers is the mechanism used to regulate the qubits in quantum processors. This is usually achieved using a series of microwave pulses, and because quantum processors work at temperatures close to absolute zero, the control pulses are usually brought into the cooled setting via broadband cables from room temperature.
As the number of qubits increases, so does the number of cables required. This restricts the potential size of a quantum processor, as the refrigerators that cool the qubits would have to become larger to house progressively more cables while also working harder to cool them down — this is ultimately a losing proposition.
A research collaboration led by Aalto University and VTT Technical Research Centre of Finland has at present created a key component of the solution to this challenge.
We have built a precise microwave source that works at the same extremely low temperature as the quantum processors, approximately -273 degrees.
Mikko Möttönen, Study Lead and Professor, Aalto University
Möttönen is also a professor at VTT Technical Research Centre of Finland.
The innovative microwave source is an on-chip device that can be combined with a quantum processor. Under a millimeter in size, it actually eliminates the need for high-frequency control cables linking different temperatures. With this low-temperature, low-power microwave source, it is quite possible to use smaller cryostats while still growing the number of qubits in a processor.
Our device produces one hundred times more power than previous versions, which is enough to control qubits and carry out quantum logic operations. It produces a very precise sine wave, oscillating over a billion times per second. As a result, errors in qubits from the microwave source are very infrequent, which is important when implementing precise quantum logic operations.
Mikko Möttönen, Study Lead and Professor, Aalto University
However, a continuous-wave microwave source, such as the one generated by this device, cannot be used as is to regulate qubits. First, microwaves have to be designed into pulses. The team is presently formulating techniques to rapidly turn the microwave source on and off.
Even without a switching solution to form pulses, an efficient, low-temperature, low-noise microwave source could be beneficial in a range of quantum technologies, such as quantum sensors.
In addition to quantum computers and sensors, the microwave source can act as a clock for other electronic devices. It can keep different devices in the same rhythm, allowing them to induce operations for several different qubits at the desired instant of time.
Mikko Möttönen, Study Lead and Professor, Aalto University
The preliminary design and the theoretical analysis were conducted by Juha Hassel and others at VTT. Hassel, who began this work at VTT, is presently the head of engineering and development at IQM, a Finnish quantum-computing hardware company.
The device was then constructed at VTT and worked by postdoctoral research Chengyu Yan and his colleagues at Aalto University using the OtaNano research infrastructure. Yan is now an associate professor at Huazhong University of Science and Technology, China.
The teams who contributed to this research are part of the Academy of Finland Centre of Excellence in Quantum Technology (QTF) and the Finnish Quantum Institute (InstituteQ).
Journal Reference:
Yan, C., et al. (2021) A low-noise on-chip coherent microwave source. Nature Electronics. doi.org/10.1038/s41928-021-00680-z.