A new super-cooled microwave source boosts the scale-up of quantum
computers
A newly designed microwave source could replace existing bulky control
systems that hinder the scalability of quantum computers

Date:
December 9, 2021
Source:
Aalto University
Summary:
Researchers have developed a circuit that produces the high-quality
microwave signals required to control quantum computers while
operating at temperatures near absolute zero. This is a key step
towards moving the control system closer to the quantum processor,
which may make it possible to greatly increase the number of qubits
in the processor.



FULL STORY ========================================================================== Researchers in Finland have developed a circuit that produces the
high-quality microwave signals required to control quantum computers while operating at temperatures near absolute zero. This is a key step towards
moving the control system closer to the quantum processor, which may make
it possible to greatly increase the number of qubits in the processor.


==========================================================================
One of the factors limiting the size of quantum computers is the
mechanism used to control the qubits in quantum processors. This is
normally accomplished using a series of microwave pulses, and because
quantum processors operate at temperatures near absolute zero, the
control pulses are normally brought into the cooled environment via
broadband cables from room temperature.

As the number of qubits grows, so does the number of cables
needed. This limits the potential size of a quantum processor, because
the refrigerators cooling the qubits would have to become larger to
accommodate more and more cables while also working harder to cool them
down -- ultimately a losing proposition.

A research consortium led by Aalto University and VTT Technical Research
Centre of Finland has now developed a key component of the solution to
this conundrum.

'We have built a precise microwave source that works at the same extremely
low temperature as the quantum processors, approximately -273 degrees,'
says Mikko Mo"tto"nen, Professor at Aalto University and VTT Technical
Research Centre of Finland, who led the team.

The new microwave source is an on-chip device that can be integrated
with a quantum processor. Less than a millimetre in size, it potentially removes the need for high-frequency control cables connecting different temperatures. With this low-power, low-temperature microwave source,
it may be possible to use smaller cryostats while still increasing 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,'
says Mo"tto"nen. '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.' However, a continuous-wave microwave source,
such as the one produced by this device, cannot be used as is to control qubits. First, the microwaves must be shaped into pulses. The team is
currently developing methods to quickly switch the microwave source on
and off.

Even without a switching solution to create pulses, an efficient,
low-noise, low-temperature microwave source could be useful 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,' explains Mo"tto"nen.

The theoretical analysis and the initial design were carried out by
Juha Hassel and others at VTT. Hassel, who started this work at VTT,
is currently the head of engineering and development at IQM, a Finnish quantum-computing hardware company. The device was then built at VTT
and operated by postdoctoral research Chengyu Yan and his colleagues
at Aalto University using the OtaNano research infrastructure. Yan is
currently an associate professor at Huazhong University of Science and Technology, China. The teams involved in this research are part of the
Academy of Finland Centre of Excellence in Quantum Technology (QTF)
and the Finnish Quantum Institute (InstituteQ).

========================================================================== Story Source: Materials provided by Aalto_University. Note: Content may
be edited for style and length.


========================================================================== Journal Reference:
1. Chengyu Yan, Juha Hassel, Visa Vesterinen, Jinli Zhang, Joni
Ikonen, Leif
Gro"nberg, Jan Goetz, Mikko Mo"tto"nen. A low-noise on-chip coherent
microwave source. Nature Electronics, 2021; DOI: 10.1038/s41928-021-
00680-z ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/12/211209124451.htm

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