Quantum computers just got an upgrade – and it’s 10× more efficient

Bits, which are the building blocks of a conventional computer, can only ever have the value of 1 or 0. By contrast, the common building blocks of a quantum computer, quantum bits or qubits, can exist in states having the value 1 and 0 simultaneously as well as all states in between in any combination. This means that a 20 qubit quantum computer can represent over a million different states simultaneously. This phenomenon, which is called superposition, is one of the key reasons that quantum computers can solve exceptionally complex problems that are beyond the capabilities of today's conventional supercomputers.

Amplifiers are essential - but cause decoherence

To be able to utilize a quantum computer's computational power, qubits must be measured and converted into interpretable information. This process requires extremely sensitive microwave amplifiers to ensure that these weak signals are accurately detected and read. However, reading quantum information is an extremely delicate business - even the slightest temperature fluctuation, noise, or electromagnetic interference can cause qubits to lose their integrity, their quantum state, rendering the information unusable. Because the amplifiers generate output in the form of heat, they also cause decoherence. As a result, researchers in this field are always in pursuit of more efficient qubit amplifiers. Now, Chalmers researchers have taken an important step forward with their new, highly efficient amplifier.

"This is the most sensitive amplifier that can be built today using transistors. We've now managed to reduce its power consumption to just one-tenth of that required by today's best amplifiers - without compromising performance. We hope and believe that this breakthrough will enable more accurate readout of qubits in the future," says Yin Zeng, a doctoral student in terahertz and millimeter wave technology at Chalmers, and the first author of the study published in the journal IEEE Transactions on Microwave Theory and Techniques.

An essential breakthrough in scaling up quantum computers

This advance could be significant in scaling up quantum computers to accommodate significantly more qubits than today. Chalmers has been actively engaged in this field for many years through a national research program, the Wallenberg Centre for Quantum Technology. As the number of qubits increases, so does the computer's computational power and capacity to handle highly complex calculations. However, larger quantum systems also require more amplifiers, leading to greater overall power consumption, which can lead to decoherence of the qubits.

"This study offers a solution in future upscaling of quantum computers where the heat generated by these qubit amplifiers poses a major limiting factor," says Jan Grahn, professor of microwave electronics at Chalmers and Yin Zeng's principal supervisor.

Activated only when needed

Unlike other low-noise amplifiers, the new amplifier developed by the Chalmers researchers is pulse-operated, meaning that it is activated only when needed for qubit amplification rather than being always switched on.

"This is the first demonstration of low-noise semiconductor amplifiers for quantum readout in pulsed operation that does not affect performance and with drastically reduced power consumption compared to the current state of the art," says Jan Grahn.

Since quantum information is transmitted in pulses, one of the key challenges was to ensure that the amplifier was activated rapidly enough to keep pace with the qubit readout. The Chalmers team addressed this by designing a smart amplifier using an algorithm that improves the operation of the amplifier. To validate their approach, they also developed a novel technique for measuring the noise and amplification of a pulse-operated low-noise microwave amplifier.

"We used genetic programming to enable smart control of the amplifier. As a result, it responded much faster to the incoming qubit pulse, in just 35 nanoseconds," says Yin Zeng.

More information about the study:

Read the article Pulsed HEMT LNA Operation for Qubit Readout in IEEE Transactions on Microwave Theory and Techniques.

The article is authored by Yin Zeng and Jan Grahn, both active at the Terahertz and Millimeter Wave Technology Laboratory at the Department of Microtechnology and Nanoscience at Chalmers University of Technology, and by Jörgen Stenarson and Peter Sobis, both active at Low Noise Factory AB.

The amplifier has been developed using the Kollberg Laboratory at Chalmers University of Technology and at Low Noise Factory AB in Gothenburg, Sweden.

The research project is funded by the Chalmers Centre for Wireless Infrastructure Technology (WiTECH) and by the Vinnova program Smarter electronic systems.