Major breakthrough in semiconductor quantum computers shrinking to desktop devices

To demonstrate the usefulness of quantum computers in real-world applications, millions of quantum bits are needed. Scalability is one of the biggest challenges for the development of future devices. One of the problems is that the quantum bits must be very close to each other on the chip in order to couple them together. Researchers at the Jülich Research Center and RWTH Aachen University have now taken a big step toward solving this problem: They have succeeded in transferring electrons (carriers of quantum information) a few microns on a semiconductor quantum chip, and their "quantum bus" (quantum bus) may be the key component for scaling to millions of quantum bits. Their "quantum bus" (quantum bus) may be a key component for scaling up to millions of quantum bits.

Semiconductor quantum chip with quantum bus

Quantum computers have the potential to greatly exceed the capabilities of traditional computers for certain tasks. But there is still a long way to go before they can help solve real-world problems. Many applications require quantum processors with millions of quantum bits. Today's prototypes present only a few such computing units.

Dr. Lars Schreiber of the JARA Institute for Quantum Information at the Jülich Research Center and RWTH Aachen University says [1], "Currently, each individual quantum bit is connected via several signal lines to a control unit about the size of a cupboard. This still works for a few quantum bits. But if you want to put millions of quantum bits on a chip, this no longer makes sense. Because that's what's necessary for quantum error correction."

At some point, the number of signal lines becomes a bottleneck. The lines take up too much space compared to the size of the tiny quantum bits. Moreover, a quantum chip cannot have millions of inputs and outputs: a modern classical chip contains only about 2,000 such inputs and outputs.Schreiber, together with colleagues at the Jülich Research Center and the RWTH Aachen University, has been conducting research for several years to find a solution to this problem.

Their overall goal is to integrate part of the control electronics directly on the chip: The approach is based on semiconductor spin quantum bits made of silicon and germanium. This type of quantum bit is relatively small. The fabrication process is essentially the same as that of a conventional silicon processor. This is considered advantageous when it comes to implementing a very large number of quantum bits. But first, some fundamental hurdles must be overcome.

Dr. Lars Schreiber (second from left) and Prof. Hendrik Bluhm (far right) with PhD students Tom Struck (far left) and Niels Focke (second from right) from the JARA Institute for Quantum Information.

Natural entanglement caused only by the proximity of particles is limited to a very small range, about 100 nanometers," says Schreiber. In order for the quantum bits to couple, they currently have to be placed very close to each other. There simply isn't room for the additional control electronics we would like to install."

To separate the quantum bits, the JARA Institute for Quantum Information (IQI) has come up with the idea of a quantum shuttle (quantum shuttle). This special component should help exchange quantum information between quantum bits over longer distances. The researchers have been working on the "quantum bus" for five years and have filed more than 10 patents. The research started as part of the European QuantERA consortium Si-QuBus and is now being continued together with industrial partners in the national project QUASAR of the German Federal Ministry of Education and Research (BMBF).

Device layout and shuttle pulses. a) Color scanning electron micrograph (SEM) of the three-gate layer design used for the device. b) Voltage trace Vi(t) applied to the electrically connected gate set during the electron shuttle. c) Electrostatic simulation of the potential difference ΔEp in an Si quantum well. d) Charge diagram where the amplitude of the shuttle pulse varies on the x-axis and the total voltage offset on the channel gates varies along the y-axis.

Prof. Hendrik Bluhm, Director of the IQI Institute, explains: "There must be about 10 microns of bridging from one quantum bit to the next. According to the theory, millions of quantum bits can be realized with such an architecture. We recently predicted this in collaboration with circuit engineers at the Central Institute for Engineering, Electronics and Analysis at the Jülich Research Center." Researchers at Delft University of Technology and Intel have come to the same conclusion.

Lars Schreiber and his team have now achieved an important step: they managed to transport an electron 5,000 times over a distance of 560 nanometers without any apparent errors: this corresponds to a distance of 2.8 millimeters. The results were published in the scientific journal npj Quantum Information [2].

An important improvement is that the electrons are driven by four simple control signals, which do not become more complex with increasing distance compared to previous methods. This is important because otherwise a large number of control electronics would be required, which would take up too much space: or they could not be integrated on the chip at all.

This achievement is based on a new way of transmitting electrons. "So far, people have tried to specifically guide electrons around individual disturbances in their path; or they have created a series of so-called 'quantum dots' and let the electrons jump from one of these quantum dots to another. Both approaches require precise signal tuning, which leads to overly complex control electronics." Lars Schreiber explains, "Instead, we generate a potential wave on which the electrons simply surf over various sources of interference. For such a uniform wave, a few control signals are sufficient: four sine wave pulses are enough."

PGI-11's quantum computer has shrunk to the size of a desktop device

As a next step, physicists now want to show that information about quantum bits encoded in electron spins is not lost during transport. Theoretical calculations have shown that this is possible in silicon over certain speed ranges. Thus, the quantum bus paves the way for scalable quantum computer architectures, which could also serve as the basis for millions of quantum bits.

Reference links:

[1]https://www.fz-juelich.de/en/news/archive/press-release/2022/key-element-for-a-scalable-quantum-computer?utm_source=miragenews&utm_medium=miragenews&utm_campaign=news