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Quantum computing takes a step forward with new qubits with holes

EU-funded researchers have created a promising new qubit system based on the interacting spins of holes confined in a nanosized silicon-germanium quantum device.

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The processing capabilities of today’s supercomputers may boggle the mind, but quantum computers are expected to surpass even the most powerful of these machines. With their enormous processing capabilities and speeds, quantum computers will be able to solve problems no processor is currently able to. The secret to a quantum computer’s processing power lies in its use of quantum bits, or qubits – subatomic particles that are the basic units of quantum information. Researchers supported by the EU-funded MaGnum and microSPIRE projects have now developed a potential new system for reliable qubits using the spin of so-called holes. The creation of these qubits is described in their study published in the journal ‘Nature Materials’.

Spinning holes

A hole is the absence of an electron in a solid material and therefore carries a positive charge. Even though holes aren’t real particles, they have many properties in common with electrons. They interact when they come close to each other and they also possess the quantum mechanical property of spin. Holes in materials such as the metalloid germanium are excellent candidates for spin qubits. The scientists built a nanostructure of various layers of germanium and silicon, making it possible for them to confine holes in a practically bi-dimensional region. The lead author of the study, Daniel Jirovec, of the Institute of Science and Technology in Austria - which coordinates MaGnum, described their collaboration with the Laboratory for Nanostructure Epitaxy and Spintronics on Silicon (L-NESS) at the Polytechnic University of Milan, coordinator of the microSPIRE project. “Our colleagues at L-NESS layered several different mixtures of silicon and germanium just a few nanometers thick on top of each other. That allows us to confine the holes to the germanium-rich layer in the middle,” explained Jirovec in a news item posted on ‘HPCwire’. “On top, we added tiny electrical wires – so-called gates – to control the movement of holes by applying voltage to them. The electrically positively charged holes react to the voltage and can be extremely precisely moved around within their layer.” The research team used this technique to move two holes close to each other so that their spins would interact, in this way creating a spin qubit. More importantly, they were able to create the qubit out of the 2 interacting hole spins using less than 10 milliteslas of magnetic field strength – a value substantially weaker than the magnetic fields of other qubit set-ups. “By using our layered germanium setup we can reduce the required magnetic field strength and therefore allow the combination of our qubit with superconductors, usually inhibited by strong magnetic fields,” stated Jirovec, remarking on the significance of this achievement. The 2-year MaGnum (Majorana bound states in Ge/SiGe heterostructures) project ended in March 2021. microSPIRE (micro-crystals Single Photon InfraREd detectors) ends in October 2021. For more information, please see: MaGnum project microSPIRE project website


MaGnum, microSPIRE, hole spin, quantum, spin qubit, germanium, silicon, magnetic field

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