Decades-old theory proved possible as QuTech's "somersaulting" spin qubits simplify control, paving way for scalable quantum computing.

Researchers at QuTech have taken a giant leap in quantum computing with the development of "somersaulting" spin qubits, an innovation that simplifies and enhances control of this advanced technology and marks a breakthrough that could revolutionize the construction of powerful quantum processors.

Quantum computing, unlike classical computing, leverages the principles of quantum mechanics to process information in ways that traditional computers cannot.

At the heart of this technology are quantum bits or qubits, which can represent and store information in multiple states simultaneously, thanks to a phenomenon known as superposition. This essentially means that, unlike conventional computer binary digits or bits (the smallest unit of data that a computer can process and store), which are either a zero or a one, qubits can be a zero, one or both at the same time.

This capability allows quantum computers to perform many calculations all at once, enabling a quantum leap in speed advantage over even the fastest modern computers.

This jump in computational power is further enhanced by something called spin transport electronics or spintronics, a field that leverages the intrinsic property of an electron – spinning.

Researchers have begun developing advanced electronics that push beyond the traditional limits of Moore's Law by using this natural process, which creates magnetism through rotation.

Utilizing what is known about the properties of qubits and the principles of spintronics, QuTech has applied these concepts to develop the next evolution of this reality-breaking technology.

Decades later

Co-authors Sasha Ivlev, Hanifa Tidjani and Chien-An Wang.

In 1998, Daniel Loss and David DiVincenzo published the seminal work "Quantum computation with quantum dots." In their original work, hopping of spins was proposed as a basis for qubit logic, but an experimental implementation ultimately remained lacking for decades.

Now, experiments have caught up with theory.

Researchers at QuTech – a collaboration between Delft University of Technology and the Netherlands Organisation for Applied Scientific Research – have demonstrated that the original "hopping gates" are indeed possible, with cutting-edge performance.

Qubits based on quantum dots are studied worldwide as they are considered a compelling platform for constructing a quantum computer.

Quantum dots are very small particles made from semiconductor materials that can capture and manipulate electrons – like trapping fireflies in glass jars – thus making them ideal for precise control in quantum computing.

The most popular approach involves locking a single electron in a quantum dot and applying a strong magnetic field to control its spin using microwave signals.

However, researchers at QuTech have demonstrated that microwave signals are not necessary. Instead, they have shown that simpler baseband signals and smaller magnetic fields can achieve universal qubit control.

This is beneficial because it can significantly simplify the control electronics required to operate future quantum processors.

The new technique involves hopping spins between quantum dots, where the unique properties of germanium allow the spins to somersault, or rotate, as they move.

Germanium jumping park

Marc Blommaert for QuTech

Co-authors Sander de Snoo and Floor van Riggelen-Doelman.

Germanium, a semiconductor with unique atomic structure and electronic properties that are particularly suitable for creating and controlling qubits, has emerged as a key material in the advancement of quantum computing.

Uncovering the full potential of this element through the development of germanium-based transistors, QuTech has been instrumental in pushing the boundaries of using this material for quantum technologies.

Now, the company has pioneered its use in facilitating the hopping of spin qubits and inducing the necessary rotations for precise control.

This research, published in prestigious journals like Nature Communications and Science, demonstrates how germanium can significantly reduce the complexity of control systems needed for quantum processors.

"Germanium has the advantage of aligning spins along different directions in different quantum dots," said Chien-An Wang, the first author of the Science paper. "We measured error rates less than a thousand for one-qubit gates and less than a hundred for two-qubit gates."

When considering the difference between hopping and somersaulting qubits, think of quantum dot arrays as a trampoline park where electron spins are like people jumping.

Typically, each person has a dedicated trampoline, but they can hop over to neighboring trampolines if available. Germanium holds a unique property: by jumping from one trampoline to the next, a person experiences a torque that makes them somersault. This property allows researchers to control the qubits effectively.

Having established control over two spins in a four-quantum dot system, the team took it a step further.

Instead of hopping spins between two quantum dots, the team also investigated hopping through several quantum dots. Analogously, this would correspond to a person hopping and somersaulting over many trampolines.

"For quantum computing, it is necessary to operate and couple large numbers of qubits with high precision," said co-author Valentin John.

Different trampolines make people experience different torques when jumping, and similarly, hopping spins between quantum dots also result in unique rotations. It is thus important to characterize and understand the variability.

"We established control routines that enables to hop spins to any quantum dot in a 10-quantum dot array, which allows us to probe key qubit metrics in extended systems," said co-author Francesco Borsoi.

Broader implications

These advancements at QuTech are not just a technical triumph but also a significant step toward making large-scale quantum computing a reality.

By simplifying the control mechanisms and reducing error rates, this research opens new possibilities for more practical and scalable quantum processors.

Principal investigator Menno Veldhorst emphasized the importance of teamwork in these breakthroughs.

"I am proud to see all the teamwork," he said. "In a time span of a year, the observation of qubit rotations due to hopping became a tool that is used by the entire group. We believe it is critical to develop efficient control schemes for the operation of future quantum computers, and this new approach is promising."

The continuous effort and collaboration at QuTech are pushing the boundaries of what is possible in quantum computing, promising a future where quantum processors are not just a theoretical concept but a practical and powerful reality.