A team at QuTech have developed acrobatic spin qubits for universal quantum logic. This feat could empower streamlined control of extensive semiconductor qubit arrays.
Quantum Dot Qubits
In 1998, Loss and DiVincenzo released the influential paper ‘quantum computation with quantum dots’. In their seminal work, the movement of spins was proposed as a foundation for qubit logic, but a practical implementation has been missing. After more than 20 years, experiments have caught up with theory. Researchers at QuTech —a collaboration between the TU Delft and TNO— have proven that the original ‘hopping gates’ are indeed feasible, with cutting-edge performance.
Advancements in Qubit Management
Qubits based on quantum dots are being explored globally, as they are deemed a convincing platform for constructing a quantum computer. The prevalent approach involves capturing a single electron and applying a sufficiently large magnetic field, enabling the electron’s spin to serve as a qubit and be managed by microwave signals.
However, in this research, the team illustrates that microwave signals are unnecessary. Instead, basic signals and minor magnetic fields are adequate to achieve comprehensive qubit control. This is advantageous as it can drastically simplify the control systems needed to operate upcoming quantum processors.
From Hopping to Acrobatic Qubits
Managing the spin necessitates hopping from one dot to another and a mechanical device capable of rotating it. Originally, the proposition of Loss and DiVincenzo involves a certain type of magnet, which proved challenging to actualize experimentally. Conversely, the team at QuTech spearheaded with germanium. This semiconductor possesses the potential to enable spin rotations independently. This is encouraged by research in Nature Communications, where Floor van Riggelen-Doelman and Corentin Déprez of the same group demonstrate that germanium can serve as a stage for hopping of spin qubits to form quantum connections. They initially detected indications of spin rotations.
When contrasting hopping and acrobatic qubits, envision quantum dot arrays as a recreational park, where electron spins resemble individuals jumping. Typically, each individual has a dedicated jumping pad, but they can hop over to neighboring pads if available. Germanium possesses a unique characteristic: simply by leaping from one jump pad to the next, an individual encounters a rotational force that induces them to perform acrobatics. This characteristic facilitates thorough control over the qubits.
Chien-An Wang, lead author of the Science article, specifies: “Germanium brings about the alignment of spins along different orientations in distinct quantum dots.” It was discovered that exceptional qubits can be created by hopping spins between such quantum dots. “We recorded error rates below one thousand for single-qubit operations and below one hundred for two-qubit operations.”
Acrobatic Qubits in a Recreational Park
Having established control over two spins in a four-quantum dot framework, the team progressed further. Rather than hopping spins between two quantum dots, the team also explored hopping across multiple quantum dots. Analogously, this would be akin to a person hopping and acrobatically navigating over numerous jump pads. Co-author Valentin John elaborates: “For quantum computing, precision in operating and coordinating large numbers of qubits is vital.”
Different jump pads induce varying forces on individuals when hopping, and similarly, hopping spins between quantum dots also results in distinct rotations. Hence, it is critical to evaluate and comprehend the diversity. Co-author Francesco Borsoi adds: “We established control routines that allow for spins to hop to any quantum dot in a 10-quantum dot grouping, enabling us to assess essential qubit characteristics in extended setups.”
Collaborative Endeavor
“I am delighted to witness the collective effort” principal investigator Menno Veldhorst summarizes. “Within a year, the observation of qubit rotations due to hopping evolved into a tool used by the entire team. We believe it is vital to develop efficient control strategies for the operation of forthcoming quantum computers, and this new approach holds promise.”
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