In 1998, Loss and DiVincenzo introduced the concept of quantum computation with quantum dots, proposing the hopping of spins as a basis for qubit logic. Although this idea remained theoretical for over two decades, the researchers at QuTech-a collaboration between TU Delft and TNO-have now demonstrated the feasibility of 'hopping gates' with advanced performance.
Quantum dot-based qubits are widely studied for quantum computing. Traditionally, controlling these qubits involves trapping a single electron and applying a large magnetic field, with microwave signals used for spin manipulation. However, the new research from QuTech shows that universal qubit control can be achieved without microwave signals. Instead, baseband signals and small magnetic fields are sufficient, greatly simplifying the control electronics for future quantum processors.
From Hopping to Somersaulting Qubits
To control the spin of an electron, it must hop from one quantum dot to another and undergo rotation. The original proposal by Loss and DiVincenzo involved a type of magnet that was challenging to implement experimentally. QuTech's team found a solution in germanium, a semiconductor that naturally allows for spin rotations. This was initially suggested by their work published in Nature Communications, where researchers Floor van Riggelen-Doelman and Corentin Deprez demonstrated that germanium could enable the hopping of spin qubits, laying the groundwork for quantum links and revealing early signs of spin rotations.
In this analogy, think of quantum dot arrays as a trampoline park with electron spins jumping between trampolines. Germanium's unique property causes these jumping spins to somersault, enabling effective qubit control. Chien-An Wang, the first author of the Science paper, noted, "Germanium has the advantage of aligning spins along different directions in different quantum dots." The team achieved error rates less than a thousand for one-qubit gates and less than a hundred for two-qubit gates.
Advancing Somersaulting Qubit Control
The researchers extended their control from two spins in a four-quantum dot system to multiple quantum dots, akin to a person somersaulting over several trampolines.
Co-author Valentin John explained, "For quantum computing, it is necessary to operate and couple large numbers of qubits with high precision." Different quantum dots induce varying rotations, making it crucial to understand and manage this variability. Co-author Francesco Borsoi added, "We established control routines that enable hopping spins to any quantum dot in a 10-quantum dot array, allowing us to probe key qubit metrics in extended systems."
Principal investigator Menno Veldhorst expressed pride in the team's efforts: "I am proud to see all the teamwork. 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."
Research Report:Operating semiconductor quantum processors with hopping spins
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