Spin qubits go trampolining
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Researchers at QuTech developed somersaulting spin qubits for universal quantum logic. This achievement may enable efficient control of large semiconductor qubit arrays
Qubits based on quantum dots are studied worldwide as they are considered a compelling platform for the construction of a quantum computer. The most popular approach is to trap a single electron and to apply a sufficiently large magnetic field, allowing the spin of the electron to be used as a qubit and controlled by microwave signals.
In this work, however, the researchers demonstrate that no microwave signals are needed. Instead, baseband signals and small magnetic fields are sufficient to achieve universal qubit control. This is beneficial because it can significantly simplify the control electronics required to operate future quantum processors.
From hopping to somersaulting qubits
Controlling the spin requires hopping from dot to dot and a physical mechanism capable of rotating it. Initially, the proposal of Loss and DiVincenzo uses a specific type of magnet, which proved difficult to realize experimentally. Instead, the group at QuTech pioneered germanium. This semiconductor conveniently may by itself already allow for spin rotations. This is motivated by work published in Nature Communications, where Floor van Riggelen-Doelman and Corentin Déprez of the same group show that germanium can serve as a platform for hopping of spin qubits as a basis to make quantum links. They observed first indications of spin rotations.
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 neighbouring trampolines if available. Germanium has a unique property: just 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.
Chien-An Wang, first-author of the Science paper, specifies: “Germanium has the advantage of aligning spins along different directions in different quantum dots.” It turned out that very good qubits can be made by hopping spins between such quantum dots. “We measured error rates less than a thousand for one-qubit gates and less than a hundred for two-qubit gates.”
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