HRL Laboratories, LLC, has published the first demonstration of universal control of encoded spin qubits. This newly emerging approach to quantum computation uses a novel silicon-based qubit device architecture, fabricated in HRL’s Malibu cleanroom, to trap single electrons in quantum dots. Spins of
three such single electrons host energy-degenerate qubit states, which are controlled by nearest-neighbor contact interactions that partially swap spin states with those of their neighbors.
Posted online ahead of publication in the journal Nature, the HRL experiment demonstrated universal control of their encoded qubits, which means the qubits can be used successfully for any kind of quantum computational algorithm implementation. The encoded silicon/silicon germanium quantum dot qubits use three electron spins and a control scheme whereby voltages applied to metal gates partially swap the directions of those electron-spins without ever aligning them in any particular direction.
“Beyond the obvious challenges of design and fabrication, a lot of robust software had to be written, for example to tune up and calibrate our control scheme,” said HRL scientist and first author Aaron Weinstein. “Significant effort was placed in developing efficient, automated routines for determining what applied voltage led to what degree of partial swap. Since thousands of such operations had to be implemented to determine error levels, each one had to be precise.
“It is hard to define what the best qubit technology is, but I think the silicon exchange-only qubit is at least the best-balanced,” said Thaddeus Ladd, HRL group leader and coauthor. “Real challenges remain in improving error, scale, speed, uniformity, crosstalk, and other aspects, but none of these requires a miracle. For many other kinds of qubits, there is at least one aspect that still looks really, really hard.
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