First demonstration of universal control of encoded spin qubits

HRL Laboratories, LLC, has published the first demonstration of universal control of encoded spin qubits. This new approach to quantum computing uses a new silicon-based qubit device architecture, fabricated in HRL’s Malibu cleanroom, to trap single electrons in quantum dots. The spins of three such single electrons host energy-degenerate qubit states, which are governed by nearest-neighbor contact interactions that partially exchange spin states with those of their neighbors.

Published online prior to publication in the journal Nature, The HRL experiment demonstrated universal control of their encoded qubits, meaning that qubits can be used successfully for any implementation of quantum computing algorithms. The encoded silicon/silicon-germanium quantum dot qubits use three electron spins and a control scheme whereby voltages applied to metal gates partially shift the directions of those electron spins without ever aligning them in any particular direction. The demonstration involved applying thousands of these precisely calibrated voltage pulses in close relation to each other over the course of a few millionths of a second. The paper is titled “Universal logic with encoded spin qubits in silicon.”

The quantum coherence offered by the isotopically enriched silicon used, the all-electric and low-crosstalk control of partial swap operations, and the configurable insensitivity of the encoding to certain error sources combine to provide a strong path toward scalable error tolerance and computational advantage. , major steps towards a commercial quantum computer.

“In addition to the obvious design and manufacturing challenges, a lot of robust software had to be written, for example to tune and calibrate our control scheme,” said HRL researcher and first author Aaron Weinstein. “Considerable effort was put into developing efficient, automated routines to determine which applied voltage led to which degree of partial swap. Since thousands of such operations had to be implemented to determine error levels, each one had to be precise. We worked hard to get all that control working with high precision.”

“This was very much a team effort,” said HRL group leader and co-author Mitch Jones. “The enabling work of talented control software, theory, device growth and fabrication teams was essential. In addition, many measurements of devices were needed to understand enough of the internal physics and to develop routines to reliably control these quantum mechanical interactions. This work and demonstration is the culmination of these measurements, made even better by the time I’ve spent working with some of the brightest scientists I’ve met.”

“It’s 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 co-author. “Real challenges remain in improving error, scale, speed, uniformity, cross-talk and other aspects, but none of these require a miracle. For many other kinds of qubits, there is at least one aspect that is still looking really, really hard out.”

Once realized at scale, quantum computers will differ from traditional supercomputers in that they use a fragile feature of quantum mechanics called quantum entanglement to perform certain calculations in a very short amount of time that would take traditional computers years or decades. Among many possible applications, one example of computation is simulating the behavior of large molecules.

Only a small amount of data is needed to describe the atoms in a molecule, but a very large workspace is needed to calculate all the quantum mechanical states that electrons in the molecule can have. Quantum chemistry simulations can dramatically affect many technological directions from materials development to drug discovery to developing processes to mitigate climate change.