First implementation! Universal Control of Silicon Encoded Spin Quantum Bits
Recently, the HRL experimental team released their first demonstration of universal control of their encoded spin quantum bits, implying that these quantum bits can be successfully implemented using any type of quantum computing algorithm. The results were published last month in Nature. ("Universal logic with encoded spin qubits in silicon")
This emerging approach to quantum computing uses a new silicon-based quantum bit device architecture fabricated in an HRL cleanroom to trap individual electrons in quantum dots. The spins of three such single electrons carry energy-simplified quantum bit states and are controlled by nearest-neighbor contact interactions while these interactions are partially exchanged with the spin states of neighboring electrons.
The encoded silicon quantum bits use three electron spins and a control scheme in which the voltage applied to the metal gate partially exchanges the orientation of these electron spins without aligning them in any particular direction. The demonstration involves the application of thousands of precisely calibrated voltage pulses in a few parts per million of a second, in strict relationship to each other.
The combination of quantum coherence provided by the experiments using isotope-rich silicon, all-electric and low-crosstalk control of the partial-swap operation, and configurable insensitivity of the encoding to certain error sources provides a powerful pathway to scalable fault tolerance and computational advantage, a major step toward commercial quantum computers.
"In addition to the obvious design and manufacturing challenges, much powerful software had to be written to tune and calibrate our control scheme." said Aaron Weinstein, first author of the experiment. "We put a tremendous amount of effort into developing efficient, automated procedures to determine how much of the applied voltage resulted in partial exchange. Because thousands of these operations must be performed to determine the error level, each operation must be precise. We worked hard to get all of these controls to work with high precision."
It was very much a team effort, and the support work of the talented control software, theory, equipment development and manufacturing teams was critical. In addition, multiple measurements of the device are required to fully understand the internal physics and develop routines to reliably control these quantum mechanical interactions.
Once implemented on a large scale, quantum computers, because they use a fragile property of quantum mechanics - quantum entanglement - will be able to perform certain calculations in very short periods of time that would take traditional computers years or decades to perform.
One example calculation among many possible applications is the simulation of large molecules. Only a small amount of data is needed to describe the atoms in a molecule, but a very large workspace is required to calculate all the quantum mechanical states that an electron in a molecule might have. Quantum chemical simulations can significantly influence many technological directions, from materials development to drug discovery to the development of processes to mitigate climate change.
