Coupling Electron Spins to Microwave Photons in Silicon – A ‘Quantum’ Leap Towards Information Processing

Quantum Computing

Scientists from Berkeley Laboratory’s Accelerator and Fusion Research Division (AFRD) have added a new dimension to the field of quantum computing by spontaneously switching electron spins from an excited state to equilibrium on demand.

The development could not only be a significant step towards processing information in a quantum computer, but it could also lead to potential enhancements in magnetic resonance techniques, which has many practical usages including biomolecular studies and medical imaging.

The experiment involved implanting a refined form of silicon with a matrix of bismuth atoms – a process commonly known as ‘doping’. The bismuth atoms embedded in the silicon sample provided the electrons with unique spin properties that enabled the experiment.

This specially developed compound was then fitted with a superconducting resonator and placed inside a resonant cavity. The material was subjected to precisely tuned microwaves to force the electrons into emitting their energy in the form of microwave particles (photons).

The release of photon causes relaxation of electron spins in a controlled and accelerated manner, thus bringing them back to the state of equilibrium from an excited state almost spontaneously, in just about 1 second. Left on their own, the natural rate of electrons flipping back from excited to relaxed state is about 10,000 years.

The researchers are hopeful that the ability of coupling individual electron spins to microwave photons could form the basis of a silicon-based quantum computer architecture. The ultimate objective of the experiment is to use the spin states of donor atoms embedded in silicon as the fundamental components of a quantum computer – the quantum bits or simply, the qubits.

Unlike the traditional bits used by the modern-day computers, which can either be 1 or 0, a qubit has the ability to behave as both 1 and 0 simultaneously – thanks to the peculiar laws of quantum mechanics.

According to Thomas Schenkel, who led the design and development of the silicon-bismuth compound, a coupled array of qubits would allow a quantum computer to be exponentially more powerful than modern computers and they will be essential for solving certain difficult calculations which are apparently impossible to process using a classical computer.

This revolutionary research has demonstrated the fact that quantum computing is possible with donor electron-spin qubits in silicon. The electrons of donor atoms can indeed maintain their spin states and coherence for the duration of time required to perform basic quantum computing.

In order to improve the rate at which the experiment could be repeated, the researchers are now trying to find a way to cause the electron spins to relax on demand. The next challenge would be to further refine the bismuth-doped silicon compound to enhance the performance and improve the overall spin quality. According to the scientists involved in the research, there is the possibility to further accelerate the electron-flipping behavior to below 1 millisecond, compared to the 1 second rate attained during the current experiment.

Only time will tell whether or not they succeed in their attempt, but the research so far has been a significant leap towards addressing the challenges associated with processing information in a quantum computer.