Quantum magnets forced to “chillax”
In what may provide a potential path to connecting data in a quantum computer, researchers have shown that excited atoms in silicon can be forced into a relaxed state on-demand using a device that serves as a microwave “tuning fork."
The team’s findings could also lead to enhancements in magnetic resonance techniques, which are widely used to explore the structure of materials and biomolecules, and for medical imaging.
The international team, which included scientists from the Atomic Energy Commission (CEA, France), the London Centre for Nanotechnology at UCL (UK), and Lawrence Berkeley National Laboratory (LBNL, USA), demonstrated how to use a specially designed superconducting cavity to dramatically increase the coupling between microwaves and a fundamental magnetic property of electrons called “spin”.
When placed in a magnetic field, a electron spin can exist in a low-energy state aligned with the field, or an excited high-energy state aligned against it. When placed in the excited state, an electron spin can, in principle, relax into the low-energy state by emitting a particle of microwave energy known as photon. However the natural rate for this microwave-emitting effect is about once every 10,000 years, making it hard to observe and of little practical use.
The team’s experiment demonstrated an accelerated, controllable relaxation of electron spins into their low-energy state and the release of a microwave photon in about 1 second – about one trillion times faster than the natural rate. The results are published in the February 15th edition of Nature.
"Our results are highly significant for quantum information processing," said Patrice Bertet, a quantum electronics scientist at CEA who led the experiment. "Indeed, they are a first step toward the strong coupling of individual electron spins to microwave photons, which could form the basis of a new spin-based quantum computer architecture."
John Morton, a professor at the London Center for Nanotechnology and co-author of the study, said:
"Our ultimate aim is to find a way to link quantum information that is fixed in one place and quantum information that can be transported by photons."
In today's computers, information is stored as individual bits, and each bit can either be a one or a zero. Quantum computers, though, could conceivably be exponentially more powerful than modern computers because they would use a different kind of bit, called a qubit, that because of the weird ways of quantum mechanics can simultaneously behave as both a one and a zero.
A coupled array of qubits would allow a quantum computer to perform many, many calculations at the same time, and electron spins are candidates for qubits in a quantum computer. The latest study shows how the microwave photons could work in concert with the spins of electrons as a way to move information around in a quantum computer.
"What we need now is ways to wire up these systems—to couple these spins together," Morton said. "We need to make coupled qubits that can perform computations."
In the experiment, conducted at CEA in France, a small sample of a highly purified form of silicon was implanted with a matrix of bismuth atoms, and a superconducting aluminum circuit was deposited on top to create a high-quality resonant cavity that allowed precise tuning of the microwaves. The electron spins of the bismuth atoms were driven into their excited high-energy state.
The microwave cavity was then tuned, like a musical tuning fork, to a particular resonance that coaxed the spins into emitting a photon as they flipped back to a relaxed state. The cavity boosted the number of states into which a photon can be emitted, which greatly increased the decay rate for the electron spins in a controllable way. “The technique is much like buying a huge number of lottery tickets to increase your chances of winning”, Morton said.
Thomas Schenkel, a physicist in Berkeley Lab’s Accelerator Technology and Applied Physics Division who developed the sample used in the experiment said that implanting the bismuth atoms into the delicate silicon framework was "like squeezing bowling balls into a lattice of ping-pong balls”.
In addition to applications in networking quantum bits in a quantum computer, this research could potentially prove useful in boosting the sensitivity of scientific techniques like nuclear magnetic resonance spectroscopy and dynamic nuclear polarization, by shortening experimental times.
"You need a way to reset spins—the ability to cause them to relax on demand to improve the rate at which you can repeat an experiment," Morton said.
Bertet said it may be possible to further accelerate the electron-flipping behavior to below 1 millisecond, compared to the 1-second rate in the latest results.
"This will then open the way to many new applications," he said.
The silicon chip containing the spins, with a superconducting aluminium resonator patterned on top of it, mounted on top of a sapphire slide so that it sits within the copper enclosure.