Two Atoms Entangled Using Microwaves

For the first time NIST physicists have linked the quantum properties of two separated ions by manipulating them with microwaves instead of the usual array of laser beams. The development could pave the way for miniaturized, easy-to-commercialize quantum computing technologies.

While microwaves have been used in past experiments to manipulate single ions, they have never been used to position ions close enough (30 µm away) to enable entanglement — a quantum phenomenon said to be crucial for transporting information and correcting errors in quantum computers.

The experiments integrated wiring for microwave sources directly on a chip-size ion trap and used a desktop-scale table of lasers, mirrors and lenses only about one-tenth the size previously required. Low-power ultraviolet lasers are still needed to cool the ions and observe experimental results but might eventually be made as small as those in portable DVD players, the physicists said.

A pair of ions is trapped by electric fields and manipulated with microwaves inside a glass chamber at the center of the apparatus. The chamber is illuminated by a green LED for visual effect. An ultraviolet laser beam used to cool the ions and detect their quantum state is colorized to appear blue. (Images: Y. Colombe/NIST)

"It's conceivable a modest-sized quantum computer could eventually look like a smartphone combined with a laser pointer-like device, while sophisticated machines might have an overall footprint comparable to a regular desktop PC," said Dietrich Leibfried, a NIST physicist.

Ions are a leading candidate for use as quantum bits (qubits) to hold information in a quantum computer. Although other promising candidates for qubits — notably superconducting circuits, or artificial atoms — are manipulated on chips with microwaves, ion qubits are at a more advanced stage experimentally in that more ions can be controlled with better accuracy and less loss of information.

In the NIST experiments, two ions hover above the middle of the square gold trap, which measures 7.4 mm on a side. Scientists manipulate and entangle the ions using microwaves fed into wires on the trap from the three thick electrodes at the lower right.

The same NIST research group previously used ions and lasers to demonstrate many basic components and processes for a quantum computer. In the latest experiments, the two ions were held by electromagnetic fields, hovering above an ion trap chip consisting of gold electrodes electroplated onto an aluminum nitride backing. Some of the electrodes were activated to create pulses of oscillating microwave radiation (1-2 GHz) around the ions. The microwaves produce magnetic fields used to rotate the ions' spins, which can be thought of as tiny bar magnets pointing in different directions. The orientation of these tiny bar magnets is one of the quantum properties used to represent information.

Scientists entangled the ions by adapting a technique they first developed with lasers. If the microwaves' magnetic fields gradually increase across the ions in just the right way, the ions' motion can be excited depending on the spin orientations, and the spins can become entangled in the process. The properties of the entangled ions are linked such that a measurement of one ion would reveal the state of the other.

The use of microwaves reduces errors introduced by instabilities in laser beam pointing and power as well as laser-induced spontaneous emissions by the ions. However, microwave operations need to be improved to enable practical quantum computations or simulations. The NIST researchers achieved entanglement 76 percent of the time, well above the minimum threshold of 50 percent defining the onset of quantum properties, but not yet competitive with the best laser-controlled operations at 99.3 percent.

The study was published in the journal Nature.

For more information, visit: www.nist.gov