University of Michigan scientists have created the first quantum microchip, which could be a giant stride in the race to produce a new generation of brawny, super-fast computers.
Working with individual ions is key to building powerful computing machines that will exploit quantum physics — instead of transistors — and trump the power of today’s most powerful supercomputers.
So, on a semiconductor chip roughly the size of a postage stamp, the Michigan scientists designed and built a device known as an ion trap, which allowed them to isolate individual charged atoms and manipulate their quantum states.
An ion expresses a positive or negative charge, depending on whether its parent atom has a missing or an extra electron. And ions are the preferred building blocks for a quantum system.
"The cadmium atom that has lost an electron becomes a negatively charged ion, which can then be controlled with an electrical field," said Daniel Stick, a doctoral student in the University of Michigan’s physics department who participated in the work.
To isolate an ion, scientists confine it in the ion trap while applying electric fields. Laser light manipulates the spin of the ion’s free electron to flip it between quantum states.
The spin of the electron dictates the value of the quantum bit, or "qubit." For example, an up-spin can represent a one, or a down-spin can represent a zero — or the qubit can occupy both states simultaneously.
This enigmatic feature of quantum mechanics is what gives the qubit a powerful advantage over the binary digit of classical computing. Known as quantum superposition, the ability of the qubit to occupy two quantum states at once means that it can execute computations at an exponentially faster rate. Each time a qubit is added to a quantum system, its computing power doubles.
The new chip, which is made of gallium arsenide, should be easily scaled and mass-produced, because it’s made using microlithography — the same process that makes microchips.
Scientists can grow the chip using multiple one-atom-thick layers in a process called molecular beam epitaxy.
The finished chip has an empty space in its center that is engineered to extremely precise dimensions. Cantilevered electrodes surround the space, which is open to allow laser beam access and observation of the trapped ion.
Laser pulses fired into vaporized cadmium prepare ions for the trap. Once an ion is trapped, it floats in electric fields supplied by the chip’s electrodes, according to Christopher Monroe, a physics professor at the University of Michigan who led the project.
A valuable feature of the quantum chip is that its size can be scaled to accommodate the objectives of a particular project. "Our target is to eventually develop a chip that can entrap 10 ions at a time," said Monroe. "But the primary goal is to prove that it works."