Gem-sized diamonds open quantum computing doors

Strange "quantum" physics usually seen only in very small things such as atoms have been induced and observed in diamonds big enough to adorn jewelry.
The hardness of diamond makes it possible to put them into entagled states that are less susceptible to disturbances that normally destroy coherence. (iStock)
Strange "quantum" physics usually seen only in very small things such as atoms have been induced and observed in diamonds big enough to adorn jewelry.

Quantum physics is known for bizarre phenomena that are very different from the behaviour we are familiar with through our interaction with objects on the human scale, which follow the laws of classical physics. For example, quantum "entanglement" connects two objects so that no matter how far away they are from one another, each object is affected by what happens to the other.

Now, scientists from the U.K., Canada and Singapore have managed to demonstrate entanglement in ordinary diamonds under conditions found in any ordinary room or laboratory.

"It's hard to understand, almost, that you can have this quantum thing that's a millimetre thick that you can hold in your hand," said Ben Sussman, a quantum physicist at the National Research Council of Canada and an adjunct professor at the University of Ottawa, who co-authored the study published online Thursday in Science.

Sussman, who largely designed and set up the experiments, said the demonstration is important in part because scientists and engineers around the world have been working hard to exploit quantum phenomena such as entanglement for use in new technologies such as quantum computing. In the future, quantum computers are expected to be able to store exponentially more information and process information exponentially faster than conventional computers.

The challenge is that many approaches to building quantum computing-technologies being tested now require unusual materials and conditions, such as very cold temperatures. What sets the new study apart is it took place at room temperature in a solid-state material.

Two diamonds (purple) were put into an entangled state using laser pulses(green). Next, by measuring the light scattered from the diamonds (red and blue), the researchers were able to confirm that they were entangled. (Science/AAAS)
Those conditions are "sort of the holy grails" for building practical quantum devices that are compact, robust and don't need special environmental controls, Sussman said.

"I think it does open a door," said Ian Walmsley, a professor of experimental physics at the University of Oxford who co-authored the paper and runs the lab where the experiments took place.

However, he and Sussman both acknowledge that the system still has issues to overcome before it can lead to commercial applications.

Walmsley said although entanglement has mostly been seen in very, very small things, it was previously known to be theoretically possible in regular-sized objects.

"We don't think there is a fundamental surprise here," he said.

However, entanglement of large-scale objects was difficult to demonstrate experimentally, and any larger systems that showed entanglement in the past were unusual, such as superconducting circuits cooled to very low temperatures.

In that context, he thinks other scientists will pay attention to the results: "They will say it's actually sort of interesting that these ambient, these everyday real-world things, can exhibit this behaviour."

Quantumness easily destroyed in big things

The reason quantum entanglement is easier to demonstrate in small objects is that a property necessary for entanglement, known as coherence, is eroded by interactions with other things, such as nearby atoms.

"Coherence is sort of the measure of the amount of quantumness in the system," Sussman said. "If there are lots of thermal things bouncing into the system, this quantumness disappears."

Walmsley said it's easier to maintain coherence in smaller objects because they can be isolated practically from disturbances. Things are trickier in larger systems that contain lots of interacting, moving parts.

Two things helped the researchers get around this in their experiment, Sussman said:

  • The hardness of the diamonds meant it was more resistant to disturbances that could destroy the coherence.
  • The extreme speed of the experiment — the researchers used laser pulses just 60 femtoseconds long, about 6/100,000ths of a nanosecond (a nanosecond is a billionth of a second) — meant there was no time for disturbances to destroy the quantum effects.

Laser pulses were used to put the two diamonds into a state where they were entangled with one another through a shared vibration known as a phonon. By measuring particles of light called photons subsequently scattered from the diamonds, the researchers confirmed that the states of the two diamonds were linked with each other — evidence that they were entangled.

The experiment was mainly conducted by Oxford University Ph.D. students K.C. Lee and Michael Sprague, a Canadian originally from Belleville, Ont.