15th August 2020
Quantum coherence breakthrough: 10,000 times longer
Universal coherence protection has been achieved in a solid-state spin qubit – a modification that allows quantum systems to stay operational ("coherent") for 10,000 times longer than before.
In the coming decades, quantum technology has the potential to revolutionise our world – vastly expanding the capabilities for research and development in communication, computing, encryption, sensing, simulations and other areas. While classical mechanics are based on the principle that a state is determined with simple binary values (zero or one), quantum technologies take advantage of quantum phenomena with multiple values simultaneously. This effect is known as quantum superposition. The result is an exponentially huge improvement in calculation speed, enabling a quantum system to perform tasks in a few seconds that might take a classical machine thousands or even millions of years to complete.
While great strides have been achieved in recent years, a major limiting factor is that quantum states require extremely quiet, stable spaces to operate. They are easily disturbed by background noise from vibrations, temperature changes, or stray electromagnetic fields. Getting them to remain stable for longer than a few millionths of a second is a big challenge.
This week, a team of scientists at the University of Chicago's Pritzker School of Molecular Engineering announced the discovery of a simple modification that allows quantum systems to stay operational – or "coherent" – 10,000 times longer than before.
The change enables quantum coherence for up to 22 milliseconds, a four orders of magnitude improvement, and far longer than any previously reported electron spin system. For comparison, the blink of an eye takes 100 milliseconds. The system can almost completely tune out some forms of temperature fluctuations, physical vibrations, and electromagnetic noise, all of which usually destroy quantum coherence.
One common approach by researchers attempting to keep quantum coherence for as long as possible is to physically isolate a system from its noisy surroundings but this can be unwieldy and complex. Another technique involves making all of the materials as pure as possible, which can be costly. For this latest new study, the University of Chicago scientists tried something different.
"With this approach, we don't try to eliminate noise in the surroundings; instead, we 'trick' the system into thinking it doesn't experience the noise," said postdoctoral researcher Kevin Miao.
The team combined electromagnetic pulses with a continuously applied, alternating magnetic field. By precisely tuning this field, they could rapidly rotate the electron spins and allow the system to "tune out" the rest of the noise.
"To get a sense of the principle, it's like sitting on a merry-go-round with people yelling all around you," explained Miao. "When the ride is still, you can hear them perfectly, but if you're rapidly spinning, the noise blurs into a background."
"This approach creates a pathway to scalability," said David Awschalom, Professor of Molecular Engineering at the Argonne National Laboratory, director of the Chicago Quantum Exchange, and lead author of the paper published this week in Science. "It should make storing quantum information in electron spin practical. Extended storage times will enable more complex operations in quantum computers and allow quantum information transmitted from spin-based devices to travel longer distances in networks."
Though their tests were run in a solid-state quantum system using silicon carbide, the scientists believe the technique should have similar effects in other types of quantum systems, such as superconducting quantum bits and molecular quantum systems. This level of versatility is unusual for such an engineering advance.
"There are a lot of candidates for quantum technology that were pushed aside because they couldn't maintain quantum coherence for long periods of time," said Miao. "Those could be re-evaluated now that we have this way to massively improve coherence."
"This breakthrough lays the groundwork for exciting new avenues of research in quantum science," said Awschalom. "The broad applicability of this discovery, coupled with a remarkably simple implementation, allows this robust coherence to impact many aspects of quantum engineering. It enables new research opportunities previously thought impractical."