Australian scientists have created the world's first-ever quantum computer circuit – one that contains all the essential components found on a classical computer chip but at the quantum scale.
The landmark discovery, published in Nature today, was nine years in the making.
"This is the most exciting discovery of my career," senior author and quantum physicist Michelle Simmons, founder of Silicon Quantum Computing and director of the Center of Excellence for Quantum Computation and Communication Technology at UNSW told ScienceAlert.
Not only did Simmons and her team create what's essentially a functional quantum processor, they also successfully tested it by modeling a small molecule in which each atom has multiple quantum states – something a traditional computer would struggle to achieve.
This suggests we're now a step closer to finally using quantum processing power to understand more about the world around us, even at the tiniest scale.
"In the 1950s, Richard Feynman said we're never going to understand how the world works – how nature works – unless we can actually start to make it at the same scale," Simmons told ScienceAlert.
"If we can start to understand materials at that level, we can design things that have never been made before.
"The question is: how do you actually control nature at that level?"
The latest invention follows the team's creation of the first ever quantum transistor in 2012.
(A transistor is a small device that controls electronic signals and forms just one part of a computer circuit. An integrated circuit is more complex as it puts lots of transistors together.)
To make this leap in quantum computing, the researchers used a scanning tunneling microscope in an ultra-high vacuum to place quantum dots with sub-nanometer precision.
The placement of each quantum dot needed to be just right so the circuit could mimic how electrons hop along a string of single- and double-bonded carbons in a polyacetylene molecule.
The trickiest parts were figuring out: exactly how many atoms of phosphorus should be in each quantum dot; exactly how far apart each dot should be; and then engineering a machine that could place the tiny dots in exactly the right arrangement inside the silicon chip.
If the quantum dots are too big, the interaction between two dots becomes "too large to independently control them", the researchers say.
If the dots are too small, then it introduces randomness because each extra phosphorus atom can substantially change the amount of energy it takes to add another electron to the dot.
The final quantum chip contained 10 quantum dots, each made up of a small number of phosphorus atoms.
Double carbon bonds were simulated by putting less distance between the quantum dots than single carbon bonds.
Polyacetylene was chosen because it's a well-known model and could therefore be used to prove that the computer was correctly simulating the movement of electrons through the molecule.
Quantum computers are needed because classical computers cannot model large molecules; they are just too complex.
For example, to create a simulation of the penicillin molecule with 41 atoms, a classical computer would need 1086 transistors, which is "more transistors than there are atoms in the observable universe".
For a quantum computer, it would only require a processor with 286 qubits (quantum bits).
Because scientists currently have limited visibility as to how molecules function at the atomic scale, there's a lot of guess work in the creation of new materials.
"One of the holy grails has always been making a high temperature superconductor," says Simmons. "People just don't know the mechanism for how it works."
Another potential application for quantum computing is the study of artificial photosynthesis, and how light is converted to chemical energy through an organic chain of reactions.
Another big problem quantum computers could help solve is the creation of fertilizers. Triple nitrogen bonds are currently broken under high temperature and pressure conditions in the presence of an iron catalyst to create fixed nitrogen for fertilizer.
Finding a different catalyst that can make fertilizer more effectively could save a lot of money and energy.
Simmons says the achievement of moving from quantum transistor to circuit in just nine years is mimicking the roadmap set by the inventors of classical computers.
The first classical computer transistor was created in 1947. The first integrated circuit was built in 1958. Those two inventions were 11 years apart; Simmons' team made that leap two years ahead of schedule.
This article was published in Nature.