Quantum computers could more easily become a reality if they incorporated the silicon semiconductor processing used by the modern electronics industry. Physicists in Australia have recently taken a new step toward that vision by reading and writing the nuclear spin state of a single phosphorus atom implanted in silicon.
In a breakthrough reported in the 18 April edition of the journal Nature, physicists have finally achieved an idea first proposed in 1998 by Bruce Kane, a physicist at the University of Maryland, in College Park. Such success could lead to quantum computers based on the same silicon-processing technology used for computer chips.
“What we are trying to do is demonstrate that there is a viable way to take the same physical platform and fabrication technology used to make any computer and mobile phone in the world, and twist it into a technology for quantum information processing,” says Andrea Morello, a quantum physicist at the University of New South Wales, in Australia.
Scientists envision quantum computers as the ideal devices for cracking modern encryption codes, searching through huge databases, and understanding the biological interactions of molecules and drugs. Quantum computing’s potential comes from harnessing the laws of quantum physics that allow the spin state of an electron or an atom’s nucleus to achieve “superposition”—existing in more than one state at a time. A classical computer bit can exist either as a 1 or a 0, but a quantum bit, or qubit, is capable of existing in multiple states at the same time.
With other quantum computing approaches, researchers have tried trapping and isolating atoms by using electromagnetic fields or superconductor materials. By comparison, Kane suggested harnessing the nuclear spin of phosphorus atoms embedded in a silicon crystal as a qubit.
Silicon-based quantum computing also offers long coherence times for electron and nuclear spins, Kane says. That means the electron spin states and nuclear spin states acting as qubits could hold on to their information for long periods of time, something that other quantum computing schemes have struggled with.
Morello and his colleagues—including UNSW’s Jarryd Pla and Andrew Dzurak as well as the University of Melbourne’s David Jamieson—liked Kane’s idea but took a slightly different approach. They first created a single-atom qubit in silicon last year by controlling the spin state of an electron bound to a phosphorus atom, rather than following Kane’s original idea of harnessing the spin state of the atom’s nucleus.
That mastery of electron spin states has now allowed the team to achieve control over a single phosphorus atom’s nucleus and demonstrate a comparatively long nuclear spin coherence time of 60 milliseconds. By manipulating the electron spin state, physicists could affect the nuclear spin state because of the coupled interaction between electron and nucleus. They could also measure changes in the nuclear spin by measuring the expected changes in the electron spin.
“When their electron spin paper came out six months ago, I knew that it was a really big accomplishment to get coherence in electron spin,” says Kane. “The nuclear spin is kind of icing on the cake—it’s expected at this point.”
But Morello and his colleagues had to overcome huge experimental challenges. To manipulate the electron spin, they created a magnetic field by running microwave power down a transmission line with a short circuit at the end. But they also needed to minimize electrical interference so that their charge detector could read the expected change in electron spin.
“The two things are hard to put together, because on the one hand you want a lot of microwave power to operate the spin, and on the other hand you want to avoid any electrical interference with the charge detector,” Morello says.
Manipulating nuclear spin proved easier than the readout process. The physicists required only a 100-megahertz frequency for the nuclear spin resonance as opposed to the 50 gigahertz for the electron spin resonance.
Electron spin still looks like the best “working qubit” for the silicon-based approach to quantum computing, Morello says. But he sees promise in using nuclear spin qubits for “quantum memory” and performing “quantum error correction” to make quantum computing a practical idea.
The mastery of nuclear spin has also given the team an idea for taking the next step toward silicon-based quantum computing—building a two-qubit logic gate based on two-qubit operations between electrons.
“We are looking at a new way to perform two-qubit operations between the electrons, enabled by the control of the nuclei,” Morello says.
The process of implanting phosphorus atoms in silicon usually means that there is some randomness in the distance between qubits and therefore some randomness in their ability to interact with one another. Physicists previously assumed that the atoms would have to be placed precisely, but the new approach shows that the scheme works even with some randomness.
“This means that the two P atoms don’t need to be placed close to each other with atomic precision—we can tolerate several nanometers of slack,” Morello explains. “We are very excited about this, because it makes the perspective of a practical…quantum computer much more realistic.”
Morello says he was confident the physicists could build working two-qubit gates fairly soon. But he also points out what he considers the biggest upcoming challenge for making any quantum computer: transporting the quantum state of a qubit across a processor consisting of many qubits. Building a silicon-based quantum computer with more than two qubits means that several two-qubit cells need to be connected, so some form of transport between the quantum gates will be crucial.
“What we would like to achieve is a ‘demonstrator quantum computer’ with around 10 qubits in, say, five years from now,” Morello says. “That prototype should contain all the basic parts, 1- and 2-qubit gates, qubit state transport, and quantum error correction. Once that has been done, it becomes a manufacturing problem more than anything else.”
A silicon-based approach to quantum computing still offers big advantages, but Kane remains cautious about whether the technology can scale up well beyond the two-qubit systems. Still, he acknowledged that he could only have imagined physicists achieving his 1998 proposal in his “wildest dreams” when he first developed the idea.
“These guys worked a dozen years to get this experiment working,” Kane says. “It’s a tribute to their perseverance.”