SYDNEY: Quantum bits based on the spin of an electron, a quantum property, could be made to interact over large distances in a key step towards large-scale quantum computing, U.S. scientists announced.
Transferring information between more than one quantum bit, or qubit – the elementary unit of information in quantum computing, similar to the current bit – is vital to perform the logic operations required for scalable quantum computing. This has been achieved between neighbouring electrons, but it has not been achieved on larger scales – the scale of millimetres rather than nanometres.
Any scalable, spin-based quantum computer “will almost certainly require long-range qubit interactions,” the researchers, from Princeton University in New Jersey, wrote in the paper, published in Nature.
Superconducting quantum bus
The researchers proposed using a ‘quantum bus’ to mediate between two spin-based qubits – in the form of a superconducting microwave circuit.
Superconducting circuits use cooled superconductors, such as aluminium, to cause electrons to condense into groups of so-called ‘Cooper pairs’ (coupled electrons) that allow the quantum effect to be observed on the macroscopic level.
The researchers showed that they could determine the spin of a single-electron qubit by measuring the microwave field inside a superconducting circuit. “The same microwave field will eventually be used to couple spin qubits that are separated by a large distance,” explained paper co-author and physicist Jason Petta from Princeton University.
“It seems now that long-distance coupling of spin qubits may be feasible. Our experiment is the first step in this direction,” he said.
Quantum bits rely on the strange behaviour of quantum mechanics in which sub-atomic particles can exist in multiple states at once – called ‘superposition’ – to achieve unprecedented processing speeds.
The state of superconducting qubits – which can be up to 1mm in size – can be altered in several ways, including using microwave signals. In this way, quantum computer scientists can create ‘information’ equivalent to the 1 and 0 of the binary code used in modern computers – or a quantum superposition of the two.
For a spin-based qubit, on the other hand, microwaves are used to control the spin of an electron to be either in an ‘up’ (0) or ‘down’ (1) state, or in a superposition.
Distant coupling has been achieved in superconducting qubits, but the benefit of doing so in a spin-based qubit is that the latter can hold its information for longer – also known as its coherence time, according to Jarryd Pla, a quantum computing researcher at the University of New South Wales in Sydney, who was not involved in the research.
Pla said ideas for coupling ‘distant’ qubits generally involve “bringing the qubits together by physically moving them around the processor […] an incredibly difficult thing to implement experimentally”.
“This work has built on advancements made in superconducting circuits to demonstrate the possibility of an alternative method of achieving long-range interactions with spins,” Pla added.
Findings will “definitely generate excitement”
Petta said the future of quantum computing, which was originally developed to crack codes but has myriad other potential uses, including in drug development, would be intricately linked with materials development.
“In the next few years, we are likely to see more activity in silicon, which is the workhorse of the semiconductor industry,” he said.
Pla, who’s research group recently published a paper in Nature reporting the first single-electron qubit in silicon, said the latest results out of the U.S. are “remarkable” and that they are “definitely going to generate some excitement in the quantum information processing community”.
Petta Group, Princeton University, homepageThe Australian Centre of Excellence for Quantum Computation & Communication Technology