A quantum leap for quantum computing
Two teams of physicists in the United States demonstrated a process that could one day be the foundation of mass-produced quantum computers exponentially faster that even the largest supercomputers in use today.
In experiments, the scientists coaxed single photons – discrete packets of electromagnetic energy with neither mass nor weight – between two superconducting circuits called quantum bits, or “qubits".
The data moved across a nanoscale circuit, known as a “quantum bus,” in the form of a 20-millimetre cable held at a super-low temperature to impede electrical resistance.
A team at the National Institute of Standards and Technology (NIST) led by Mika Sillanpaa devised the cable, which also stored the data for a brief 10 nanoseconds, showing that data storage was possible.
A team led by Johannes Majer at Yale University got the two qubits to talk to each other. Both studies were published in the British journal Nature.
This is not the first time that scientists have succeeded in linking one qubit with another, said Majer. But it is the first time they have done so over a relatively long distance, via the equivalent of a micro-scale computer chip.
The new configuration introduces several features that could help pave the way to mass produced, ultra-powerful quantum computers.
One is scalability – the possibility to create a network of multiple solid-state qubits, something that has yet to be achieved.
“These papers verify that interactions between superconducting qubits can be mediated through a quantum bus that is in principle scalable,” commented Raymond Laflamme, Director of the Institute for Quantum Computing at the University of Waterloo in Canada, after reviewing the papers.
“This demonstrates an increasing quantum control of these systems that is essential for quantum information processors and is an important step towards building quantum computers."
Another advantage is the possibility of using existing chip technology for the fabrication of the qubits and the circuitry connecting them.
Quantum computing is a qualitative breakthrough compared to current technology. Instead of using the binary digits 0 and 1 to hold information, quantum computing is based on a principle of quantum mechanics that governs changes of state, called superposition, that occur at atomic level.
As with current digital technology, a qubit can be expressed as a 0 or 1. But in a counter-intuitive twist, it can also hold both values at the same time, allowing for a huge increase in the number of simultaneous calculations – provided it can be controlled and accessed.
Harnessing the photons that carry the data at an atomic level, said Majer has posed a major challenge.
Photons, which move at the speed of light, are so perishable that as soon as you see one it disappears, for its contact with the retina expends the energy that made it exist.
They are visible in the middle of the electromagnetic spectrum in the form of light, but are also present in X-rays, at one extreme of the spectrum, and gamma-rays, at the other.
“We have to control electrical signals corresponding to one single proton,” said Majer, pointing out that a cell phone emits about a hundred billion trillion photons – that's a "1” with 23 zeros after it – every second. Other experts described the findings as an important breakthrough.
“They are really great papers and the techniques are impressive,” noted Laflamme.
But they also cautioned that quantum laptops are still a long way off, pointing to two major limitations in the experiments. Scaling up to a network of interacting qubits remains out of reach for now.
“For a real quantum computer, many more coupled qubits have to be realised, at least hundreds if not thousands,” acknowledged Majer.
The other is a problem of temperature: for the experiment to work, the cable was cooled to 10 milli-degrees above absolute zero, or minus 273.13 C (minus 459.63 F).
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