Quantum computer users may soon have to wrestle with their own version of the "PC or Mac?" question. A design based on superconducting electrical circuits has now performed two benchmark feats, suggesting it will be a serious competitor to rival setups using photons or ions.
"The number of runners in the race has just gone up to three," says Andrew White of the University of Queensland, Australia, who builds quantum computers based on photons and was not involved in the new result.
The defining feature of a quantum computer is that it uses quantum bits or qubits. Unlike ordinary bits, these can exist in multiple states at once, known as a superposition. They can also be entangled with each other, so their quantum states are linked, allowing them to be in a sort of "super" superposition of quantum states.
This means quantum computers could perform multiple calculations simultaneously, making them much faster than ordinary computers at some tasks.
Previously, setups using photons or trapped ions as qubits have made the most headway in early calculations. Now Matteo Mariantoni of the University of California, Santa Barbara, and colleagues have boosted the computing power of a rival design, first demonstrated in 2003, that uses tiny, superconducting wires instead.
Wire loops
Mariantoni's team used a chip embedded with micrometre-sized loops of wire made of a mixture of aluminium and rhenium. When these wires were cooled to within a whisker of absolute zero, they became superconducting, meaning their electrons coupled up as structures called "cooper pairs".
The pairs in each wire were made to resonate as an ensemble. Because each ensemble could exist as a superposition of multiple different resonating states, they acted as qubits.
Mariantoni's team entangled these qubit wires using a second type of wire, known as a bus, that snaked all around the chip. First they tuned this bus so that it took on some of the quantum information in one of the qubits. Then they transferred this information to further qubit wires, thus entangling the qubits.
Benchmark tests
The design made strides in solving calculations often used as benchmarks for testing quantum computers' capabilities.
It ran a calculation known as the quantum Fourier transform, which is a central component of the most famous quantum algorithm, known as Shor's. If Shor's were run on a system with enough qubits, it would allow huge numbers to be factorised quickly. That has not happened yet, but if it ever did, it would cause many current encryption systems to break down, since they rely on the fact that ordinary computers can't do this.
The researchers also used entangled qubits to create a system known as a "Toffoli OR phase gate", which is a critical step towards building codes that do quantum error correction. This required entangling three qubits – a first for superconducting quantum circuits. "Getting three bits to play well together is hard," says White.
Ordinary chips
The advances may seem like baby steps, since both Shor's algorithm and the Toffoli gate have been realised with relatively low numbers of photons and trapped ions.
But the reason the new result is exciting is that it could be hard to scale up these systems, which tend to be delicate and require specialised equipment, while the superconducting system uses chips like an ordinary computer. "The beautiful thing about a solid circuit is that it's something you can write using lithographic technology," says White. "It looks much easier than say ion traps or photonic approaches."
But future quantum computers might not come down to an either/or choice like that between a Mac and a PC. Instead, true to their quantum nature, they may be "superpositions" of different designs. "I don't think anybody knows what the best architecture will be," says White. "Probably we will end up using hybrids of the various approaches."
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