Quantum simulator progress

Friday 14th April 2017
Encoding 1 Qubit in an entangled state with several particles. Courtesy https://scitechdaily.com/

 In research reported on the arXiv server, China physicists are claiming the lead in the race to couple together increasing numbers of superconducting qubits, and can entangle 10 qubits to one another via a central resonator –  beating the previous record by one qubit. They claim the result paves the way for quantum simulators to calculate the behavior of small molecules and quantum-mechanical systems much more efficiently than the most powerful conventional computers. Quantum computing  it is claimed is unrealistic in the foreseeable future.

Which leaves the by-stander wondering if IBM can pull off a hat trick with its current quantum build.  The UK is working on high performance Think Tank.  With Quantum simulators,  superconducting circuits create qubits by superimposing two electrical currents, and hold out the promise to fabricate many qubits on a single chip through the use of silicon-based manufacturing technology.  A multi-institutional group led by J (left) ian-Wei Pan of the University of Science and Technology of China in Hefei, built the circuit consisting of 10 qubits, each half a millimetre across, made from aluminium slivers on to sapphire substrate.

Where the UK is concerned the major players can be found set into a landscape approach to the whole quantum technology (see below).

The qubits act as non-linear LC oscillators, arranged in a circle around a component known as a bus resonator. Initially, qubits are put into a superposition state of two oscillating currents with different amplitudes by supplying each of them with a very low-energy microwave pulse. To avoid interference at this stage, each qubit is set to a different oscillation frequency.  But for the qubits to interact with each another, they need the same frequency. The bus allows qubits to transfer energy from one to another, but does not absorb any of that energy itself.

The end result of this process, says team member (right) Haohua Wang of Zhejiang University, is "some kind of magical interaction". To establish just how entangled their qubits were, the researchers used quantum tomography to discover the probability of detecting each of the thousands of possible states that the entanglement could generate. Their measured probability distribution yielded the correct state, on average, about two thirds of the time. The fact that this "fidelity" was above 50% meant that their qubits were "entangled for sure" says Wang.

According to Shibiao Zheng of Fuzhou University, (left) who designed the entangling protocol, the key  set-up ingredient  is the bus that allows them to generate entanglement "very quickly". The previous record of nine for the number of entangled qubits in a superconducting circuit was held by John Martinis and colleagues at the University of California, Santa Barbara and Google. That group uses a different architecture for their system. Instead of linking qubits  by a central hub they lay them out in a row,  connecting each to its nearest neighbour. Doing so allows the use an error-correction scheme  that they developed, known as surface code.

High fidelity
Error correction will be vital for the functioning of any large-scale quantum computer in order to overcome decoherence, that is the destruction of delicate quantum states by outside interference.  Involving the addition of qubits to provide cross-checking, error correction relies on each gate operation introducing very little error. Otherwise, errors would simply spiral out of control. In 2015, Martinis and co-workers showed that superconducting quantum computers could in principle be scaled up, when they built two-qubit gates with a fidelity above that required by surface code – introducing errors less than 1% of the time.

Martinis praises Pan and colleagues for their "nicely done experiment", in particular for their speedy entangling and "good single-qubit operation". But it is hard to know how much of an advance they have really made, he argues, until they fully measure the fidelity of their single-qubit gates or their entangling gate. "The hard thing is to scale up with good gate fidelity," he says.

Wang says that the Chinese collaboration is working on an error-correction scheme for their bus-centred architecture. But he argues that in addition to exceeding the error thresholds for individual gates, it is also important to demonstrate the precise operation of many highly entangled qubits. "We have a global coupling between qubits," he says. "And that turns out to be very useful."

Quantum simulator
Wang agrees that construction of a universal quantum computer – one that would perform any quantum algorithm far quicker than conventional computers could – is not realistic for the foreseeable future, given the many millions of qubits such a device is likely to need. Currently Wang and his colleagues have a more modest aim in mind: the development of a "quantum simulator" consisting of perhaps 50 qubits, which could outperform classical computers when it comes to simulating the behavior of small molecules and other quantum systems.

Xiaobo Zhu  (right) of the University of Science and Technology of China, who was in charge of fabricating the 10-qubit device, says that the collaboration aims to build the simulator within the next "5–10 years", noting that this is similar to the timescale quoted by other groups including the one of Martinis. "We are trying to catch up with the best groups in the world," he says.

The UK landscape approach can be viewed graphically

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