Quantum Computer Building Block is 200 Times Faster

University of New South Wales (UNSW) Professor Michelle Simmons and her team have developed the fastest two-qubit gate between atoms suspended in silicon. A two-qubit gate is the central building block of any quantum computer – and the UNSW team’s version of it is the fastest that’s ever been demonstrated in silicon, completing an operation in 0.8 nanoseconds, which is ~200 times faster than other existing silicon spin-based two-qubit gates.

The next major goal is building a 10-qubit quantum integrated circuit which they target within 3-4 years.

Nature – A two-qubit gate between phosphorus donor electrons in silicon

Using a scanning tunneling microscope to precision-place and encapsulate phosphorus atoms in silicon, the team first had to work out the optimal distance between two qubits to enable the crucial operation.

“Our fabrication technique allows us to place the qubits exactly where we want them. This allows us to engineer our two-qubit gate to be as fast as possible,” says study lead co-author Sam Gorman from CQC2T.

“Not only have we brought the qubits closer together since our last breakthrough, but we have learnt to control every aspect of the device design with sub-nanometer precision to maintain the high fidelities.”

In the Simmons’ group approach, a two-qubit gate is an operation between two electron spins – comparable to the role that classical logic gates play in conventional electronics. For the first time, the team was able to build a two-qubit gate by placing two atom qubits closer together than ever before, and then – in real-time – controllably observing and measuring their spin states.

The team’s unique approach to quantum computing requires not only the placement of individual atom qubits in silicon but all the associated circuitry to initialize, control and read-out the qubits at the nanoscale – a concept that requires such exquisite precision it was long thought to be impossible. But with this major milestone, the team is now positioned to translate their technology into scalable processors.

Abstract
Electron spin qubits formed by atoms in silicon have large (tens of millielectronvolts) orbital energies and weak spin–orbit coupling, giving rise to isolated electron spin ground states with coherence times of seconds. High-fidelity (more than 99.9 per cent) coherent control of such qubits has been demonstrated, promising an attractive platform for quantum computing. However, inter-qubit coupling—which is essential for realizing large-scale circuits in atom-based qubits—has not yet been achieved. Exchange interactions between electron spins promise fast (gigahertz) gate operations with two-qubit gates, as recently demonstrated in gate-defined silicon quantum dots. However, creating a tunable exchange interaction between two electrons bound to phosphorus atom qubits has not been possible until now. This is because it is difficult to determine the atomic distance required to turn the exchange interaction on and off while aligning the atomic circuitry for high-fidelity, independent spin readout. Here we report a fast (about 800 picoseconds) 𝐒𝐖𝐀𝐏‾‾‾‾‾‾‾√ two-qubit exchange gate between phosphorus donor electron spin qubits in silicon using independent single-shot spin readout with a readout fidelity of about 94 per cent on a complete set of basis states. By engineering qubit placement on the atomic scale, we provide a route to the realization and efficient characterization of multi-qubit quantum circuits based on donor qubits in silicon.

SOURCES- Nature, University of New South Wales
Written By Christina Wong

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