A team of Australian engineers has built a quantum logic gate in silicon for the first time, making calculations between two qubits of information possible – and thereby clearing the final hurdle to making silicon quantum computers a reality.

“What we have is a game changer,” said team leader Andrew Dzurak, Scientia Professor and Director of the Australian National Fabrication Facility at UNSW.

“We’ve demonstrated a two-qubit logic gate – the central building block of a quantum computer – and, significantly, done it in silicon. Because we use essentially the same device technology as existing computer chips, we believe it will be much easier to manufacture a full-scale processor chip than for any of the leading designs, which rely on more exotic technologies.

“This makes the building of a quantum computer much more feasible, since it is based on the same manufacturing technology as today’s computer industry,” he added.

*Lead author Menno Veldhorst (left) and project leader Andrew Dzurak (right) in the UNSW laboratory where the experiments were performed.*

* Silicon two-qubit logic device, incorporating SET read-out and selective qubit control.*

*Exchange spin funnel*

Nature – A two-qubit logic gate in silicon

The advance represents the final physical component needed to realise the promise of super-powerful silicon quantum computers, which harness the science of the very small – the strange behaviour of subatomic particles – to solve computing challenges that are beyond the reach of even today’s fastest supercomputers.

In classical computers, data is rendered as binary bits, which are always in one of two states: 0 or 1. However, a quantum bit (or ‘qubit’) can exist in both of these states at once, a condition known as a superposition. A qubit operation exploits this quantum weirdness by allowing many computations to be performed in parallel (a two-qubit system performs the operation on 4 values, a three-qubit system on 8, and so on).

“If quantum computers are to become a reality, the ability to conduct one- and two-qubit calculations are essential,” said Dzurak, who jointly led the team in 2012 that demonstrated the first ever silicon qubit, also reported in Nature.

Until now, it had not been possible to make two quantum bits ‘talk’ to each other – and thereby create a logic gate – using silicon. But the UNSW team – working with Professor Kohei M. Itoh of Japan’s Keio University – has done just that for the first time.

The result means that all of the physical building blocks for a silicon-based quantum computer have now been successfully constructed, allowing engineers to finally begin the task of designing and building a functioning quantum computer.

“Despite this enormous global interest and investment, quantum computing has – like Schrödinger’s cat – been simultaneously possible (in theory) but seemingly impossible (in physical reality),” said Professor Mark Hoffman, UNSW’s Dean of Engineering.

“The advance our UNSW team has made could, we believe, be the inflection point that changes that Schrödinger’s paradigm,” he added. “The technology – devised, tested and patented by our team – has the potential to take quantum computing across the threshold from the theoretical to the real.”

A key advantage of the UNSW approach is that it reconfigured the ‘transistors’ used to define the bits in existing silicon chips, and turned them into qubits. “The silicon chip in your smartphone or tablet already has around one billion transistors on it, with each transistor less than 100 billionths of a metre in size,” said Dr Menno Veldhorst, a UNSW Research Fellow and the lead author of the Nature paper.

“We’ve morphed those silicon transistors into quantum bits by ensuring that each has only one electron associated with it. We then store the binary code of 0 or 1 on the ‘spin’ of the electron, which is associated with the electron’s tiny magnetic field,” he added.

Dzurak noted that the team had recently “patented a design for a full-scale quantum computer chip that would allow for millions of our qubits, all doing the types of calculations that we’ve just experimentally demonstrated”.

He said that a key next step for the project is to identify the right industry partners to work with to manufacture the full-scale quantum processor chip.

Such a full-scale quantum processor would have major applications in the finance, security and healthcare sectors, allowing the identification and development of new medicines by greatly accelerating the computer-aided design of pharmaceutical compounds (and minimising lengthy trial and error testing); the development of new, lighter and stronger materials spanning consumer electronics to aircraft; and faster information searching through large databases.

**Abstract – two qubit logic gate in silicon**

Quantum computation requires qubits that can be coupled in a scalable manner, together with universal and high-fidelity one- and two-qubit logic gates. Many physical realizations of qubits exist, including single photons, trapped ions, superconducting circuits, single defects or atoms in diamond and silicon, and semiconductor quantum dots, with single-qubit fidelities that exceed the stringent thresholds required for fault-tolerant quantum computing. Despite this, high-fidelity two-qubit gates in the solid state that can be manufactured using standard lithographic techniques have so far been limited to superconducting qubits, owing to the difficulties of coupling qubits and dephasing in semiconductor systems. Here we present a two-qubit logic gate, which uses single spins in isotopically enriched silicon1 and is realized by performing single- and two-qubit operations in a quantum dot system using the exchange interaction, as envisaged in the Loss–DiVincenzo proposal. We realize CNOT gates via controlled-phase operations combined with single-qubit operations. Direct gate-voltage control provides single-qubit addressability, together with a switchable exchange interaction that is used in the two-qubit controlled-phase gate. By independently reading out both qubits, we measure clear anticorrelations in the two-spin probabilities of the CNOT gate

SOURCES- UNSW, Nature