Light shines on the new path of quantum technology

Australian scientists have for the first time demonstrated the protection of correlated states between matched photons – light energy packets – using the intriguing physical concept of topology. This experimental breakthrough paves the way for the construction of a new type of quantum bit, the building blocks of quantum computers.

The research, developed in close collaboration with Israeli colleagues, is published today in the prestigious journal, Science, recognition of the fundamental importance of this work.

"We can now propose a way to build robust entangled states for logic gates using protected photon pairs," said Andrea Blanco-Redondo, senior author of the Nano Institute at the University of Sydney.

Logic gates are the switches needed to use the algorithms written for quantum computers. Conventional computing switches are in the simple binary form of zero or one. Quantum switches exist in a state of "superposition" that combines zero and one.

Protecting quantum information long enough for quantum machines to perform useful calculations is one of the greatest challenges of modern physics. Useful quantum computers will require millions or billions of qubits to process information. Until now, the best experimental devices have about 20 qubits.

To unleash the potential of quantum technology, scientists need to find a way to protect the entangled superposition of quantum bits, or qubits, at the nanoscale. Attempts to achieve this goal using superconductors and trapped ions have been shown to be promising, but they are extremely sensitive to electromagnetic interference, making them devilishly difficult to transform into useful machines.

The use of photons (light energy packets) rather than electrons has been one of the proposed alternatives for building logic gates to compute quantum algorithms.

Photons, unlike electrons, are well isolated from the thermal and electromagnetic environment. However, the scaling of quantum devices based on photonic bits has been limited due to loss of diffusion and other errors; until now.

"What we have done is to develop a new silicon nanowire lattice structure, creating a special symmetry that confers an unusual robustness to photon correlation.This symmetry allows both to create and guide these correlated states, called "fashions," said Dr. Blanco-Redondo, researcher Messel at the Faculty of Physics.

"This robustness stems from the underlying topology, global network property that remains unchanged against the disorder."

The correlation that this produces is necessary to build entangled states for quantum gates.

The channels, or waveguides, made using silicon nanowires of a width of only 500 nanometers, were aligned in pairs with a deliberate defect of symmetry in the center, creating two structures in lattice with different topologies and an intermediate "edge".

This topology makes it possible to create special modes in which the photons can associate, called "edge modes". These modes allow the information carried by the paired photons to be transported in a robust manner that would otherwise have been scattered and lost on a uniform network.

Dr. Blanco-Redondo has designed and conducted the experiment in the Sydney Nanoscience Hub with Dr. Bryn Bell, previously at the University of Sydney and now at the University of Oxford.

The photons were created by high-intensity ultra-short laser pulses, the same underlying technology for which Donna Strickland and Gerard Mourou were awarded the Nobel Prize in Physics 2018.

This research is the latest in recent discoveries on the topological states of matter. These topological features provide protection for classical and quantum information in fields as diverse as electromagnetism, condensed matter, acoustics and cold atoms.

Microsoft Quantum laboratories, including Sydney, are pursuing the development of electronic qubits, in which quantum information is topologically protected by the knotting of quasi-particles known as Majorana fermions. This is a bit like half weave states induced by the interaction of superconductors and semiconductor metals.

Topologically protected states have already been demonstrated for single photons.

However, Dr. Blanco-Redondo said, "Quantum information systems will rely on multi-photon states, emphasizing the importance of this discovery for further development."

She added that the next step will be to improve the protection of photon entanglement to create robust and scalable quantum logic gates.

Professor Stephen Bartlett, theoretical quantum physicist at Sydney Nano and not related to the study, said: "The result of Dr. Blanco-Redondo is exciting at the fundamental level because it shows the existence of protected modes attached at the limit of a topologically ordered material.

"What it means for quantum computing is not clear because it is still in its infancy, but we hope that the protection afforded by these edge modes could be used to protect photons from the types of noise problematic for quantum applications. "

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More information:
"Topological protection of two-photon states" Science (2018). science.sciencemag.org/cgi/doi… 1126 / science.aau4296