Protected Biphoton States

This is an artist’s impression of correlated photons on a nanowire lattice with a topological defect. (Source: S. Zentilomo, Univ. of Sydney)

Scientists in Australia have for the first time demon­strated the protection of corre­lated states between paired photons using the intriguing physical concept of topo­logy. This experi­mental breakthrough opens a pathway to build a new type of quantum bit, the building blocks for quantum computers. “We can now propose a pathway to build robust entangled states for logic gates using protected pairs of photons,” said Andrea Blanco-Redondo at the Univer­sity of Sydney Nano Institute.

Classical compu­tational switches are in simple binary forms of zero or one. Quantum switches exist in a state of super­position that combine zero and one. Protecting quantum infor­mation long enough so that quantum machines can perform useful calcu­lations is one of the biggest challenges in modern physics. Useful quantum computers will require millions or billions of qubits to process information. So far, 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 at the nanoscale. Attempts to achieve this using super­conductors and trapped ions have shown promise, but they are highly susceptible to electro­magnetic inter­ference, making them devilishly difficult to scale up into useful machines. The use of photons rather than electrons has been one proposed alter­native upon which to build logic gates that can calcu­late quantum algorithms.

Photons are well isolated from the thermal and electro­magnetic environment. However, scaling quantum devices based on photonic qubits has been limited due to scat­tering loss and other errors; until now. “What we have done is develop a novel lattice structure of silicon nanowires, creating a parti­cular symmetry that provides unusual robustness to the photons’ corre­lation. The symmetry both helps create and guide these correlated states,” said Blanco-Redondo. “This robust­ness stems from the underlying topo­logy, a global property of the lattice that remains unchanged against disorder.”

The corre­lation this produces is needed to build entangled states for quantum gates. Waveguides, made using silicon nano­wires just 500 nanometres wide, were lined up in pairs with a deliberate defect in symmetry through the middle, creating two lattice structures with different topo­logies and an inter­vening edge. This topology allows for the creation of special modes in which the photons can pair up, building edge modes. These modes allow infor­mation carried by the paired photons to be trans­ported in a robust fashion that other­wise would have been scattered and lost across a uniform lattice.

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 2018 Nobel Prize in Physics. This research is the latest in the flourishing of disco­veries in the past decade on topo­logical states of matter. These topo­logical features offer protection for classical and quantum infor­mation in fields as diverse as electro­magnetism, condensed matter, acoustics and cold atoms.

Microsoft Quantum Labo­ratories, including the one in Sydney, are pursuing the development of electron-based qubits where quantum information is topo­logically protected via the knotting of quasi­particles known as Majorana fermions. This is a bit like braiding half electron states induced through the interaction of super­conductors and semi­conducting metals.

Topolo­gically protected states have previously been demon­strated for single photons. However, Blanco-Redondo said: “Quantum infor­mation systems will rely on multiphoton states, high­lighting the importance of this discovery for further development.” She said the next step will be to improve protec­tion of the photon ent­anglement to create robust, scalable quantum logic gates.

Stephen Bartlett, a theoretical quantum physicist at Sydney Nano who is uncon­nected to the study, said: “Blanco-Redondo’s result is exciting at a fundamental level because it shows the existence of protected modes attached to the boundary of a topo­logically ordered material. What it means for quantum computing is unclear as it is still early days. But the hope is that the protec­tion offered by these edge modes could be used to protect photons from the types of noise that are proble­matic for quantum appli­cations.” (Source: U Sydney)

Reference: A. Blanco-Redondo et al.: Topological protection of biphoton states, Science 362, 568 (2018); DOI: 10.1126/science.aau4296

Link: Institute of Photonics and Optical Science (IPOS), The Sydney Nano Institute, School of Physics, University of Sydney, Sydney, Australia

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