Lighting the way forward

With the support of Singapore’s Ministry of Education, NTU researchers are using nanophotonics to bring out the best in the worlds of quantum and topological science.

If you have ever undergone laser surgery to correct your vision, enjoyed high-speed data transmission over an optic fibre or simply scanned an item with a barcode reader at the supermarket, you have made use of photonics, the science of controlling light. But as our ability to control light becomes more precise, even more applications are appearing on the horizon.

“Now that we can manipulate structures at the nanometre scale, we can create materials that interact with light in a completely new way,” says Assoc Prof Cesare Soci of NTU’s School of Physical and Mathematical Sciences. “We can now tune fundamental properties such as absorption, reflection and scattering at will and even create materials with new properties.”

Building on significant support from Singapore’s Ministry of Education (MOE) in 2012, which helped to establish the Centre for Disruptive Photonic Technologies (CDPT) at NTU’s The Photonics Institute (TPI), researchers received another substantial MOE grant in 2017 to use their expertise in nanophotonics to develop quantum and topological technologies.

“Both of these are new paradigms of nanophotonics,” says Assoc Prof Soci, who is Deputy Director of CDPT along with Director Prof Nikolay Zheludev and Co-Director Prof Shen Zexiang. “What we are trying to do is deploy these technologies at nanoscales and use all the capabilities we have in nanofabrication to build integrated systems.”

A quantum leap in capabilities
The 2017 grant, involving seven principal investigators led by world-renowned nanophotonics expert Prof Zheludev, is focused on three main areas: quantum nanophotonics, topological nanophotonics, and nanophotonic materials with novel quantum and topological properties.

“Here at NTU, we have deep expertise in all three areas. Combining these different areas of expertise in one project will push the entire field forward, which is exactly the goal of this grant,” says Asst Prof Gao Weibo, whose work in quantum nanophotonics seeks to achieve spin-photon entanglement, the coupling between a quantum spin and a single photon of light.

Harnessing this coupling would underpin the construction of a quantum computer. To this end, Asst Prof Gao and his team are using germanium or silicon vacancies (or defects) in the atomic structure of diamond crystals as qubits, the basic building blocks of quantum computation.

Unlike the conventional method of using qubits based on trapped ions, integrating defect-based qubits with existing silicon-based telecommunications is easier as both diamond and silicon are based on mature solid-state technologies, making the process much more scalable. The challenge, however, is that the coherence time of germanium or silicon defect-based qubits—the time-scale over which interactions with the environment destroy the information carried by a qubit—is relatively short at room temperature.

To enhance the coherence time of these spin defects, Asst Prof Gao keeps temperatures in the milli-Kelvin range close to absolute zero (0 Kelvin, or −273.15 °C). “The new funding will allow us to build a quantum materials and devices characterisation facility that includes a milli-Kelvin cryostat with a confocal microscope—one of a few such facilities in the world,” he explains. “Furthermore, with matching funds from NTU, we are also building a quantum devices manufacturing facility to create the defects as well as cavities, which will allow us to enhance device performance.”

As difficult as it may be to create and stabilise a single qubit, the real power of quantum computing will only be unleashed if many qubits can be connected in a network. To move to the next step of achieving spin-photon entanglement, Asst Prof Gao will be working with his colleagues to achieve better collection and emission efficiency.

Of muffins and doughnuts
While Asst Prof Gao is working on coupling nanophotonics to quantum technology, Assoc Prof Chong Yidong has taken a mathematical approach to furthering our ability to manipulate light. Just as semiconductor materials are used to control the flow of electricity, photonic crystals can be used to control the flow of light, he explains. Instead of simply insulating against light, however, topological nanophotonics can create totally new states of light called topological edge modes.

Topology, a branch of mathematics, is concerned with the properties of space that remain the same after deformation. For example, a sphere can be formed into a muffin while staying the same in topological terms, but a muffin cannot be reshaped into a doughnut without puncturing a hole, making muffins and doughnuts topologically distinct.

“It was proven about ten years ago that not all possible insulators of light fall into the same topological class. All the photonic structures that people were previously looking at were basically different types of muffins or spheres. But once photonic topological insulators were discovered, it was like finding that doughnuts also exist,” Assoc Prof Chong says.

“When you take a topological material and surround it with a conventional photonic insulator, it is like wrapping a muffin around a doughnut. Because these two types of photonic media are topologically distinct, you are mathematically guaranteed that something funny happens at the interface between the two materials. That ‘something funny’ is the existence of what we call a topological edge mode.”

Once a topological edge mode is created, it allows light to flow in strange and exotic patterns. As its properties are determined by mathematical topological incompatibility rather than fine details such as the shape of the interface, topological edge modes are extremely robust and can survive deformation of the system or disorders caused by imperfections.

When applied to lasers, topological edge modes allow for the tightly controlled amplification of light, resulting in extremely stable lasers, Assoc Prof Chong says. Apart from developing new types of topological lasers, his group is currently investigating the use of topological nanophotonics to create new robust types of non-linear optical devices that can change the frequency of light.

Material over matter
As promising as both quantum and topological nanophotonics may be, research into new materials and devices is required to realise their potential. “Topological photonics have been experimentally demonstrated but every single iteration is a big technical challenge,” Assoc Prof Chong says.

The team is also exploring the use of topological insulator crystals, which have atomically thin metallic surfaces, to create new nanophotonic devices. “We want to use these metallic surfaces for plasmonics, which is normally done with conventional metals such as gold or silver,” says Assoc Prof Soci. “Plasmonics involves the use of rapid oscillations of electrons at the surfaces and interfaces of conducting materials such as metals.”

To find other materials that can support quantum and topological nanophotonics, part of the grant will be used to establish a materials discovery facility. In addition to searching for new topological crystals, the facility will also study interfaces of topological insulators with other exotic materials, like superconductors, Assoc Prof Soci adds.

While applications of these fundamental research findings might be ten or more years away, NTU researchers are also actively engaged in discussions with industry partners through the LUX Photonics Consortium, which is supported by the National Research Foundation and led by TPI’s Co-Director Prof Tjin Swee Chuan.

Acting as a node for the photonics research community, the Consortium connects researchers at NTU, the National University of Singapore and Singapore’s Agency for Science, Technology and Research, with industry players such as global semiconductor company KLA-Tencor.

“As leaders in the fields of quantum and topological nanophotonics, we were the first to organise a series of workshops bringing together researchers in the fields of quantum and topological nanophotonics. After a first edition in 2016 and a second one in 2018, we are planning for another edition of the workshop in 2019,” says Assoc Prof Soci.

“We are very happy that a number of groups around the world are now taking our approach of finding connections between the quantum and topological aspects, not only to advance the fundamental field of nanophotonics but also to find applications that will greatly impact our everyday lives, whether it is faster telecommunications or more efficient computing.”

The article appeared first in NTU’s research & innovation magazine Pushing Frontiers (issue #14, December 2018).

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