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Electronic, optical and quantum systems are never perfect. Energy can leak from them, which is a ubiquitous and undesirable phenomenon that leads to various problems physicists refer to as decoherence, noise and loss. 

Of the three, particularly energy loss can prevent making the most of new technologies in extreme cases. For example, there is quantum information loss in qubits, the kernels of quantum calculation power, which severely limits the scaling-up of quantum computers.

Conventional wisdom for physicists and engineers is to minimize opportunities for loss to arise in the first place. This is difficult enough, and troubles only increase if the systems need to be coupled to their environment for functionality. An alternative but largely unexplored strategy is to use loss itself as a tool for creating exotic behaviour inside the system. Now researchers from Aalto University, the famed Nokia Bell Labs and the University of Central Florida used the alternative approach to create loss-protected optical systems which can operate perfectly even in noisy environments.

Elizabeth Pereira, Doctoral Researcher at Aalto University in the Nokia Industrial Doctoral School in Quantum Technology, created a theoretical design together with her supervisor, Department of Applied Physics Assistant Professor Jose Lado. A group led by Andrea Blanco-Redondo at UCF, in collaboration with Hongwei Li at Nokia Bell Labs, then demonstrated the design experimentally. 

The paper was published in the prestigious journal Nature Materials:

The design centres around a state of matter known as a non-Hermitian topological state, which emerged from carefully engineered energy losses. These states are protected from loss and noise precisely by the engineered loss itself. This protection leads to photonic edge excitations, which are the result of a mathematical protection associated with the description of the system, known as a topological protection. The team then realized the design experimentally inside a programmable and integrated photonics platform. 

“Controlling how devices couple to the environment provides a new strategy to engineer collective excitations that make new functionalities possible. While loss is often considered a nuisance, carefully controlling it allows us to engineer exotic phenomena, which would not be possible in perfect devices,” Lado says.

The discovery has potential both for fundamental science and industrial applications.

For fundamental science, it enables exploring so called non-Hermitian phenomena: states of matter that are not allowed in closed environments but which might prove a way to poke at some of the most fundamental constrains in physics research.

For industry, creating topological excitations driven by engineered losses might make it possible to build devices that operate perfectly even in noisy environments. It would be a significant boon for developing more robust laser, optical and quantum technologies.

“Photonic systems are ideal platforms for exploiting the control of losses to engineer unconventional states. In my doctoral work, I first performed a theoretical design of this system, and it has been hugely rewarding to show that such a strategy works in devices experimentally,” Pereira says.

The study is a result of the Nokia Industrial Doctoral School in Quantum Technology, a long-term effort to train highly skilled PhDs in the field of quantum technology between Nokia and Aalto. The focus of the Nokia Industrial Doctoral School in Quantum Technology is on industrially relevant topics that range from fundamental science to applied research.