From quantum losses to quantum gains

A study by Dr. Pablo Martinez Azcona, a postdoctoral researcher in the Quantum dynamics and Control team of Prof. Aurélia Chenu, has unveiled a surprising way to manage noise in quantum systems. In collaboration with Dr. Aritra Kundu, formerly from the same team, Prof. Adolfo Del Campo and Avadh Saxena (from Los Alamos National Laboratory, US), the team have researched a novel phenomenon, dubbed “antidephasing”, and challenged a conventional understanding of how noise affects delicate quantum properties. Their latest discovery could significantly impact the development of qubits, the building blocks of quantum computers.

Understanding quantum noise

So, what exactly is “noise” in this context? In quantum computing, noise refers to unwanted disturbances or unpredictable fluctuations that can disrupt the delicate quantum properties of a system. Traditionally, such noise is highly detrimental. It causes quantum systems to “decohere,” meaning they lose their quantum characteristics like superposition (being in multiple states at once) or entanglement (where the quantum states of two or more particles are linked in such a way that they cannot be described independently of each other). 

The traditional setup in quantum mechanics is to consider a quantum system perfectly isolated. However, recently, it became possible to engineer more complicated systems, which physicists call “non-Hermitian”, where the systems have losses (as if we poked a hole in a bucket of water). Such systems can show phenomena that closed systems could not.

Decoherence as the main obstacle

This loss of quantum properties, often called “decoherence”, is a major obstacle that hinders the creation of practical quantum computers. Dr. Martinez Azcona’s work, however, explores a counter-intuitive scenario where this very noise can actually be beneficial. His theoretical model uses classical white noise to represent these unpredictable fluctuations on non-Hermitian systems. These are an unavoidable part of current experimental quantum setups, such as those found in superconducting qubits or trapped ions. 

He explains this complex concept using a relatable analogy: “Imagine two buckets of water representing two possible states of a quantum bit, or qubit. One bucket has a hole, and we’re introducing noise into the amount of water that leaks out”. His team found that when noise affects the size of these “holes”, the system behaves differently. Instead of always losing quantum properties, the system can partially preserve them.

Noise-induced phases: when noise stabilises the system

One of the most interesting findings is the existence of a “noise-induced phase,” where the system’s dynamics surprisingly stabilises. It is a strange phenomenon where the “bucket with a hole starts to fill up” because of the noise. This contrasts sharply with typical scenarios where losses inevitably lead to decay.

This theoretical breakthrough holds significant practical implications, particularly for Noisy Intermediate-Scale Quantum (NISQ) devices, which are current experimental realisations of quantum technology and are inherently subject to noise. Dr. Martinez Azcona’s research provides insights into understanding which quantum properties are resistant to noise and what can be done to mitigate its effects. The findings also suggest new avenues for preparing quantum states that are currently difficult to achieve, such as “magic” states. 

Furthermore, the model can help to explain puzzling experimental observations such as extra sources of decoherence in non-Hermitian qubits, which previous theories couldn’t fully explain. Indeed, the antidephasing model could provide a better fit to experimental data than alternative models. 

Overall, Dr. Martinez Azcona’s research paves the way for a deeper understanding of open quantum systems and offers new strategies for controlling and leveraging noise in future quantum technologies.

The research paper, “Quantum Dynamics with Stochastic Non-Hermitian Hamiltonians,” has recently been published in “Physical Review Letters (PRL)”. The project was funded by the Luxembourg National Research Fund (FNR).