Exploring light fields and a quantum emitter

A new study in Physical Examination Letters illuminates the intricacies of energy exchange within two-party quantum systems, providing in-depth insight into quantum coherence, pure phase shift effects, and the potential impact on future quantum technologies.

In quantum systems, particle behavior and energy transfer are governed by probability distributions and wave functions, thereby adding layers of complexity to the understanding of energy exchange.

Exploring energy exchange in quantum systems inherently involves addressing the complexities arising from quantum decoherence and the scales at which quantum systems operate, by introducing sensitivity.

Despite these challenges, the study of energy exchanges in quantum systems is essential for advancing quantum technologies and understanding fundamental aspects of quantum mechanics.

The researchers aim to bridge the gap between theoretical predictions and experimental observations in quantum optics and thermodynamics. By exploring energy exchanges within bipartite quantum systems, the study strives to provide a comprehensive framework for understanding the complex dynamics at play.

“Since my doctorate and the beginning of my academic career, I have acquired training in experimental quantum optics, which I have kept while moving on to theory. I became involved in quantum thermodynamics ten years ago and I “I have been working to bridge the gaps between “These results represent a great realization of these efforts,” Professor Alexia Auffèves, visiting professor-researcher at the Singapore Quantum Technology Center and co-author of the study, told Phys.org.

Unitary and correlation energy

Bipartite systems refer to quantum systems composed of two distinct entities or subsystems, often exhibiting quantum entanglement and superposition. Energy exchanges within these systems, such as those studied in the research, provide insight into quantum dynamics.

According to Professor Auffèves, the theorist behind the study: “When two quantum systems are coupled but otherwise isolated, they can exchange energy in two ways: either by exerting a force on each other, or by becoming entangled. We call these exchange energies “unitary” and “correlation” respectively.

This distinction highlights the dual nature of energy interactions within two-part systems, with unitary energy involving forces and correlation energy resulting from entanglement.

Understanding the dynamics within these systems is crucial to advancing quantum mechanics and developing applications like quantum computing. In particular, bipartite systems are essential components of quantum gates and algorithmic operations, forming the foundation of emerging quantum technologies.

Professor Auffèves further develops the research axis: “We studied experimentally and theoretically these energy exchanges, first between a qubit and a light field, and then between two light fields coupled by a beam splitter.”

Part 1: Spontaneous emission of a qubit

In the first part of the study, the researchers focused on the spontaneous emission of a qubit, represented by a quantum dot. Quantum dots are nanoscale semiconductors exhibiting quantum mechanical properties.

It is often called an artificial atom because, like atoms, it has a discrete energy level. The quantum dot was placed in a reservoir of empty electromagnetic modes, meaning there was no disturbance or interaction due to electromagnetic fields.

“Previous theoretical results obtained in my group predict that the amount of unit energy transferred to the vacuum field should be proportional to the initial quantum coherence of the qubit,” explained Professor Auffèves.

Simply put, when the qubit is initially prepared in an equal superposition of ground and excited states, the unit energy transfer to the vacuum field is maximized.

In such a scenario, the unit energy transferred is equal to half of the total energy released by the qubit. In contrast, if the qubit is initially inverted, only the correlation energy is transferred to the field. This dependence on the initial quantum state of the qubit highlights the complex nature of energy transfers in quantum systems.

The results of the first part were precisely what the researchers expected. As Professor Auffèves points out, “the experiments reported in the article perfectly meet our expectations. They involve as a qubit a quantum dot coupled to a leaky semiconductor microcavity.”

“The unit energy received by the field, that is to say the energy locked in the coherent component of the emitted field, is measured using a homodyne device. The level of experimental control is such that l The unit energy almost reaches the theoretical limit, regardless of the theoretical limit.initial state of the quantum dot.

This means the team could precisely measure and understand how the quantum field exchanges energy during this process.

Part 2: Coupling of two light fields

For the second part, the researchers examined the energy exchanges between the emitted light field and a coherent reference field. The two fields were tightly coupled using a beam splitter, a device commonly used in quantum optics to manipulate the paths of light beams.

The study involved a quantum system reminiscent of linear photonic quantum computing, incorporating light field interference via beam splitters.

“Unlike the first case, this study was uncharted territory. This sparked an exciting dialogue between theory and experiment to extend our concepts of unit energies and correlation to this new situation and study new behaviors and patterns,” said the Professor Auffèves.

The quantitative analysis revealed a significant result: unit energy transfers were found to be dependent on the purity and coherence of the emitted field. This implies that the characteristics of the light field, particularly its purity and coherence, play a crucial role in determining the nature and magnitude of unit energy exchanges.

“In both cases, we see that the unit energy (respectively correlation energy) received by a light field is equal to the change in energy of the coherent (respectively incoherent) component of this field,” explains Professor Auffèves.

Quantum applications and beyond

“The framework we started to build in this paper could play a key role in future energy analyzes of photonic quantum computing,” said Professor Auffèves.

Understanding energy and entropy exchanges is crucial to improving processes such as entanglement generation and quantum gates. Managing pure phase shift at higher temperatures, as the study reveals, becomes vital for efficient unitary energy exchange, necessary for implementing quantum gates.

Speaking of future research, Professor Auffèves wants to focus on the fundamental side of things by exploring quantum optics with energetic and entropic tools.

“For example, by extracting optical signatures of irreversibility, or conversely, by detecting the quantum character of a field with energy figures of merit. On the practical side, it will be important to evaluate if and how the concepts of unitary energy and correlation impact the energy cost of macroscopic and comprehensive quantum technologies,” she concluded.

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