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Quantum sensing of charge states in hybrid quantum dot-microwave systems

Nord, Lukas LU (2026) FYSM34 20261
Department of Physics
Mathematical Physics
Abstract
Inspired by the experimental work performed in the research group led by Ville Maisi at Lund University, this thesis investigates a hybrid quantum setup in circuit quantum electrodynamics (cQED). The system consists of a microwave resonator coupled to a double quantum dot (DQD) acting as an internal sensor to indirectly detect the state of an external quantum system. Using input-output theory and quantum Langevin equations, we derive the reflection coefficient of the coupled system, utilizing a mean-field approximation to determine steady-state solutions. The performance and sensitivity of the measurement scheme are quantified using Quantum Fisher Information (QFI) to identify optimal probing regimes for an external system modeled as... (More)
Inspired by the experimental work performed in the research group led by Ville Maisi at Lund University, this thesis investigates a hybrid quantum setup in circuit quantum electrodynamics (cQED). The system consists of a microwave resonator coupled to a double quantum dot (DQD) acting as an internal sensor to indirectly detect the state of an external quantum system. Using input-output theory and quantum Langevin equations, we derive the reflection coefficient of the coupled system, utilizing a mean-field approximation to determine steady-state solutions. The performance and sensitivity of the measurement scheme are quantified using Quantum Fisher Information (QFI) to identify optimal probing regimes for an external system modeled as either a single quantum dot or a secondary two-level qubit. Our results show that the external system induces state-dependent frequency shifts in the sensor qubit, which produce distortions in the reflection spectrum. We find that sensitivity is maximized at probing frequencies corresponding to the steepest slopes of resonance features, in which the reflection is most sensitive to the external system. Ultimately, this work demonstrates that cQED-based hybrid systems pro- vide a versatile platform for indirect state discrimination and provides a foundation for future studies into the quantum limits of sensing. (Less)
Popular Abstract
Measuring the Invisible - Using “Artificial Atoms” to Sense the Quantum World

The Challenge of Looking Without Touching
In the world of quantum mechanics, the act of looking at something can change it forever. This is a hurdle for building quantum computers because touching a particle to measure it risks destroying its fragile information. This research explores a way to bypass this problem using a hybrid system that acts like a microscopic translator. This setup allows us to sense a quantum state without making direct contact.

The Microscopic Translator
Imagine you want to know if a door is open in a dark room without entering. Instead, you listen to a bell in the hallway. If the open door changes the way the bell echoes, you can... (More)
Measuring the Invisible - Using “Artificial Atoms” to Sense the Quantum World

The Challenge of Looking Without Touching
In the world of quantum mechanics, the act of looking at something can change it forever. This is a hurdle for building quantum computers because touching a particle to measure it risks destroying its fragile information. This research explores a way to bypass this problem using a hybrid system that acts like a microscopic translator. This setup allows us to sense a quantum state without making direct contact.

The Microscopic Translator
Imagine you want to know if a door is open in a dark room without entering. Instead, you listen to a bell in the hallway. If the open door changes the way the bell echoes, you can determine the door’s state just by listening from the outside. In this study, the hallway is a microwave resonator, which is a tiny device that traps light. The bell is a Double Quantum Dot, which is an artificial atom made of semiconductor material. By coupling these, we create a sensor that detects an external system, the door, by looking at how microwaves bounce off the device.

How the Sensing Works
The process relies on a chain reaction of physical interactions.
• Interaction. The external system, such as a single quantum dot or a qubit, is coupled to the internal sensor.
• Frequency Shift. When the state of the external system changes, it induces a frequency shift in the sensor.
• The Echo. Because the sensor is coupled to the resonator, this shift changes how microwaves are reflected.
• Measurement. By measuring the reflection coefficient, we determine the state of the external system purely through its influence on the resonator frequency.

Results
A key finding is that the most sensitive measurements do not happen where the signal is loudest. Using Quantum Fisher Information, the research identified that the best results come from probing the system at the steepest slopes of the resonance features. At these specific frequencies, even a tiny change in the external system causes a massive change in the reflected signal. This analysis demonstrates that these hybrid systems provide a versatile platform for indirect state discrimination. This allows us to identify the state of a system without interacting with it directly.

Why This Matters
This work provides a blueprint for building more sensitive quantum sensors. These are vital for technologies like quantum computers, where scientists must monitor bits with extreme precision and minimal interference. By understanding these limits, we move closer to making stable, large-scale quantum technology a reality. (Less)
Please use this url to cite or link to this publication:
author
Nord, Lukas LU
supervisor
organization
course
FYSM34 20261
year
type
H2 - Master's Degree (Two Years)
subject
keywords
cQED, Quantum, sensing, hybrid systems
language
English
id
9231824
date added to LUP
2026-06-09 10:01:53
date last changed
2026-06-09 10:01:53
@misc{9231824,
  abstract     = {{Inspired by the experimental work performed in the research group led by Ville Maisi at Lund University, this thesis investigates a hybrid quantum setup in circuit quantum electrodynamics (cQED). The system consists of a microwave resonator coupled to a double quantum dot (DQD) acting as an internal sensor to indirectly detect the state of an external quantum system. Using input-output theory and quantum Langevin equations, we derive the reflection coefficient of the coupled system, utilizing a mean-field approximation to determine steady-state solutions. The performance and sensitivity of the measurement scheme are quantified using Quantum Fisher Information (QFI) to identify optimal probing regimes for an external system modeled as either a single quantum dot or a secondary two-level qubit. Our results show that the external system induces state-dependent frequency shifts in the sensor qubit, which produce distortions in the reflection spectrum. We find that sensitivity is maximized at probing frequencies corresponding to the steepest slopes of resonance features, in which the reflection is most sensitive to the external system. Ultimately, this work demonstrates that cQED-based hybrid systems pro- vide a versatile platform for indirect state discrimination and provides a foundation for future studies into the quantum limits of sensing.}},
  author       = {{Nord, Lukas}},
  language     = {{eng}},
  note         = {{Student Paper}},
  title        = {{Quantum sensing of charge states in hybrid quantum dot-microwave systems}},
  year         = {{2026}},
}