Effects of the environment on quantum systems: decoherence, bound states and high impedance in superconducting circuits
Doctoral thesis, 2021

Superconducting circuits in the quantum regime represent a viable platform for microwave quantum optics, quantum simulations and quantum computing. In the last two decades, a large effort brought this architecture from an academic curiosity to concrete technology. 

In this thesis, we study the effects of the environment on superconducting circuits. We consider mainly two typologies of the environment. On one hand, we study the classical baths inevitably coupled to the circuits, in particular the substrate where they are fabricated and the highly attenuated coaxial lines used for controlling them, which are the main sources for decoherence. On the other hand, we study structured electromagnetic environments that shape the density of states for the circuits, modifying their energy structure and their excitation properties.  

Defects on the substrate mechanically and electrically coupled to superconducting circuits, behave as a bath of two-level systems. We investigate the effects of the bath on a qubit fabricated on silicon. From a time trace with more than 2000 measurements of T1 and T2 (every 3 min for 60 h), we statistically infer a Lorentzian resonance signature of the bath. Moreover, measuring the residual population of the first excited state of the qubit, and tuning the photonic population in the line, we assess the thermal state of the bath, measuring a temperature of 56 mK. Furthermore, we investigate the mechanical coupling of the bath, saturating its state, strongly pumping neighbouring modes in a high finesse mechanical resonator.
On a piezoelectric substrate, the travelling phonons, carry an electric component together with a lattice deformation. Therefore, superconducting circuits can be coupled to a phononic waveguide through which they release part of their energy. We design, fabricate and measure superconducting resonators on gallium arsenide, demonstrating the electromechanical coupling as the main source of decoherence.

Concentrating on the effects of the photonic bath in the coaxial line, we design a qubit with a very large coupling to this bath compared to the bath of two-level fluctuators. In this limit, the scattering of a coherent photon by the qubit linearly depends on the photonic bath population. In this regime, the qubit can be used as a primary thermometer; we measured injected calibrated noise and the photon occupation of our input lines at different temperatures. 

Finally, we implemented a slow-waveguide made of a linear chain of high impedance resonators. The excitation of two transmon qubits coupled to the waveguide is dressed with a photonic component, generating the hybrid excitation of atom-photon bound state. We spectroscopically investigated the first and second excitation subspaces of the system, and we demonstrated full frequency and time domain control, of these bound states.

These results may help to improve the performance of superconducting circuits and their setup. Moreover, we hope that our experiment can provide tools for quantum thermodynamics and quantum simulation.


atom photon bound state

circuit QED

quantum thermodynamics

two-level systems

surface acoustic wave

high impedance

superconducting circuits


Kollektorn, lecture room, Kemivägen 9, MC2-huset
Opponent: Prof. Johannes Fink, Institute of Science and Technology (IST), Vienna, Austria


Marco Scigliuzzo

Chalmers, Microtechnology and Nanoscience (MC2), Quantum Technology

Acoustic spectral hole-burning in a two-level system ensemble

npj Quantum Information,; Vol. 7(2021)

Journal article

Primary Thermometry of Propagating Microwaves in the Quantum Regime

Physical Review X,; Vol. 10(2020)

Journal article

Phononic loss in superconducting resonators on piezoelectric substrates

New Journal of Physics,; Vol. 22(2020)

Journal article

Decoherence benchmarking of superconducting qubits


Journal article

Extensible quantum simulation architecture based on atom-photon bound states in an array of high-impedance resonators

Measuring the temperature of a superconducting qubit and its surrounding two-level fluctuators

I början av 1900-talet utveckades kvantmekaniken för att förklara experimentella resultat som stred mot de klassiska fysiklagarna. Teorin utvecklades med åren, och effekterna är har gett oss många viktiga elektronikkomponenter. Transistorer, lasrar, ljus emitterande dioder (LED) och solceller har på ett avgörande sätt format vårt samhälle. I slutet av 1900-talet påvisade vetenskapsmän möjligheten med att utforma och bygga artificiella atomer med supraledande kretsar. Tillsammans med framsteg inom andra områden visade detta vägen för den andra kvantrevolutionen. Forskare kunde utforma energistrukturen (ljusets färger som den artificiella atomen interagerar med). De kunde bestämma hur starkt den interagerar med ljuset och till och med bestämma atomens dimensioner i jämförelse med ljusets våglängd (i byggandet av jätteatomer). Dessa artificiella atomer kan också användas som kvantbitar, de grundläggade byggstenarna i en kvantdator.


I den här avhandlingen undersöker vi effekterna av omgivningen på supraledande artificiella atomer, särskilt dekoherens, dvs förlusten av energi eller information, och effekten av en strukturerad elektromagnetisk omgivning på atomens excitationer. 


Vi visar att huvudkällan till dekoherens beror på substratet där kretsarna är fabricerade: defekter på kisel absorberar energi från atomen, medan excitationer på gallium arsenid konverteras till mekaniska vågor. Vi mäter temperaturen på defekternas och höjer avsiktiligt deras temperatur, samt pumpar närliggande moder på en ytakustisk resonator. Dessutom demonstrerar vi att fotonernas termiska tillstånd i en mätledning påverkar kvantbitens spridning av mikrovågor.


Slutligen implementerar vi en vågledare där ljuset saktas ner och nästintill stannar. När en atom är kopplad till vågledaren lokaliserar sig ett moln av fotoner kring atomerna, och bildar ett bundet tillstånd av atomen och fotonerna. I vårt experiment kontrollerar vi två bundna tillstånd, vi mäter deras spektrum och vi ser till att de växelverkar med varandra.


Dessa resultat kan användas för att förbättra prestandan på supraledande kretsar i kvantregimen vilket kan ge bättre atrificialle atomer för kvanttermodynamik och kvantakustik och desutom bättre kvantbitar för kvantdatorer och kvantsimuleringar.

At the beginning of the 20th century, physicists developed quantum mechanics to explain experimental results that violated the “classical” laws of physics. This theory evolved over the years, and its effects became visible in many devices used in everyday life. Transistors, lasers, light-emitting diodes and solar cells, have deeply shaped our society. Towards the end of the 20th century, scientists demonstrated the possibility to design and build artificial atoms made from superconducting circuits. Together with developments in other fields, this opened the way to the second quantum revolution. Researchers could design the energy structure (the colors of light that the artificial atom interacts with), they could decide how strong it interacts with light and even decide the dimensions of the atom compared to the light wavelength (building so-called giant atoms). These artificial atoms can also be used as qubits, the basic building blocks of quantum computers


In this thesis, we study the effects of the environment on superconducting artificial atoms, in particular decoherence, the loss of energy or information, and the effect of a structured electromagnetic environment on the atomic excitations.


We show that the main source of decoherence depends on the substrate where the circuits are fabricated: defects on silicon absorb the energy from the atom, while on gallium arsenide the excitation is converted into mechanical waves. We measure the temperature of the defect bath and we purposely warm its temperature up, pumping modes close in frequency in a surface acoustic wave resonator. Moreover, we demonstrate that the thermal state of the photons in the measuring lines affects the qubit scattering of microwaves.


We implement a waveguide where the light slows down almost to stop. When an atom is coupled to the waveguide, a cloud of photons localizes around the atom, producing an atom-photon bound state. In our experiment, we control two bound states, we measure their spectrum and we make them interact.


These results can be used to improve the performance of superconducting circuits in the quantum regime and they could provide important building blocks for quantum thermodynamics, quantum acoustic as well as for quantum computing and simulation.

Quantum Sound: Generating and manipulating phonons at the quantum level

Swedish Research Council (VR) (2015-00152), 2016-01-01 -- 2025-12-31.

An Open Superconducting Quantum Computer (OpenSuperQ)

European Commission (EC) (829363), 2018-10-01 -- 2021-09-30.

Areas of Advance

Nanoscience and Nanotechnology (SO 2010-2017, EI 2018-)

Subject Categories

Physical Sciences

Electrical Engineering, Electronic Engineering, Information Engineering

Nano Technology


Nanofabrication Laboratory



Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5001


Chalmers University of Technology

Kollektorn, lecture room, Kemivägen 9, MC2-huset

Opponent: Prof. Johannes Fink, Institute of Science and Technology (IST), Vienna, Austria

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