QSE Quantum Seminar: "Utilizing and controlling collective states in superconducting waveguide QED"

Please join us for the QSE Center Quantum Seminar with Gerhard Kirchmair from the Institute for Experimental Physics, University of Innsbruck and Institute for Quantum Optics and Quantum Information, Austrian Academy of Sciences who will give a talk about "Utilizing and controlling collective states in superconducting waveguide QED" on Wednesday July 29th from 12:00 to 13:30.
Location: 

Pizzas will be available at 12:00. All PhDs, postdocs, students, group leaders, and PIs are welcome to join us.

TITLE: "Utilizing and controlling collective states in superconducting waveguide QED"

ABSTRACT: 
Propagating microwave photons in waveguides couple well to superconducting qubits and mediate long-range interactions between distant qubits causing the emergence of collective states. Of particular interest are dark or subradiant states, which are protected from decoherence as they decouple from the 
waveguide environment. I will show how we can use collective states of two qubits in the waveguide to build a primary thermometer which can distinguish the influence of a local bath from the temperature of the mode it should measure [1].Controlling individual qubits in waveguide QED system poses a challenge if one does not have access to individual addressing ports. I show that with the use of dispersion we can achieve spatial addressing of superconducting qubits separated by a subwavelength distance within a microwave waveguide. This approach relies on a self-focusing effect to create a position-dependent interaction between a chirped pulse and an individual qubit [2].Furthermore, I will show the progress in building and controlling a setup where we extend the system to larger arrays of transmon qubits in a planar implementation. The goal is to have 8 Transmon qubits, coupled to a coplanar waveguide. The qubits will be spaced out by a distance that corresponds to half of the emission wavelength. In such a system it will be possible to create subradiant states with up to four excitations in different configurations [3,4].
[1] Sharafiev et.al. Phys. Rev. Lett. 134, 213602 (2025)
[2] Zanner et.al. Phys. Rev. Applied 24, 014051 (2025)
[3] Sheremet et.al. Rev. Mod. Phys. 95, 015002 (2023)
[4] Holzinger et.al., Phys. Rev. Let. 129, 253601 (2022)

BIO:
Gerhard Kirchmair was born in 1981 in Hall i.T., Austria. He studied physics at the University of Innsbruck and did his diploma thesis in the group of Prof. Rainer Blatt, on building a laser for precision spectroscopy on Calcium ions. For his PhD work, he continued in the group of Prof. Blatt, working on realizing quantum information protocols and quantum simulation experiments with calcium ions in a linear Paul trap. He received his PhD in 2010, with Presidential Honors, on the subject “Quantum non-demolition measurements and quantum simulation”. From 2010 on, he joined the group of Prof. Robert Schoelkopf as a post-doctoral associate at Yale University. There he was working on superconducting circuits coupled to Josephson junction qubits with a focus on implementing quantum optics experiments. He contributed to developing the 3D circuit QED architecture which increased coherence times of superconducting qubits by a factor of 100. He used this architecture to create setups for QIP and quantum optics experiments. These experiments led to the observation of the single photon Kerr effect and the generation of microwave cat states consisting of more than 100 photons. He is professor for experimental physics at the University of Innsbruck and Scientific Director at the Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences. His research group works on superconducting circuits for quantum optics and quantum simulation experiments and on interfacing microwave circuits with micromechanical systems.