# Mr. Andrés Rosario Hamann

### Research student - PhD

Andrés sucessfully completed his thesis on "Rectangular waveguide QED with superconducting qubits" and joind Quantum Device Lab at ETH Zürich as a postdoctoral research fellow.

Andrés started his Physics studies at the Pontificia Universidad Católica del Perú (PUCP), where he received a MSc diploma for theoretical studies on the relationship between entanglement and non-Markovianity. After working on polarimetric measurements on single and entangled photons pairs at the Quantum Optics lab of PUCP, he started a PhD at the SQDlab. His PhD project aims to generate stable entanglement on superconducting quantum circuits. He is currently working on nonreciprocal phenomena in waveguide quantum electrodynamics and optimising the fabrication of superconducting quantum circuits at the SQDlab.

### 2018

### 2017

2017

, Phys. Rev. A, 96, pp. 53817

Recent theoretical studies of a pair of atoms in a 1D waveguide find that the system responds asymmetrically to incident fields from opposing directions at low powers. Since there is no explicit time-reversal symmetry breaking elements in the device, this has caused some debate. Here we show that the asymmetry arises from the formation of a quasi-dark-state of the two atoms, which saturates at extremely low power. In this case the nonlinear saturability explicitly breaks the assumptions of the Lorentz reciprocity theorem. Moreover, we show that the statistics of the output field from the driven system can be explained by a very simple stochastic mirror model and that at steady state, the two atoms and the local field are driven to an entangled, tripartite |W⟩ state. Because of this, we argue that the device is better understood as a saturable Yagi-Uda antenna, a distributed system of differentially-tuned dipoles that couples asymmetrically to external fields.

### 2016

2016

, Phys. Rev. Appl., 6, pp. 14014

The ability to determine whether a multi-level quantum system is in a certain state while preserving quantum coherence between all orthorgonal states is necessary to realize binary-outcome compatible measurements which are, in turn, a prerequisite for testing the contextuality of quantum mechanics. In this paper, we use a three-level superconducting system (a qutrit) coupled to a microwave cavity to explore different regimes of quantum measurement. In particular, we engineer the dispersive shifts of the cavity frequency to be identical for the first and second excited states of the qutrit which allows us to realize a strong projective binary-outcome measurement onto its ground state with a fidelity of 94.3%. Complemented with standard microwave control and low-noise parametric amplification, this scheme can be used to create sets of compatible measurements to reveal the contextual nature of superconducting circuits.

### The ARC Centre of Excellence for Engineered Quantum Systems

This Centre of Excellence seeks to initiate the Quantum Era in the 21st century by engineering designer quantum systems.