# Publications

### 2021

2021

, Phys. Rev. Research, 3, 043211

Microwave circulators play an important role in quantum technology based on superconducting circuits. The conventional circulator design, which employs ferrite materials, is bulky and involves strong magnetic fields, rendering it unsuitable for integration on superconducting chips.

2021

, Applied Physics Letters, 119, 091101

Superconducting quantum circuits are one of the leading quantum computing platforms. To advance superconducting quantum computing to a point of practical importance, it is critical to identify and address material imperfections that lead to decoherence.

### 2020

2020

, Phys. Rev. Lett., 124, pp. 240501

Initialization of a qubit in a pure state is a prerequisite for quantum computer operation. Qubits are commonly initialized by cooling to their ground states through passive thermalization or by using active reset protocols.

2020

, Phys. Rev. Lett, 124, pp. 70401

Quantum mechanics postulates that measuring the qubit's wave function results in its collapse, with the recorded discrete outcome designating the particular eigenstate the qubit collapsed into. We show this picture breaks down when the qubit is strongly driven during measurement. More specifically, for a fast evolving qubit the measurement returns the time-averaged expectation value of the measurement operator, erasing information about the initial state of the qubit, while completely suppressing the measurement back-action. We call this regime `quantum rifling', as the fast spinning of the Bloch vector protects it from deflection into either of its two eigenstates. We study this phenomenon with two superconducting qubits coupled to the same probe field and demonstrate that quantum rifling allows us to measure either one of the two qubits on demand while protecting the state of the other from measurement back-action. Our results allow for the implementation of selective read out multiplexing of several qubits, contributing to efficient scaling up of quantum processors for future quantum technologies.

### 2019

2019

, Phys. Rev. Lett., 123, pp. 150501

We present a technique to measure the transfer function of a control line using a qubit as a vector network analyzer. Our method requires coupling the line under test to the the longitudinal component of the Hamiltonian of the qubit and the ability to induce Rabi oscillations through simultaneous driving of the transversal component. We used this technique to characterize the 'flux' control of a superconducting Transmon qubit in the range of 8 to 400\,MHz. Our method can be used for the qubit 'flux' line calibration to increase the fidelity of entangling gates for the quantum processor. The qubit can be also used as a microscopic probe of the electro-magnetic fields on a chip.

### 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.