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Publications

2021

Operating a passive on-chip superconducting circulator: Device control and quasiparticle effects
Thanh Le Dat et al, 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.
Approximations in transmon simulation
Jones Tyler et al, 2021
Physical Review Applied, 16, 054039

Classical simulations of time-dependent quantum systems are widely used in quantum control research. In particular, these simulations are commonly used to host iterative optimal control algorithms.
Neural networks for on-the-fly single-shot state classification
Navarathna Rohit et al, 2021
Applied Physics Letters

Neural networks have proven to be efficient for a number of practical applications ranging from image recognition to identifying phase transitions in quantum physics models.
Near-field terahertz nanoscopy of coplanar microwave resonators
Guo Xiao et al, 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

Measuring effective temperatures of qubits using correlations
Kulikov A, Navarathna R and Fedorov A, 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.
Probabilistic motional averaging
Karpov D et al, 2020
EPJ B, 93, pp. 49

In a continuous measurement scheme a spin-1/2 particle can be measured and simultaneously driven by an external resonant signal.
Quantum rifling: protecting a qubit from measurement back-action
Szombati D. et al, 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

In situ characterization of qubit control lines: a qubit as a vector network analyzer
Jerger M. et al, 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

Nonreciprocity Realized with Quantum Nonlinearity
Hamann A.R. et al, 2018
Phys. Rev. Lett., 121, pp. 123601

Our experiment comprises the first experimental implemenation of a so called "optical rectifier" or a non-reciprocal device based on quantum nonlinearity of two-level atoms. Our device is also the simplest possible device which can behave non-reciprocally and shows the connection between non-reciprocty and entangelement.

2017

Nonreciprocal Atomic Scattering: A saturable, quantum Yagi-Uda antenna
Müller C. et al, 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. 

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