Christian Kraglund Andersen

Strong Coupling Cavity QED with Gate-Defined Double Quantum Dots Enabled by a High Impedance Resonator

The strong coupling limit of cavity quantum electrodynamics (QED) implies the capability of a matter-like quantum system to coherently transform an individual excitation into a single photon within a resonant structure. This not only enables essential processes required for quantum information processing but also allows for fundamental studies of matter-light interaction. In this work we demonstrate strong coupling between the charge degree of freedom in a gate-detuned GaAs double quantum dot (DQD) and a frequency-tunable high impedance resonator realized using an array of superconducting quantum interference devices (SQUIDs). In the resonant regime, we resolve the vacuum Rabi mode splitting of size

Ultrastrong coupling dynamics with a transmon qubit

2g/2\pi = 238

Detection of Weak Microwave Fields with an Underdamped Josephson Junction

MHz at a resonator linewidth

Resonator reset in circuit QED by optimal control for large open quantum systems

\kappa/2\pi = 12

Quantum annealing with all-to-all connected nonlinear oscillators

MHz and a DQD charge qubit dephasing rate of

Effective description of tunneling in a time-dependent potential with applications to voltage switching in Josephson junctions

\gamma_2/2\pi = 80

Multi-frequency modes in superconducting resonators: Bridging frequency gaps in off-resonant couplings

MHz extracted independently from microwave spectroscopy in the dispersive regime. Our measurements indicate a viable path towards using circuit based cavity QED for quantum information processing in semiconductor nano-structures.

Quantized resonator field coupled to a current-biased Josephson junction in circuit QED

The interaction of light and matter is often described by the exchange of single excitations. When the coupling strength is a significant fraction of the system frequencies, the number of excitations are no longer preserved and that simple picture breaks down. This regime is known as the ultrastrong coupling regime and is characterized by non-trivial light-matter eigenstates and complex dynamics. In this work, we propose to use a an array Josephson junctions to increase the impedance of the light mode enabling ultrastrong coupling to a transmon qubit. We show that the resulting dynamics can be generated and probed by taking advantage of the multi-mode structure of the junction array. This proposal relies on the frequency tunability of the transmon and, crucially, on the use of a low frequency mode of the array, which allows for non-adiabatic changes of the ground state.

Closing a quantum feedback loop inside a cryostat: Autonomous state preparation and long-time memory of a superconducting qubit

We have constructed a microwave detector based on the voltage switching of an underdamped Josephson junction, that is positioned at a current antinode of a {\lambda}/4 coplanar waveguide resonator. By measuring the switching current and the transmission through a waveguide capacitively coupled to the resonator at different drive frequencies and temperatures we are able to fully characterize the system and assess its detection efficiency and sensitivity. Testing the detector by applying a classical microwave field with the strength of a single photon yielded a sensitivity parameter of 0.5 in qualitative agreement with theoretical calculations.

Low-Latency Digital Signal Processing for Feedback and Feedforward in Quantum Computing and Communication

We study an implementation of the open GRAPE (Gradient Ascent Pulse Engineering) algorithm well suited for large open quantum systems. While typical implementations of optimal control algorithms for open quantum systems rely on explicit matrix exponential calculations, our implementation avoids these operations leading to a polynomial speed-up of the open GRAPE algorithm in cases of interest. This speed-up, as well as the reduced memory requirements of our implementation, are illustrated by comparison to a standard implementation of open GRAPE. As a practical example, we apply this open-system optimization method to active reset of a readout resonator in circuit QED. In this problem, the shape of a microwave pulse is optimized such as to empty the cavity from measurement photons as fast as possible. Using our open GRAPE implementation, we obtain pulse shapes leading to a reset time over four times faster than passive reset.