Controlling Cavity Magnon Polariton Properties in Frequency and Time Domain

The cavity-magnon-polariton (CMP) is the quasi-particle of a hybrid system that connects photonic excitations to magnonic ones. With this connection, the CMP is an ideal candidate to build a bridge between the promising fields of quantum computing and magnonics. However, for the CMP to fulfill its role as an effective interface, a robust control of its underlying physical properties is imperative. Here, we show how the coupling strength can be manipulated within the experiment by a second continuous drive tone applied directly to the magnon system. Furthermore, switching into the time-domain, we demonstrate a coherent control over the different CMP modes based on ns-short pulses to both oscillating systems. At last, frequency fluctuations are investigated with the future goal of improving magnon coherence times

Controlled manipulation of atoms in Rydberg quantum states for application in experiments with antihydrogen

Antihydrogen atoms are synthesized via charge-exchange or three-body-recombination processes for stringent CPT symmetry and gravity tests on antimatter at CERN's Antiproton Decelerator. While the atoms are produced in a wide range of highly excited Rydberg states, experiments rely on ground state antihydrogen which are currently obtained only via spontaneous emission. Due to radiative lifetimes of the order of milliseconds, the decay toward ground state is slow. Magnetic neutral-atom traps allow to hold onto the initially formed antihydrogen. Here, ground state atoms are obtained via spontaneous decay along the trapping potential and spectroscopy measurements can be performed inside the trap. Typical traps capture maximum temperatures (for ground state atoms) of roughly 500mK. State-of-the-art formation temperatures, however, lie around 40K. Due to the large difference between formation and trappable temperature, currently best achieved trapping fractions of antihydrogen amount to only 0.01%. As an alternative, the formed anti-atoms can be extracted out of their formation region into a beam allowing for close to field-free measurements. At the given velocities, the Rydberg radiative decay toward ground state is too slow to establish a significant population of the ground state through spontaneous emission even within a few meter long beam path. In addition, an efficient beam formation would require antihydrogen in ground state close to the formation region. This work thus deals with the controlled manipulation of Rydberg atoms to stimulate their decay toward ground state. In order to address the initial distribution of Rydberg levels with deexcitation lasers, state-mixing can be achieved through pulsed electric fields that are added to the already present magnetic one in state-of-the-art formation traps. Another possibility lies in mixing the states with light in the THz and microwave frequency range. In both cases, visible lasers allow to very efficiently transfer the mixed population toward the ground state via intermediate strongly bound states that decay on a few nanosecond timescales. In a three-body-recombination reaction the capture of positrons into a bound state of antiprotons can be enhanced relying on stimulated radiative recombination. This approach can be combined with the developed techniques to deexcite bound levels. In trap experiments use-cases of deexcitation for cooling within a magnetic field gradient to enhance the trapping fraction of antihydrogen atoms exist. The second part of the thesis deals with the experimental implementation of the methods theoretically identified in part one. Deexcitation light sources are tested and a hydrogen Rydberg beamline for a proof-of-principle experiment is designed, built and commissioned. Different Rydberg beam production schemes are discussed and experimentally assessed. Rydberg state formation is observed within a microwave discharge plasma and a hydrogen 2s to Rydberg excitation laser is commissioned

Control of the Coupling Strength and the Linewidth of a Cavity-Magnon Polariton

The full coherent control of hybridized systems such as strongly coupled cavity photon-magnon states is a crucial step to enable future information processing technologies. Thus, it is particularly interesting to engineer deliberate control mechanisms such as the full control of the coupling strength as a measure for coherent information exchange. In this work, we employ cavity resonator spectroscopy to demonstrate the complete control of the coupling strength of hybridized cavity photon-magnon states. For this, we use two driving microwave inputs which can be tuned at will. Here, only the first input couples directly to the cavity resonator photons, whilst the second tone exclusively acts as a direct input for the magnons. For these inputs, both the relative phase $\phi$ and amplitude $\delta_0$ can be independently controlled. We demonstrate that for specific quadratures between both tones, we can increase the coupling strength, close the anticrossing gap, and enter a regime of level merging. At the transition, the total amplitude is enhanced by a factor of 1000 and we observe an additional linewidth decrease of $13\%$ at resonance due to level merging. Such control of the coupling, and hence linewidth, open up an avenue to enable or suppress an exchange of information and bridging the gap between quantum information and spintronics applications.Comment: 9 pages, 6 figure

Transmon Qubit in a Magnetic Field: Evolution of Coherence and Transition Frequency

We report on spectroscopic and time-domain measurements on a fixed-frequency concentric transmon qubit in an applied in-plane magnetic field to explore its limits of magnetic field compatibility. We demonstrate quantum coherence of the qubit up to field values of $B={40}\,\mathrm{mT}$, even without an optimized chip design or material combination of the qubit. The dephasing rate $\Gamma_\varphi$ is shown to be not affected by the magnetic field in a broad range of the qubit transition frequency. For the evolution of the qubit transition frequency, we find the unintended second junction created in the shadow angle evaporation process to be non-negligible and deduce an analytic formula for the field-dependent qubit energies. We discuss the relevant field-dependent loss channels, which can not be distinguished by our measurements, inviting further theoretical and experimental investigation. Using well-known and well-studied standard components of the superconducting quantum architecture, we are able to reach a field regime relevant for quantum sensing and hybrid applications of magnetic spins and spin systems.Comment: 9 pages, 8 figure

Probing the Tavis-Cummings level splitting with intermediate-scale superconducting circuits

We demonstrate the local control of up to eight two-level systems interacting strongly with a microwave cavity. Following calibration, the frequency of each individual two-level system (qubit) is tunable without influencing the others. Bringing the qubits one by one on resonance with the cavity, we observe the collective coupling strength of the qubit ensemble. The splitting scales up with the square root of the number of the qubits, which is the hallmark of the Tavis-Cummings model. The local control circuitry causes a bypass shunting the resonator, and a Fano interference in the microwave readout, whose contribution can be calibrated away to recover the pure cavity spectrum. The simulator's attainable size of dressed states with up to five qubits is limited by reduced signal visibility, and -- if uncalibrated -- by off-resonance shifts of sub-components. Our work demonstrates control and readout of quantum coherent mesoscopic multi-qubit system of intermediate scale under conditions of noise

Waveguide Bandgap Engineering with an Array of Superconducting Qubits

Waveguide quantum electrodynamics offers a wide range of possibilities to effectively engineer interactions between artificial atoms via a one-dimensional open waveguide. While these interactions have been experimentally studied in the few qubit limit, the collective properties of such systems for larger arrays of qubits in a metamaterial configuration has so far not been addressed. Here, we experimentally study a metamaterial made of eight superconducting transmon qubits with local frequency control coupled to the mode continuum of a waveguide. By consecutively tuning the qubits to a common resonance frequency we observe the formation of super- and subradiant states, as well as the emergence of a polaritonic bandgap. Making use of the qubits quantum nonlinearity, we demonstrate control over the latter by inducing a transparency window in the bandgap region of the ensemble. The circuit of this work extends experiments with one and two qubits towards a full-blown quantum metamaterial, thus paving the way for large-scale applications in superconducting waveguide quantum electrodynamics.Comment: 7 pages, 4 figure

Waveguide bandgap engineering with an array of superconducting qubits

Waveguide quantum electrodynamics offers a wide range of possibilities to effectively engineer interactions between artificial atoms via a one-dimensional open waveguide. While these interactions have been experimentally studied in the few qubit limit, the collective properties of such systems for larger arrays of qubits in a metamaterial configuration has so far not been addressed. Here, we experimentally study a metamaterial made of eight superconducting transmon qubits with local frequency control coupled to the mode continuum of a waveguide. By consecutively tuning the qubits to a common resonance frequency we observe the formation of super- and subradiant states, as well as the emergence of a polaritonic bandgap. Making use of the qubits quantum nonlinearity, we demonstrate control over the latter by inducing a transparency window in the bandgap region of the ensemble. The circuit of this work extends experiments with one and two qubits toward a full-blown quantum metamaterial, thus paving the way for large-scale applications in superconducting waveguide quantum electrodynamics

Transmon qubit in a magnetic field: Evolution of coherence and transition frequency

We report on spectroscopic and time-domain measurements on a fixed-frequency concentric transmon qubit in an applied in-plane magnetic field to explore its limits of magnetic field compatibility. We demonstrate quantum coherence of the qubit up to field values of $B=40\,\mathrm{mT}$, even without an optimized chip design or material combination of the qubit. The dephasing rate $\Gamma_\varphi$ is shown to be unaffected by the magnetic field in a broad range of the qubit transition frequency. For the evolution of the qubit transition frequency, we find the unintended second junction created in the shadow angle evaporation process to be non-negligible and deduce an analytic formula for the field-dependent qubit energies. We discuss the relevant field-dependent loss channels, which cannot be distinguished by our measurements, inviting further theoretical and experimental investigation. Using well-known and well-studied standard components of the superconducting quantum architecture, we are able to reach a field regime relevant for quantum sensing and hybrid applications of magnetic spins and spin systems

Magnons at low excitations: Observation of incoherent coupling to a bath of two-level-systems

Collective magnetic excitation modes, magnons, can be coherently coupled to microwave photons in the single excitation limit. This allows for access to quantum properties of magnons and opens up a range of applications in quantum information processing, with the intrinsic magnon linewidth representing the coherence time of a quantum resonator. Our measurement system consists of a yttrium iron garnet (YIG) sphere and a three-dimensional (3D) microwave cavity at temperatures and excitation powers typical for superconducting quantum circuit experiments. We perform spectroscopic measurements to determine the limiting factor of magnon coherence at these experimental conditions. Using the input-output formalism, we extract the magnon linewidth $\kappa_\mathrm{m}$. We attribute the limitations of the coherence time at lowest temperatures and excitation powers to incoherent losses into a bath of near-resonance two-level systems (TLSs), a generic loss mechanism known from superconducting circuits under these experimental conditions. We find that the TLSs saturate when increasing the excitation power from quantum excitation to multi-photon excitation and their contribution to the linewidth vanishes. At higher temperatures, the TLSs saturate thermally and the magnon linewidth decreases as well