Ellipsometry measurements were done on thin amorphous MoGe layers with a Ge content of 20%. Capping the MoGe layer with at least 20 nm of sputtered SiO2 prevents oxidation of the layer. We find...Show moreEllipsometry measurements were done on thin amorphous MoGe layers with a Ge content of 20%. Capping the MoGe layer with at least 20 nm of sputtered SiO2 prevents oxidation of the layer. We find that the refractive index of the sputtered oxide for wavelengths longer than 680 nm is given by n(l) = 1.44606 + 60, 682/l2. An estimate of the optical properties of MoGe as a function of wavelength is made using an iterative procedure applied to the data of a 300 nm thick MoGe film capped with 60 nm of sputtered oxide. A comparison with ellipsometry data for thin film MoGe samples show that the data are inconsistent with the model despite the fact that multiple sources for systematic errors were eliminated.Show less
An optimal design of a superconducting single photon detector depends on the optical properties of the superconducting material. Here we describe transmission and reflection measurements on thin...Show moreAn optimal design of a superconducting single photon detector depends on the optical properties of the superconducting material. Here we describe transmission and reflection measurements on thin-film amorphous MoGe deposited on fused silica (SiO2) substrates and ellipsometry of thin amorphous MoGe films on Si(100) substrates. From the ellipsometry we find a simple relation between the dielectric constant and the thickness d for film thicknesses between 5 and 30 nm. This simple relation entails a surprisingly strong dependence of the dielectric constant on thickness found in films that are much thicker than the electron mean free path (0.5 nm) derived from resistivity measurements in literature. Reflection and transmission measurements of thin-film MoGe on SiO2 show similar thickness dependence. However, the roughness of the SiO2 substrate has significant influence on the measurements for the thinner films (<15 nm).Show less
The photonic bandstructure of triangular photonic crystal can contain Dirac cones if the structure has C3V symmetry. By breaking the symmetry in a continuous way, a stopgap is created. We present a...Show moreThe photonic bandstructure of triangular photonic crystal can contain Dirac cones if the structure has C3V symmetry. By breaking the symmetry in a continuous way, a stopgap is created. We present a systematic method to optimise this gap guided by intuition. As a result, we create a relatively large complete photonic bandgap (CPBG) of 17.2% for a disconnected structure and 15.6% for a connected (self-supporting) structure. The continuous breaking of the symmetry gives rise to non-trivial topology with protected edge modes. These edge modes are present at the interface of two topologically distinct materials with a topological Chern number C = 1 2 . The topological edge modes require C = 1 and their properties and interactions can be understood in therms of pseudo-spin. We illustrate this by visualising the pseudo-spin in our structure.Show less
In order to help reduce energy consumption in buildings, porous films of the thermochromic material vanadium dioxide (VO2) have been studied for smart window applications, by simulations and...Show moreIn order to help reduce energy consumption in buildings, porous films of the thermochromic material vanadium dioxide (VO2) have been studied for smart window applications, by simulations and experiments. It was found that, for films with pores of less than 100 nm in size, an effective medium approximation, such as the Maxwell-Garnett formula, can be used to approximate the the porous film by a homogeneous film with an effective dielectric permittivity. Using the transfer matrix method, the film color, luminous transmission and solar modulation of porous films on a glass substrate have been studied. It was found that the transmission color of the film can be modified by varying the thickness and porosity of the film, but the color will always be yellowish. The solar modulation varies greatly with film thickness and porosity, and has a maximum of 0.205 at a porosity of 0.8 and film thickness of 360 nm. The luminous transmission depends mostly on the total amount of VO2 in the film. A trade-off has to be made between a high luminous transmission, and a high solar modulation. In general, the best optical performance can be achieved for highly porous films between 100 and 300 nm thick. A porous VO2 film on a glass substrate has been characterized with ellipsometry. It was found that the film thickness is approximately 40 nm, the film consists of a single layer of grains with a feature size of <100 nm. The obtained optical constants of the film are in good agreement with previous research in literature, except at wavelengths above 1000 nm in the high temperature state, indicating less effective thermochromic properties. Further research is needed to determine the causes.Show less
It has been a long term goal of physicists to control macroscopic quantum superposition states - cat states - since these connect to a number of open fundamental questions in physics: the...Show moreIt has been a long term goal of physicists to control macroscopic quantum superposition states - cat states - since these connect to a number of open fundamental questions in physics: the transition from the quantum to the classical world, the quantum measurement problem, and the area between quantum physics and theory of general relativity. Optomechanics has been identified as a method for generating cat states, however, this is yet to be achieved. The scientific community has developed increasingly improved optomechanical systems. About a decade ago, a promising optomechanical system has been demonstrated that consists of a high-stress silicon nitride membrane in the middle of a Fabry-Pérot cavity. This project concerns the development of a membrane-in-the-middle device for our lab. Our main focus lies on developing an understanding about the connection between system design and optomechanical performance. In addition, we demonstrate optomechanics for our device, and show that the initial optomechanical parameters are good. The availability of clearly defined methods for improving upon the current system parameters implies that we are moving in the right direction towards quantum optomechanical experiments.Show less
Quantum computing promises to deliver exponential speed-up over classical machines in solving specific problems. However, quantum information is susceptible to decoherence and errors, and fault...Show moreQuantum computing promises to deliver exponential speed-up over classical machines in solving specific problems. However, quantum information is susceptible to decoherence and errors, and fault-tolerant quantum computing (FTQC) is the only realistic approach. In FTQC, operations are performed on logical qubits which are encoded in physical qubits such that errors are correctable or trackable. Specifically, in the repetition code for quantum error correction (QEC), qubits are encoded in Greenberger-Horne-Zeilinger (GHZ)-type states to be protected from bit-flip or phase-flip errors. It is important not to leave the protected subspace at any time to meet the basic requirement for FTQC. Previous demonstrations of the repetition code in various physical systems have circumvented this requirement, detecting errors at the cost of decoding the logical qubit. Using five superconducting qubits in circuit quantum electrodynamics, we demonstrate quantum bit-flip error detection at the logical-qubit level by stabiliser measurements for the first time. These stabilisers are assessed by their ability to generate GHZ-type entanglement: projecting a maximal superposition state into the subspaces being stabilised, while maintaining the coherence within each. To further characterise the error detection, we intentionally apply errors on all qubits and assess the fidelities both in the encoded subspace and at the logical-qubit level. Although current fidelities of the stabiliser measurements preclude improvements by error detection over idling, this demonstration is a critical step towards larger codes based on stabiliser measurements in the paradigm of FTQC.Show less
