Decoherence in microscopic two-level systems and in analog quantum simulators

Quantum simulation is a promising field for first applications of quantum computers. In the first part of this work we study effects of decoherence in analog quantum simulators using many-body methods for systems out of equilibrium. In the second part we study the interplay between two important sources of decoherence, namely quasiparticle excitations in superconductors and low-energy excitations in amorphous layers. The latter are described within the phenomenological tunneling-system model

Europe's migration 'partnerships' with Africa need a new direction in 2017

2016 was the deadliest year for migrants crossing the Mediterranean; more than 4,800 people died on their way to Europe, despite the fact that fewer attempted the journey than in previous years. While European migration policies tend to be fragmented, there has been a shift to stepping up a "Fortress Europe" approach. But closing borders won't work in the long run. Curbing illegal migration and sealing borders will not stop migration from occurring, but rather improve the business of traffickers and make migrants' journeys more dangerous. A year ago, more than 60 heads of states and government met in Malta for the first summit between the European Union (EU) and the African Union (AU) solely dedicated to the topic of migration. Contrary to initial hopes, the Action Plan and Political Declaration that were agreed upon did not make a significant difference to previous EU policies on migration, which tend to be broad and vague. The EU approach does not take account of the reality of migration, as we have recently pointed out. Nine out of ten international refugees remain in the Global South. And the main burden of the refugee crisis is not in Europe, but among the poorest countries in the world

Die Migrationspolitik der EU in Afrika braucht einen Richtungswechsel

Entgegen der weitverbreiteten Annahme eines "Exodus" aus Afrika bleibt die Mehrheit der afrikanischen Flüchtlinge und Migranten bislang auf ihrem Heimatkontinent. Doch in Zukunft wird der Migrationsdruck aufgrund des großen Bevölkerungswachstums steigen. Um die damit verbundenen Herausforderungen zu bewältigen und Potenziale zu nutzen, bedarf es eines Richtungswechsels der europäischen Migrationspolitik in Afrika

Decoherence spectroscopy with individual two-level tunneling defects

Recent progress with microfabricated quantum devices has revealed that an ubiquitous source of noise originates in tunneling material defects that give rise to a sparse bath of parasitic two-level systems (TLSs). For superconducting qubits, TLSs residing on electrode surfaces and in tunnel junctions account for a major part of decoherence and thus pose a serious roadblock to the realization of solid-state quantum processors. Here, we utilize a superconducting qubit to explore the quantum state evolution of coherently operated TLSs in order to shed new light on their individual properties and environmental interactions. We identify a frequency-dependence of TLS energy relaxation rates that can be explained by a coupling to phononic modes rather than by anticipated mutual TLS interactions. Most investigated TLSs are found to be free of pure dephasing at their energy degeneracy points, around which their Ramsey and spin-echo dephasing rates scale linearly and quadratically with asymmetry energy, respectively. We provide an explanation based on the standard tunneling model, and identify interaction with incoherent low-frequency (thermal) TLSs as the major mechanism of the pure dephasing in coherent high-frequency TLS.Comment: 6 pages, 3 figures, supplementary material availabl

Quantum simulation of the spin-boson model with a microwave circuit

We consider superconducting circuits for the purpose of simulating the spin-boson model. The spin-boson model consists of a single two-level system coupled to bosonic modes. In most cases, the model is considered in a limit where the bosonic modes are sufficiently dense to form a continuous spectral bath. A very well known case is the ohmic bath, where the density of states grows linearly with the frequency. In the limit of weak coupling or large temperature, this problem can be solved numerically. If the coupling is strong, the bosonic modes can become sufficiently excited to make a classical simulation impossible. Here, we discuss how a quantum simulation of this problem can be performed by coupling a superconducting qubit to a set of microwave resonators. We demonstrate a possible implementation of a continuous spectral bath with individual bath resonators coupling strongly to the qubit. Applying a microwave drive scheme potentially allows us to access the strong-coupling regime of the spin-boson model. We discuss how the resulting spin relaxation dynamics with different initialization conditions can be probed by standard qubit-readout techniques from circuit quantum electrodynamics.Comment: 23 pages, 10 figure