Influence of coherence time drift on the secret key rate

Quantum communication provides a plethora of new possibilities compared to the realm of classical communication. Since the channels used are noisy, losses are unavoidable, and quantum repeaters are needed to transmit a signal over longer distances to overcome these exponential losses. To increase the performance of these repeaters, cutoff times...

Security and Practical Models for Quantum Key Distribution

This report reviews two quantum key distribution (QKD) protocols: the BB84 protocol and the measurement device independent (MDI) QKD protocol. The goal of this report is to recreate the security proof of the BB84 protocol, to generate the secret key rate for a practical application of the BB84 protocol, both with and without decoy states, and to...

Detecting and controlling spin pairs near NV centres in diamond

The development of quantum computers is a monumental challenge for modern physics. One proposed pathway toward a fully scalable and fault-tolerant quantum computer involves the development of a quantum network. Such a network would have applications ranging from distributed quantum computation to fundamentally secure quantum communication. The...

Towards a fault-tolerant one-way quantum repeater

Quantum communication can enable new features that are provably impossible with classical communication alone. However, the optical fibers used to send the quantum information are inherently lossy. To overcome the exponential losses over distance so-called quantum repeaters are needed to amplify the signal. As opposed to memory-based approaches,...

Synchronized quantum network emulator using discrete event simulation

Uncomputability in Information Theory

We present a powerful approach for learning about uncomputability and undecidability in informationtheory. Our approach is to use automata from automata theory that have undecidable properties toconstruct channels for which an information-theoretic quantity is uncomputable or undecidable. Wedemonstrate this approach by showing that, for channels...

Inequalities and Quantum Entanglement: From the Cauchy-Schwarz Inequality to Non-linear Entanglement Witnesses

In this thesis, two topics are studied: mathematical inequalities and non-linear quantum entanglement witnesses. First, various inequalities, like the Cauchy-Schwarz inequality (on finite dimensional vector spaces) and Jensen's inequality, along with their extensions and generalisations, are proved and discussed. The intimate relationship...

Building blocks of quantum repeater networks

Local multi-qubit Clifford equivalence of graph states

In the range of applications opened by quantum technology, often a highly entangled source state is needed as an input for a protocol (target state). The easiest example of such an application is QKD, other examples are quantum secret sharing and measurement based quantum computing. Generating a non-local entangled state is however not trivial....

Entropy production in quantum is different

Currently, 'time' does not play any essential role in quantum information theory. In this sense, quantum information theory is underdeveloped similarly to how quantum physics was underdeveloped before Erwin Schrödinger introduced his famous equation for the evolution of a quantum wave function. In this review article, we cope with the problem...

Optimizing quantum entanglement distillation

Quantum entanglement is a physical resource that is essential for many quantum information processing tasks, such as quantum communication and quantum computing. Although entanglement is essential for practical implementations in those fields, it is hard to create and transmit entanglement reliably. External factors introduce noise which may...

A universal test for gravitational decoherence

Quantum mechanics and the theory of gravity are presently not compatible. A particular question is whether gravity causes decoherence. Several models for gravitational decoherence have been proposed, not all of which can be described quantum mechanically. Since quantum mechanics may need to be modified, one may question the use of quantum...

Contextuality without nonlocality in a superconducting quantum system

Classical realism demands that system properties exist independently of whether they are measured, while noncontextuality demands that the results of measurements do not depend on what other measurements are performed in conjunction with them. The Bell–Kochen–Specker theorem states that noncontextual realism cannot reproduce the measurement...

Loophole-free Bell test using electron spins in diamond: Second experiment and additional analysis

The recently reported violation of a Bell inequality using entangled electronic spins in diamonds (Hensen et al., Nature 526, 682–686) provided the first loophole-free evidence against local-realist theories of nature. Here we report on data from a second Bell experiment using the same experimental<br/>setup with minor modifications. We find a...