In the past several decades, tremendous progress has been made in physics and engineering towards realize practical quantum computers. Several small-scale systems have already been demonstrated which utilize separate, off-the-shelf hardware components to directly control qubits. However, as higher-quality qubits continue to spur the development of larger-scale systems capable of performing complex computations, it is becoming increasingly important to consider how control architectures should be adapted to keep pace. This motivates the design of a scalable control architecture capable of dynamically orchestrating quantum programs on a large number of qubits while maintaining the stringent requirements imposed by qubit coherence times and operation in dilution refrigerators.

In this thesis, several contributions are made towards realizing a control architecture for large-scale quantum computing systems based on distributed Nitrogen-vacancy (NV) centers in diamond. A configurable design is presented which significantly increases system throughput by parallelizing execution of instructions performed on separate distributed nodes. A detailed analysis of the dependencies between instructions in quantum programs at an algorithmic level is also used to develop an heuristic function for evaluating the performance of different hardware designs. Furthermore, the demand for increased complexity of computations is addressed by a microarchitectural design which translates high-level quantum instructions into a set of signals used to configure special-purpose signal generators. Taken together, these microarchitectural components enable highly-efficient generation of signals for performing arbitrary single-qubit rotation gates, a key step towards achieving universal quantum computation.

Additionally, detailed evaluations of the control architecture enhancements are provided. For example, run time analysis is performed on a set of benchmark quantum algorithms to show the significant speedup capabilities and performance tradeoffs of the control architecture's parallel execution model. Moreover, the successful integration of the control architecture with the pre-existing signal generators is verified by executing parameterized microinstructions which configure the signal generators to produce amplitude-modulated, phase-coherent microwave and radio-frequency signals. Finally, the full system functionality is verified using several microprograms which each implement higher-level quantum instructions for single-qubit rotation gates. These demonstrations bridge the end-to-end connection between quantum algorithms and physically-realizable quantum gates in hardware.

Quantum computing is a promising means of satisfying the ever-increasing need for more and faster computations. While some specific applications can already benefit from quantum computing today, a large-scale general-purpose quantum computer is yet to be developed. Many different technologies are being explored to reach this goal, color centers in diamond being a promising candidate. This thesis focuses on a specific kind of color center: the nitrogen-vacancy center (NV center).
Building on top of existing work, a quantum instruction set architecture (QISA) for a quantum computer based on NV centers is designed. A compiler targeting this QISA is designed and implemented using the OpenQL framework. In this process alternative approaches are also explored and contributions useful outside the context of NV centers are made. The final compiler is able to take a quantum circuit operating on logical qubits and transform it into a sequence of QISA instructions operating on physical qubits. This functionality integrates seamlessly with the existing passes in OpenQL, allowing for further extension and the possibility to make use of future developments.
Additional functionality includes optimization, scheduling, and hardware-oriented features such as quantization of operands. Each step is easily configurable using a multitude of scripts and data files, and additional steps can be added in the future if needed.
The functionality of the developed compiler is demonstrated by compiling several circuits, some of which are validated by passing their output through a simulator.
The compiler configuration as used in these tests is able to transform logical circuit into simple physical circuits, but the compiler provides all the functionality to use more complicated logical qubits which incorporate quantum error correction.

The world of quantum computing is still quickly developing and growing as it is searching for the best technology to implement quantum bits. One of these technologies is a diamond-based quantum computer using nitrogen-vacancy (NV) centers. In order to sustain the mentioned growth, the limits of electronics as well as computer engineering need to be explored. Consequently, an Instruction Set Architecture (ISA) defining the functionality of the NV centers must be constructed that, in turn, controls the electronics needed to operate the qubits within the NV
center.


In this thesis, we created a simulator to verify the functionality of the ISA and, more importantly, utilize the simulator to explore the capabilities of a diamond-based quantum computer. The simulator mimics the interaction between components and qubits in a diamond-based quantum computer and maps these interactions onto quantum mechanical instructions, such as qubit rotations. The simulator is implemented by modeling the electronics based on literature and creating interaction between the components using an event scheduler. The event scheduler is part of the NetSquid framework in which this simulator is built. The framework has models implemented to represent qubits and perform operations on qubits as well. The qubit representation model represents the electron and carbon nuclei qubits. The quantum operations are used to perform quantum mechanical operations on the qubits.


The simulator is tested for physical accuracy using single diamond tests from literature, namely magnetic biasing, rabi cycle checking, and charge resonance checks. The simulator returned an accurate value for the Larmor frequency and Rabi frequency meaning that it was close to the expected value. For example, setting the Larmor frequency to 1.72GHz and using the magnetic biasing sequence to determine the Larmor frequency resulted in the same 1.72GHz$value.
After single diamond tests have been performed, the control of multiple diamonds and their qubits is tested using a quantum algorithm, the surface-7 code. The simulator accurately mimics the expected behavior of the noiseless execution of the surface-7 code. The results of the simulator follow the trend of the expected values.


The final product of this thesis is a simulator that can accurately describe the interaction between an ISA and the components of the diamond-based system-architecture in the noiseless case. The simulator provides a setup for a future simulator with noise sources implemented, resulting in a simulator that can be used to determine the limits of electronic properties or to predict the outcome of lab experiments.

In the field of quantum information theory, it is well-known that the purely quantum phenomenon called quantum entanglement can boost the capacity of a quantum channel, which is called the superadditivity of the capacity. Shor showed in his breakthrough paper on the equivalence of additivity conjectures that this superadditivity is equivalent to the subadditivity of the minimum output entropy of quantum channels, and Hastings gave a high-dimensional and probabilistic counterexample to the minimum output entropy additivity conjecture. Since then, researchers have endeavored to classify which quantum channels have subadditive minimum output entropy and to find deterministically constructed quantum channels with subadditive minimum output entropy. It is well-known that determining the minimum output entropy of an arbitrary quantum channel is a hard problem (in fact, it has been shown to be NP-complete), which in turn makes it difficult to determine whether a quantum channel has subadditive minimum output entropy.

In this thesis, we utilize the representation theory of compact (partition) quantum groups to construct so-called covariant Clebsch-Gordan quantum channels and analyze their minimum output entropy. We review the work of Brannan and Collins in the case of the free orthogonal quantum groups $O_N^+$, and introduce a similar analysis for the quantum permutation group $S_N^+$. Afterwards, we specialize to the subfamily of lowest-weight Clebsch-Gordan channels, and we show that, in the case where one embeds the fundamental representation of $O_N^+$ in the tensor product of two irreducible representations of $O_N^+$, the associated lowest-weight quantum channels have sufficient additional structure to analytically compute their minimum output entropy in terms of a recurrence relation.

Lastly, we present an analysis of certain numerical methods that can be utilized to bound the minimum output entropy from below and from above in low dimensions: we use epsilon-covers for lower bounds, and we use a modified version of the derivative-free optimization method called Particle Swarm Optimization over the unit sphere, hybridized with gradient descent, to find upper bounds. We show a proof-of-concept by applying the epsilon-cover to three $S_N^+$ channels with small input dimensions, but also note that the exponential scaling of epsilon-covers with the input dimension makes them intractable with higher input dimensions. We benchmark the Particle Swarm Optimization enriched with gradient-descent on the highest-weight Clebsch-Gordan channels for which it can be shown that the minimum output entropy is zero, and we see that the optimization technique performs adequately. We also apply the optimization scheme to the tensor product of two $S_N^+$-channels, but do not find any violation of the minimum output entropy additivity conjecture.

Throughout the work, the goal was to develop the physical layer of a quantum network stack. The layer of the network stack connected to the physical layer is the link layer. For entanglement-based quantum networks, we demonstrated the successful operation of a link layer and a physical layer. The physical layer’s entanglement generation procedure—implemented here using two NV center-based quantum network nodes—is abstracted by the link layer into a robust platform-independent service that can be utilized to run quantum networking applications. Other quantum network platforms using the approaches provided here (which are not unique to our diamond devices) will accelerate the development of large-scale and heterogeneous quantum networks.

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A vision is developed on how successful university-industry collaborations can be established in the field of quantum technology development. By looking at the stakeholders, purpose and type of collaboration, and influencing factors, a three-step model is developed to move from awareness to acceptance to collaboration. By overcoming the existing barriers, university-industry collaborations can be successfully realized.

High fidelity GHZ states among remote nodes is a precious commodity which can allow for non-local stabilizer measurements and thus pave the way for a modular fault-tolerant quantum computer. To this end, we extend the high fidelity intracavity gate introduced by Borregaard et al. (2015) to distributed paradigm, consisting of SnV-inspired atomic states in cavities connected by fibers. The adiabatic dynamics of this system can be solved efficiently using the effective operator formalism of Reiter and Sørensen (2012). We develop a Python framework that enables the analytical calculation of these effective dynamics reliably and swiftly, while being versatile and easily modifiable. The possibilities of this framework are showcased by obtaining results for a symmetric distributed setup and verifying its scalability. We present the ways that it can be optimized while taking into consideration experimentally inspired constraints, and proceed to optimize it for GHZ generation in color centers. These optimized gates are compared against an emission based protocol using the GHZ creation simulations of the Modicum protocol. As a byproduct of our investigation, we identify a specific set of Hamiltonians which, under certain conditions, can generate GHZ states with a single multi-qubit entangling gate.

Quantum networks allow multiple devices to exchange information encoded within quantum systems.
Such quantum networks use classical control messages to coordinate entanglement between nodes.
Third parties which can forge such control messages may interfere with the workings of quantum links, however:
They may either perform fraudulent requests for entanglement, destroying local quantum memory of nodes as a result, or interfere with the inner workings of protocols within the quantum stack targeting the availability of the link(s).
We, therefore, conclude that all link and physical layer control messages must be transmitted via authenticated channels.
Typically one uses a Message Authentication Code (MAC) to do so, which takes as input a message and outputs a tag which is transmitted alongside the message.
Additionally, it takes as input a unique number each time (nonce) to prevent replay-type attacks.
In this work, we first investigate the use of information-theoretic MACs combined with quantum key distribution (QKD) to authenticate control messages. We find that the fastest QKD solutions provide key material at a rate that is sufficient to not become a bottleneck in current quantum links.
Second, we survey multiple computationally secure MAC solutions and benchmarks to get an indication of their performance when authenticating short messages.
While not information-theoretically secure, their latency is generally speaking greater than or equal to that of information-theoretic solutions.
Finally, we augment the existing simulation of a single quantum link by Dahlberg et al. by inserting delays based on the performance of these MACs.
The performance of the link is evaluated using the mean throughput:
The rate at which successfully entangled pairs are delivered.
We find that the introduction of transmission time overhead, without any authentication, causes a noticeable decrease in throughput of the link.
When considering an authenticated channel that uses SipHash (a popular MAC) we find that throughput decreases even further, though less significantly.
Therefore, the overall decrease in throughput appears to not be detrimental to the working of the quantum link, which remains functional even when the classical channel is authenticated.

The first part of this thesis provides a mathematical description for bipartite quantum correlations, aiming to analyze the geometry of several sets of correlations. We explain why quantum entanglement can be used to simulate shared randomness: C_loc(Γ) ⊆ C_q^d(Γ) for a sufficiently large d. The known bound for this dimension d in the literature is d ≥ dim(C_loc(Γ))+1, but we improve this by showing that the inclusion is always true for d ≥ dim(C_loc(Γ)). For the proof of this bound, we show that the set C_private(Γ) of correlations using private randomness is connected, which allows the use of an improved version of Carathéodory’s Theorem. In the second part of this thesis, we define and analyze a see-saw method to determine the state and measurement operators that reconstruct both the correlation itself as its entanglement dimension, by solving consecutive semidefinite programs. One of the strengths of the algorithm is its generality: it applies to different dimensions, question sets, and answer sets. Some numerical experiments demonstrated that the method can indeed reconstruct quantum correlations, although some highly entangled correlations failed to be reconstructed due to the computationallimitations. The numerical experiments motivated several new theorems, for example the fact that every correlation with |A|= 1 or |B|= 1 has entanglement dimension 1, which means that it can be written as a private randomness correlation. The proof of this result is based on the earlier described improvement for the dimension d.

Topos theory and quantum mechanics are both known for having a logic that is different from ordinary logic. With this in mind, much work has been done on unifying these two fields. Loveridge, Dridi and Raussendorf apply this unification to measurement-based quantum computation [14], revealing links between computation, contextuality and the failure of the law of excluded middle in the topoi associated with each computation. We review their work, fill in gaps, follow their research suggestion and have some criticism. Our main original finding is a formula, in the formal language of the topos associated with a computation, that expresses that the computation is deterministic.

The quantum internet will allow for communication via qubits, enabling for improved clock synchronization, blind quantum computing and quantum key distribution. Key components of such a quantum network are quantum repeaters, which help to overcome the exponential loss of photons in optical fibers. In this thesis we focus on one type of quantum repeater based on atomic ensembles. In particular, we build a versatile and elaborate simulation model based on a discrete event simulator for quantum networks, capable of simulating quantum repeater chains of arbitrary length with a vast range of tunable simulation parameters. We validate this model by comparing it to an analytical one and investigate the effects of additional sources of noise that cannot be taken into account in the analytical model. Furthermore, we give a detailed theoretical overview and performance comparison of two types of quantum memories based on atomic ensembles: atomic frequency comb and electromagnetically induced transparency. Finally, we present two integer linear programming formulations for determining the optimal positioning of quantum repeaters in two spatial dimensions and demonstrate their use on a European-scale network.

The testing of quantum applications can be approached from three perspectives. The first one concerns the certification of the accuracy of the quantum device where the application is run. The second one has to do with the classical verification of the result output by the application. Yet a third one addresses the problem from the software engineering perspective. As new quantum applications that run in Noisy Intermediate-Scale Quantum devices are developed, there is an increasing need for tools that can help to find bugs and verify that these applications work as expected. In this thesis we design and develop such a tool. We introduce QuTAF, a test automation framework for quantum applications that is based in the robot framework. To the best of our knowledge, this is the first test automation framework software developed and used for testing quantum applications in a real quantum node. We prove that our QuTAF is capable of detecting minor deficiencies in current state-of-the-art quantum hardware, by running tests for small quantum applications
executed in a networked quantum node. We also simulate and test two different
failure scenarios to validate the capabilities of our QuTAF. We simulate quantum
devices affected by depolarizing and dephasing noise, and find that our QuTAF is able to detect errors introduced by an increase in the depolarization probability, but is otherwise insensitive to the errors produced by the dephasing of quantum states. We also simulate bugs present in the quantum programs, and prove that our QuTAF is able to correctly identify these as failing test cases.

A network of devices capable of transmitting quantum information called a quantum network has promising applications with vast benefits. One of the most near term achievable application is the quantum key distribution. It could be used for sharing a secret key between two end-users through any insecure authenticated communication channel. Other than sharing a secret key, a quantum network can be used for connecting quantum computers. Quantum computers have the potential to solve computational problems that their classical counterparts would require significantly more time to do so. By connecting such devices, distributed computation problems such as leader election or distributed consensus between nodes can be solved securely.

For such applications to be put into practice on a large scale, there are open practical questions still to be answered. In a quantum network, a data qubit containing quantum information is transmitted by an operation called quantum teleportation. For it, two nodes need to share entanglement. Quantum teleportation then ensures the safe transmission of a qubit by using the shared entanglement and the exchange of two classical bits. This operation, however, consumes entanglement between the nodes, changing the network topology with each served request.

In quantum networks, certain nodes share entanglement between each other. Routing in quantum networks entails determining which of these shared entanglements are used to transmit quantum information. As this proves to be a difficult task, we study quantum networks and in particular consider the problem of routing entanglement in quantum networks.

The average latency for routing entanglement in a quantum network has been studied so far using a distributed routing approach. Thus, we present numerical simulation results of centralised routing for the average latency of demands in two existing entanglement generation models. In the first, shared entanglement between nodes is created on-demand. In the second, certain nodes pre-share entanglement before the demand comes. Our results show the intuition that using a centralised routing approach in the on-demand model results in drastically more average latency than in the model with pre-shared entanglement.

Since some nodes might not be informed about the change in topology, we study the effect of the propagation of information about the network topology to nodes in the network. Our observations based on simulation results show that the average latency is significantly higher if the information about the topology is not propagated well in the network. As propagating information to all nodes in a network would be a significant load for the network, we consider information propagation within a radius and still observe a considerable decrease in average latency.

At last, we present a technique called link prediction which can be performed by any node without the need for information propagation and still achieve a considerable decrease in average latency. It can be effectively used to predict the change of topology in a quantum network by using knowledge about the network topology for a certain future point in time and previous knowledge about the network traffic.

This document describes the design process the prototype of a measurement light bulb which is able to project spherical harmonics of orders 0-1-2. Photographs of these projections can be combined to represent many light distributions. The measurement light bulb can be used in the field of research focused on the computation of the illumination impact of lighting, more specifically, simulating different light sources. Our prototype consists of a laser beam rotating over two axes, which allows the device to project onto a sphere around itself. The device can be controlled wirelessly using Bluetooth. The lamp can project spherical harmonics in a resolution of 4 degree and 256 monochrome light levels. The time one projection takes is about one second, which allows for quick measurements. The dimensions and the weight of the lamp are such that it is portable. At this stage, the prototype can only operate in a dark environment, due to the use of a low powered laser. In this document, the focus will be on the design of control, communication and the PCB.

Quantum computation is becoming an increasingly interesting field, especially with the rise of real quantum computers. However, current quantum processors contain a few tens of error-prone qubits and the realization of large-scale quantum computers is still very challenging. Therefore, quantum computer simulators are particularly suitable for testing and analysing quantum algorithms without having a real quantum computer at one's disposal. In this thesis, different quantum algorithms such as Grover's and Shor's algorithm as well as key quantum routines such as the Quantum Fourier Transform (QFT) and a quantum adder/subtractor are described and analysed (optimal number of iterations, time complexity). Some of them have been implemented for an arbitrary number of qubits and have been simulated using two different quantum simulators, the QX simulator developed at QuTech and the Liquid simulator from Microsoft. In addition, how errors affect the success rate of the algorithms has been investigated.

Building a working quantum computer brings many challenges to overcome. For quanum processors, promising qubits seem to be NV-centres in diamond. Such processor is divided into processing units (PUs), each comprising an NV-centre with several carbon spin qubits, and they are interconnected with each other by optical links. The qubits within a PU can be modelled as nodes in a star graph and the optical connections as connections between the centres of the star graphs, such that the centres with their interconnections form a complete graph. We call the overall graph a fully connected star graph. In order to execute a quantum circuit, multiple pairs of qubits should be brought together in PUs at different times, which requires planning. Sorting the qubits is done via swaps. A swap within a PU can be performed easily, however, swapping between different PUs is done by teleportation, which is much more costly. Therefore, we neglect the costs of swaps within PUs. Parallellisation of swaps between PUs is done in a step. The less steps are used, the faster the quantum circuit is executed. In practice, as we are interested in fast computations, we often want to minimize the number of steps. The following problem is addressed. Given a current distribution of the qubits over the nodes in the fully connected star graph, and given a wanted, permuted distribution, minimize the number of swaps and/or steps to route the qubits from the current to the wanted distribution. At the moment, no efficient algorithm exists to sort the qubits especially in fully connected star graphs. Efficient solutions are presented in the form of algorithms. It is shown that qubit moves in fully connected star graphs can be scheduled efficiently, such that the number of moves is minimized. Guide lines are given to minimize the execution time too.