1,721,122 research outputs found
Quantumness in optomechanics
Full text linkCavity optomechanics has become an established and equally promising branch in quantum optics. Thanks to the interaction between matter and electromagnetic radiation, it has proved to be an optimal platform for a range of scopes, from weak force sensing to the study of non-classicality of mechanical motion. Besides, the capability to isolate genuine quantum features of the interaction represents a test ground to address many important questions regarding decoherence, quantum-to-classical transitions and the interface between quantum mechanics and gravity.
The first part of the research embedded in this thesis is addressed towards the clear identification and characterisation of quantum features in optomechanics. The main model we will refer to is a deformable Fabry-P\'erot cavity where one of the two mirrors moves under the radiation pressure of light. After having properly assessed the quantum peculiarities of the system, and also having revised some intakes from past literature, we will focus on the study of mechanical non-linearities, as they have been proved to be a key resource to bring out and enhance quantum properties. These investigations provide the basis to eventually propose a method to deterministically prepare and measure macroscopic quantum superposition states of the movable mirror. Such massive quantum states play a key role to inspect the foundations of physics, e.g. to test the collapse of the wave function and phenomenological models of quantum gravity, as well as to develop new enhanced quantum technologies.Open Acces
Optimal control of nonclassical light in optomechanics
No full textNonclassical light stands out for its ability to improve quantum metrology, cryptography, and optical
quantum computation. Yet, generation of certain types of nonclassical light, especially Fock states,
remains a considerable challenge. Optomechanical systems, characterized by a nonlinear interac-
tion between light and a mechanical oscillator, have emerged as a promising avenue to address this
challenge. However, these systems also introduce challenges due to the intricate interplay between
mechanical and optical components, and the interference of cavity dissipation. This thesis introduces
innovative driving strategies that address the two limitations.
First, a driving scheme is proposed to generate separable optomechanical states, while maintaining the
nonlinear nature of the system dynamics. This is illustrated with driving profiles leading to an optical
two-photon Fock state and its superpositions with the vacuum. While the study mainly focuses on an
ideal situation where the system dissipations are neglected, I also show that the fidelity of the final
states remain high even when perturbed by the environment.
Then, I consider a more general case where the optical dissipation is no longer negligible. A novel
driving scheme is crafted to suppress destructive interference of Fock state amplitudes of more than a
single photon. As a result, strong photon-blockade effect can be realized in a time orders of magnitude
shorter than in existing schemes that achieve photon blockade in the steady state. Moreover, the driving
scheme provides the flexibility to adjust the duration of photon blockade, as well as to increase the
single-photon occupation at the cost of weaker photon blockade.
Towards the end, the existing study is extended to include the dynamics of external components. In
the study, I consider a simple case where leaked light is directed into a resonant cavity. The range
of system parameters where the output cavity also experiences pronounced photon blockade is then
identified.Open Acces
Detection and typicality of bound entangled states
No full textWe derive an explicit analytic estimate for the entanglement of a large class of bipartite quantum states, which extends into bound entanglement regions. This is done by using an efficiently computable concurrence lower bound, which is further employed to numerically construct a volume of 3 x 3 bound entangled states.
Complete positivity of non-Markovian quantum dynamics
Full text linkThe hierarchy equations of motion provide an elegant formalism for the description of non-Markovian quantum dynamics, which has successfully been applied to a broad variety of physical problems ranging from light-harvesting complexes to resonant tunnelling junctions. When these equations are derived from microscopic models of system and environment, then they typically expand to an infinite series of coupled differential equations. The truncation of this series is accompanied by an approximation of the dynamics and can give rise to a violation of complete positivity.
The goal of this thesis is to investigate under what conditions a set of hierarchy equations of motion induces completely positive dynamics. Such conditions are in particular crucial for a phenomenological understanding of the equations of motion. Several approaches to this problem are proposed and compared for classical systems before a generalisation to quantum mechanical problems is carried out. The main result is a concise set of operator inequalities that are sufficient for complete positivity and that can be implemented efficiently in terms of a semi-definite program. The applicability of this technique is demonstrated by various explicit examples.
For truncated hierarchy equations for which a violation of complete positivity is inevitable the framework is extended such that the degree of this violation can be estimated and the validity of the truncation can be gauged. In particular in the presence of initial system-environment correlations a dynamical process can also be considered physically reasonable, when the time evolution is not completely positive and it is shown how hierarchy equations that describe such scenarios can be identified. Eventually, the framework is altered such that it gives rise to conditions on hierarchy equations that are sufficient not only for completely positive but even Markovian dynamics.Open Acces
Optimal implementation of quantum channels on noisy hardware
No full textExperimental progress in the development of quantum computers has now led to the establishment of controllable devices at scales approaching those necessary for quantum advantage. As such, a considerable amount of interest has grown around developing practical strategies for implementing useful algorithms on these platforms, a task which is made difficult by the considerable noise rates of current implementations. This thesis expands upon this work, developing strategies for implementing high-fidelity gates on noisy superconducting qubits. Throughout, the practical utility of the strategies is prioritised, with their viability verified through implementations on real hardware.
First, a variational quantum gate optimisation (VQGO) routine is developed that utilises quantum control to maximise an implemented gate's fidelity. To do this, a novel fidelity measure, the fidelity is developed, analysed and established to be more practical than state-of-the-art fidelity measures for this purpose. This practicality is demonstrated experimentally through the successful optimisation of a three qubit gate.
The VQGO scheme is then extended by developing a pulse-level control scheme that takes advantage of the natural entangling operations in the experimental device. This is highly successful, yielding very high fidelity two and three qubit gates, although it is found that parameter drift precludes the scheme from realising a more complex Floquet-engineered gate.
Finally, the analysis of the physics underpinning the experimental platform is extended through the development of a characterisation and calibration protocol aimed at facilitating analogue quantum simulations on the device. The viability of the platform for such a purpose is experimentally assessed against a set of necessary criteria, during which two unexpected noise sources are identified and characterised. The characterisation and calibration protocols are shown to be practical and useful, with the viability of the platform for analogue quantum simulation being contingent on the development of strategies for overcoming these noise sources.Open Acces
Optimally driven quantum systems
Full text linkPeriodically driven quantum systems offer an exceptional platform for quantum simulations due to the possibility to approximate their dynamics in terms of time-independent effective Hamiltonians. This, together with the recent experimental advances, has situated driven systems at center stage of engineered quantum-mechanical devices.
The aim of this thesis is to develop theoretical methods in order to design optimal quantum simulations with driven systems. By applying the derived tools to experimentally relevant models, the applicability and significance of the methods are furthermore demonstrated.
First, we introduce a method to derive accurate effective Hamiltonians by merging two seemingly unrelated tools: Floquet theory and flow equations. With this, the required analytical identification of the effective Hamiltonian in terms of the system's parameters is achieved.
Second, we identify structural properties that determine the accessible effective dynamics of a system of particles on shaken optical lattices, which is arguably one of the most remarkable systems for many-body quantum simulations. In particular, we identify fundamental symmetries of the underlying lattice geometry that determine the emergence of new tunneling processes.
Third, we develop an optimal control scheme to design polychromatic driving protocols that optimally simulate specifically targeted dynamics. We apply this scheme to demonstrate an optimal realization of Raman transitions with a Lambda system, a building block in many quantum simulations. Then, we employ it to implement a topological Chern insulator through suitably engineering the geometry-dependent tunneling of particles on a shaken hexagonal lattice. Hereby, a realistic route to experimentally test strongly-correlated topological phases of matter is provided.
By determining structural properties of driven systems and suitable driving protocols, the methods described in this thesis open substantial possibilities for the development of optimal quantum simulations and, ultimately, reliable quantum technologies.Open Acces
Optimal control of dissipative quantum dynamics
Full text linkIn this thesis, we develop a perturbative approximation for the solution of Lindblad master equations with time-dependent generators that satisfies the fundamental property of complete positivity, as essential for quantum simulations and optimal control of open quantum systems. By probing our method to several explicit examples we show that ensuring this property not only improves the accuracy of the perturbative approximation substantially, but also it permits to read off the effective dissipative processes . Subsequently, we design optimal entangling quantum gates for trapped ions mediated by a bus mode which is subject to decoherence. We show that suitably designed polychromatic control pulses, help to suppress the qubit-phonon entanglement substantially while maintaining the mediated interaction. This leads to a considerable reduction in the gate infidelity, in particular, for multi-qubit gates this yields a significant improvement in the gate performance.Open Acces
Quantum coherence in trapped ions
Full text linkThe techniques of ion trapping have been indispensable for the study of single particles and have greatly increased our understanding of the physical world. With a high degree of control over single quantum systems made possible by recent technical developments, trapped ion researchers are now looking to observe increasingly complex quantum dynamics. There is nothing fundamentally limiting the size such systems can scale to, if the remaining engineering challenges can be overcome, and there is consequently enormous potential for our understanding of physical processes on the boundary between simple quantum and large scale classical effects. The transition between quantum and classical dynamics is characterised by decoherence, which reduces the purity of quantum states as they couple to the environment. Investigation into the mechanisms of decoherence requires the development of novel tools for its classification, which motivates the experiments presented in this work. These show that multi-level coherence in the motional state of a trapped ion can be verified from interference patterns which extend the Ramsey technique. The metric uses simple operations and is shown not to be able to produce false positives, making it of interest in the study of coherence in noisy intermediate scale systems. The motional state of a trapped ion cannot be directly measured, and the scheme shows it is possible to extract information about a quantum system coupled to the measurement basis. Coherence between quantum states can enhance information processing and sensing, which is driving the development of techniques for preserving coherence in the presence of noise. This motivates the project begun in this thesis, which is to design and implement control fields for robust ion trap quantum logic. The rapid progress in the scale of ion trap devices expected in the coming years means new leaps in our understanding of the physical world could happen at any moment.Open Acces
High-fidelity quantum logic on trapped ions with microwave radiation
Full text linkThis thesis describes the development of technology and experimental techniques for scalable trapped-ion quantum computing. An architecture for scalable quantum computing
is presented in which 171Yb+ ions are trapped in linear R.F. Paul traps and their hyper-fine states are used to form qubits. Quantum logical operations can be carried out by applying R.F. and microwave fields. This thesis presents significant improvements in the experimental setup used for quantum computing, as well as new theoretical developments and experimental demonstrations of new high- fidelity quantum control methods.
A new procedure for carrying out high- delity two qubit gates using static magnetic
eld gradients and resonant microwave elds is presented. It is shown that this has the
potential to enable signi cantly improved delities compared to current methods. Improved
methods for measurement and statistical analysis in trapped-ion experiments are
also shown. These should enable more accurate measurement of two-qubit gate delities,
which becomes increasingly important as delities increase.
A new system for generating arbitrary R.F. and microwave waveforms is presented,
including a bespoke software control system allowing the user to de ne custom pulse
sequences, which represents a signi cant improvement in the scalability of the quantum
control system. Experimental developments towards a system for carrying out on-chip
trapped-ion quantum logic in a static magnetic eld gradient are also presented.
A new technique for generating multi-level control methods is introduced, based on
reducing a multi-level system to an e ective two-level system. High- delity quantum
control methods generated using this technique, which form a will form a key part of the
scalable quantum computing architecture, are implemented experimentally.Open Acces
Robust laser-free entanglement with trapped ions
Full text linkTrapped ions with microwave radiation are a promising platform for universal quantum
computing. However, a major obstacle in the way of scalability is the coupling of the qubits
to their noisy environment. This thesis offers means to improve the fidelity of two-qubit
entangling gates. To this end, we investigate noise from classical control hardware and
study quantum control methods that increase the gate’s robustness.
The noise spectrums of classical control hardware typically exhibit non-Markovian
behaviour. Therefore, a transfer function in frequency space is derived for each source,
transforming hardware noise to qubit-frame noise. It is found that voltage noise on the
electrodes is a significant contribution to decoherence as it displaces the ions within the
static magnetic field gradient. We propose and demonstrate a voltage noise cancellation
scheme that is compatible with microfabricated surface traps.
We then identify a library of quantum control methods that increase the robustness
of a bichromatic interaction to both spin and motional decoherence. We also propose a
novel σz ⊗ σz entangling gate which makes use of the intrinsic J-coupling interaction of
ions in a static magnetic gradient. The resulting interaction is virtually insensitive to
motional decoherence, which alleviates stringent experimental requirements. We finally
demonstrate a bichromatic interaction that is simultaneously robust to spin and motional
decoherence, by means of continuous dynamical decoupling and phase modulation on the
sidebands.
Recalling that noise in the ion’s position couples into magnetic field noise due to the
static magnetic field gradient, we use this noise mechanism as the basis of a promising
electric field sensor. We experimentally demonstrate AC electrometry with a sensitivity
of S = 7.0(5)mVm−1Hz−1/2. Noise spectroscopy was also demonstrated and was limited
by the noise floor, where the minimum sensitivity was 545 nVm−1Hz−1/2.Open Acces
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