This thesis concerns possible implementations of quantum computing schemes and tries to overcome some standard limitations. The central result is a technique we call reservoir engineering that is applied to optical cavity QED based quantum computing. The usual problem for quantum computing with atomic qubits in cavities is scalability as this requires either the coupling of photons leaking from cavities,using linear optics elements and measurements or shuttling of ions into and out of cavities. We propose an alternative that applies strong dissipative coupling to an environment as a control on fibre-coupled cavity systems. The control mechanism is effectively an overdamping of certain common cavity modes that restricts the time evolution of the qubit-cavity system onto a smaller subsystem consisting of only one common cavity modes. Within this subsystem, we then show that it is possible to implement quantum computing schemes that apply otherwise only to atomic qubits in the same cavity.
This thesis concerns possible implementations of quantum computing schemes and tries to overcome some standard limitations. The central result is a technique we call reservoir engineering that is applied to optical cavity QED based quantum computing. The usual problem for quantum computing with atomic qubits in cavities is scalability as this requires either the coupling of photons leaking from cavities,using linear optics elements and measurements or shuttling of ions into and out of cavities. We propose an alternative that applies strong dissipative coupling to an environment as a control on fibre-coupled cavity systems. The control mechanism is effectively an overdamping of certain common cavity modes that restricts the time evolution of the qubit-cavity system onto a smaller subsystem consisting of only one common cavity modes. Within this subsystem, we then show that it is possible to implement quantum computing schemes that apply otherwise only to atomic qubits in the same cavity.
We show that it is possible to employ reservoir engineering to turn two distant and relatively bad cavities into one good cavity with a tunable spontaneous decay rate. As a result, quantum computing schemes, which would otherwise require the shuttling of atomic qubits in and out of an optical resonator, can now be applied to distant quantum dots. To illustrate this we transform a recent proposal to entangle two qubits via the observation of macroscopic fluorescence signals [J. Metz et al., Phys. Rev. Lett. 97, 040503 (2006)] to the electron-spin states of two semiconductor quantum dots. Our scheme requires neither the coherent control of qubit-qubit interactions nor the detection of single photons. Moreover, the scheme is relatively robust against spin-bath couplings, parameter fluctuations, and the spontaneous emission of photons.
Engineered quantum systems are poised to revolutionize information science in the near future. A persistent challenge in applied quantum technology is creating controllable, quantum interactions while preventing information loss to the environment, decoherence. In this thesis, we realize mesoscopic superconducting circuits whose macroscopic collective degrees of freedom, such as voltages and currents, behave quantum mechanically. We couple these mesoscopic devices to microwave cavities forming a cavity quantum electrodynamics (QED) architecture comprised entirely of circuit elements. This application of cavity QED is dubbed Circuit QED and is an interdisciplinary field seated at the intersection of electrical engineering, superconductivity, quantum optics, and quantum information science. Two popular methods for taming active quantum systems in the presence of decoherence are discrete feedback conditioned on an ancillary system or quantum reservoir engineering. Quantum reservoir engineering maintains a desired subset of a Hilbert space through a combination of drives and designed entropy evacuation. Circuit QED provides a favorable platform for investigating quantum reservoir engineering proposals. A major advancement of this thesis is the development of a quantum reservoir engineering protocol which maintains the quantum state of a microwave cavity in the presence of decoherence. This thesis synthesizes strongly coupled, coherent devices whose solutions to its driven, dissipative Hamiltonian are predicted a priori. This work lays the foundation for future advancements in cavity centered quantum reservoir engineering protocols realizing hardware efficient circuit QED designs.
The coupling of a single ion to an optical cavity is a promising route towards scalable quantum technologies. This study presents the design, initial construction and simulations of two different ion traps for quantum computation with novel capabilities. The first system is an end cap system that utilises Fabry Perot cavities formed by the facets of fibres. The small mode volumes (cavity length ~ 300 µm) of the fibre cavities are ideal to achieve high cooperativity and coupling strength. This trap features the novel ability to mechanically adjust the position of the ion by perturbing/distorting the trapping potential with grounded side electrodes. A capability crucial to maximising the ion-cavity coupling. The design incorporates a number of improvements over the previous iteration by Takahashi et al. such as increased optical access, increased mode matching and improved mechanical stability of the cavity. To verify and test the design modifications; Pound-Drever-Hall cavity locking was set up. The test set up failed to lock to an error signal due to a mechanically unstable translation stage. The other design presented is a linear micro trap intended to trap strings of ions and does not use fibre based cavities. This design features multiple different regions reserved for storage, computation and communication and aims to implement a shuttling scheme that will allow the selective loading of single ions from a reservoir of ions into the desired regions. The fabrication process would utilise methods in microfabrication. Simulations of the trapping dynamics were used to optimise the electrode geometries and splitting protocol.
In this paper, we consider the application of quantum reservoir engineering in quantum metrology. More precisely, we are concerned with a system setup where a sequence of atoms constructing the “time-varying” quantum reservoir interact, in turn, with the trapped field in a cavity through the Jaynes–Cummings Hamiltonian. In particular, we were able to manipulate the initial states of reservoir atoms in order to enhance estimation precision regarding the coupling strength between each atom and the cavity (the coupling strength between each atom and the cavity was assumed to be identical). The novelty of this work lies in alternately preparing the atoms at two different states in a pairwise manner, such that the cavity could converge into a squeezed state with photonic loss to the environment taken into account. The control scheme proposed here thus leads to higher precision compared to the previous work where reservoir atoms were initialized at the same state, which drove the cavity to a coherent state. Detailed theoretical analysis and numerical simulations are also provided. In addition, this system setup and the associated control scheme are easily implemented for quantum metrology, since no entanglement is required for the preparation of atom states, and the final cavity state can stay steady.
A primary challenge in quantum science and technology is to isolate the fragile quantum states from their environment in order to prevent the irreversible leakage of energy and information which causes decoherence. In the late 1990s, however, a new paradigm emerged, in which the environment itself, as well as the coupling to this environment of the quantum state, are engineered in ways which, counterintuitively, facilitate the creation and stabilization of desired quantum properties. This paradigm, coined reservoir engineering, has been pursued experimentally in various implementations, in which the states of trapped ions, atomic clouds or artificial atoms based on superconducting circuits were driven into a desired target state using a dissipative reservoir. In all these cases, the cold electromagnetic environment served as the engineered reservoir, capitalizing on the tremendous progress in the ability to shape the modes of the electromagnetic field using masers and lasers. In optomechanics, which is the study of the coupling between light and mechanical motion, a similar scenario has been prevalent. Light is used to read out and control the mechanical motion, and in the past few years, various optomechanical architectures both in the optical and microwave domain have enabled to push mechanical systems into the quantum regime. These results can be interpreted in the context of reservoir engineering: cooling and many other optomechanical phenomena exploit the cold, dissipative nature of light. This thesis reports on the development of a multimode superconducting circuit with a mechanically compliant element - fitting into the field of circuit cavity electromechanics -, with which we pursue two themes. In the first set of experiments, reversing the roles of dissipation described above, we engineer the mechanical oscillator into a cold, dissipative environment for microwave light. The mechanical element is the fundamental mode of a free-standing top electrode of a capacitor, 100 nm thick and 32 micron in diameter made of aluminum with an effective mass of around 170 pg, vibrating at a resonance frequency of 5.5 MHz. The dissipative mechanical reservoir is prepared using an auxiliary microwave mode with engineered parameters.We utilize it to control the properties - in particular, the susceptibility - of the electromagnetic field and to perform low-noise amplification close to the quantum limit. The noise analysis reveals that the reservoir is close to its quantum ground state with a mean thermal occupation number well below 1, demonstrating its utility as a resource in the quantum regime. We also show that the system can be driven to the parametric instability threshold, above which self-sustained oscillations of the microwave field (masing) is observed. Finally, we demonstrate injection locking of the maser using an external seed pump. In a second set of experiments, a similar electromechanical circuit is used to implement a microwave isolator based on optomechanical interactions. The overarching theme is that dissipation of the mechanical oscillator, albeit intrinsic and not engineered via an auxiliary mode s a key ingredient for the device to function as a nonreciprocal frequency converter and isolator. In these experiments, an additional mechanical mode is used, such that a 4-mode optomechanical plaquette is realized.
Spin squeezing has important applications in the field of quantum metrology and quantum information processing. Here we propose that coherent population trapping is well suitable for establishing cavity dissipation mechanism and generating a spin-squeezed state. An ensemble of N double Λ -type atoms is placed inside the two-mode optical cavity, where one Λ subsystem is driven resonantly by two strong control fields to form a dark resonance and the other Λ subsystem is coupled by two cavity vacuum fields and two external fields with large detunings. Due to the dark resonance, the atoms are trapped in a dark state and one has the maximal coherence between the two ground states. Two double off-resonance stimulated Raman scattering interactions are induced between fields and dressed atoms to establish a dissipative quantum dynamical process based on a collective cavity reservoir. As a result, strong stable spin squeezing is generated, which is verified by our numerical and analytical results. Published by the American Physical Society 2024