A path towards quantum supremacy with superconducting qubits

A key milestone on the path towards building a quantum computer will be the demonstration of an algorithm which exceeds the capabilities of any classical computer - achieving so called quantum supremacy. The challenge in developing such an algorithm lies in balancing computational complexity with experimental feasibility, particularly in the presence of errors. In this thesis, we design superconducting qubits and algorithms with the goal of finding such a balance. We implement a wide variety of control protocols, including parametric drive, adiabatic control, floquet evolution and time-domain spectroscopy. These tools are used to the study topological invariants, quantum chaos, quantum statistical mechanics, chiral symmetry breaking and many-body localization. We present experimental techniques for characterizing the complexity and fidelity of these algorithms and show that quantum supremacy is achievable using existing technology

Bosonic Quantum Simulation in Circuit Quantum Electrodynamics

The development of controllable quantum machines is largely motivated by a desire to simulate quantum systems beyond the capabilities of classical computers. Investigating intrinsically multi-level model bosonic systems, using conventional quantum processors based on two-level qubits is inefficient and incurs a potentially prohibitive mapping overhead in the current near-term intermediate-scale quantum (NISQ) era. This motivates the development of hybrid quantum processors that contain multiple types of degrees of freedom, such that one can leverage an optimal one-to-one mapping between the model system and simulator. Circuit quantum electrodynamics (cQED) has emerged as a leading platform for quantum information processing owing to the immense flexibility of engineering high fidelity coherent interactions and measurements. In cQED, microwave photons act as bosonic particles confined within a nonlinear network of electromagnetic modes. Controlling these photons serves as the basis for a hardware efficient platform for simulation of naturally bosonic systems. In this thesis, we present two experiments that encapsulate this idea by simulating molecular dynamics in two different regimes of electronic-nuclear coupling: adiabatic and nonadiabatic. In the first experiment, we implement a boson sampling protocol for estimating Franck-Condon factors associated with photoelectron spectra. Importantly, we fulfill the scalability requirement by developing a novel single-shot number-resolved quantum non-demolition detector for microwave photons. In the second experiment, we develop and employ a model for simulating dissipative nonadiabatic dynamics through a conical intersection as a basis for modeling photochemical reactions. We directly observe branching of a coherent wave-packet upon passage through the conical intersection, revealing the competition between coherent evolution and dissipation in this system. The tools developed for the experiments in this thesis serve as a basis for implementing more complex bosonic simulations

Quantum Light with Quantum Dots in III-V Photonic Integrated Circuits: Towards Scalable Quantum Computing Architectures

The work in this thesis is motivated by the goal of creating scalable quantum computers, and equally by the physical understanding that develops alongside and follows from this. The fields of physics and technology are symbiotic, and quantum information processing is a prime example. The field has the potential to test quantum mechanics in new and profound ways. Here we approach the technological problem by building upon the foundations laid by the semiconductor chip manufacturing industry. This architecture is based on the III-V semiconductors Gallium Arsenide and Indium Arsenide. Combining the two we can create chip-embedded atom-like light sources -- quantum dots -- that can produce quantum photonic states in lithographically etched nanoscale waveguides and cavities. We demonstrate the integration of quantum light sources and single-mode beam splitters in the same on-chip device. These are the two primary ingredients that are needed to produce the entangled states that are the basis of this type of quantum computing. Next we look at the quantum light source in more detail, showing that with cavity-enhancement we can significantly mitigate the detrimental dephasing associated with nanostructures. The source can be used as a means to produce coherently scattered photons in the waveguides. More importantly, the on-demand photons obtained from pulsed excitation are more indistinguishable and thus more suitable for quantum information carrying and processing. Through experiments and simulations, we investigate some aspects of single-photon sources under pulsed excitation, including emission rate, emission number probabilities, and indistinguishability. A new technique to measure very short lifetimes is demonstrated and examined theoretically. Finally we look at preliminary steps to extend the platform further. The inclusion of photonic crystals and superconducting nanowires provides on-chip filters and detectors, and etched diode structures enable electrical excitation and tunability of the circuit components. These show some clear paths that the work can continue to evolve along

Quantum effects and novel physics in rotating frames

The birth of quantum physics and general relativity were two revolutions in physics. But a century later, scientists have not yet united the two theories. Attempts to combine them are mostly theoretical; controlled experiments have historically been neglected due to the comparative weakness of gravity and the corresponding precision or extreme scales assumed needed to test quantum gravity effects. We take a new approach, inspired by Einstein’s equivalence of gravitational fields and accelerated frames. Non-inertial frames can be controlled in the lab, and allows us to experimentally test new frame-dependent effects and already-established quantum effects in new regimes. This frame-dependence is fundamentally interesting by itself, but also provides parallels to curved spacetime effects. To that effect, I have carried out experiments in rotating frames and shown new effects. I have combined mechanical rotation with acoustics, sending sound waves through a rotating absorber. With this, I was the first to show experimental proof of the Zel’dovich effect: the amplification of waves carrying angular momentum by a rotating object. It is theorised the Zel’dovich effect should also generate electromagnetic waves out of the quantum vacuum, however the conditions are much harder to meet. I have also done optics experiments to show how rotation can affect quantum entanglement. The Hong-Ou-Mandel effect was used as a witness for antisymmetric entanglement between photons. The symmetry of frequency entangled photon pairs can be manipulated by introducing path superpositions and controlling their phase difference. Through experiment I established that to witness antisymmetry with the Hong-Ou-Mandel effect it was much easier in the regime where the superposed paths had path length differences outwith the single-photon coherence length. Within a rotating frame, a rotation-dependent phase difference between counterpropagating beams of light appears, called the Sagnac effect. Combining Sagnac interferometers with a Hong-Ou-Mandel interferometer on a rotating platform, I have shown how rotation can control the entanglement symmetry of photon pairs. The success of these experiments can be built on in future experiments exploring quantum effects in rotating frames and curved spacetimes. Identifying these effects has relevance in fundamental physics and to new technologies e.g. quantum communication, as it scales up to satellites in the curved spacetime around the rotating Earth

Coupling Rydberg atoms to superconducting microwave circuits

A hybrid circuit quantum electro-dynamics (circuit QED) setup consisting of helium atoms in high-n Rydberg states and superconducting co-planar waveguide (CPW) microwave resonators has been developed with the goal of performing hybrid quantum optics experiments with applications in quantum information processing. In this thesis an overview of the field of cavity QED is introduced, and numerical methods to calculate the atomic energy level structure and transition dipole moments in electric and magnetic fields are described. Using this background information, a new method for efficiently preparing high-n circular Rydberg states is presented. This was required to ac- cess circular-state–to–circular-state transition frequencies in experiments which are compatible with superconducting CPW microwave resonators. Experimental and numerical results demonstrating the implementation of this method for the preparation of the |n = 70, l = 69, ml = +69⟩ circular state in helium are reported. The design and fabrication of λ/4 superconducting CPW microwave resonators, compatible with these high-n circular Rydberg states of helium is then described. The effects of microwave driving power, temperature and magnetic field on the characteristics of these resonators are presented. Finally, experiments in which helium Rydberg atoms have been coherently coupled to the microwave field of a superconducting co-planar waveguide resonator are reported

Walker Percy and the Magic of Naming: The Semeiotic Fabric of Life

Walker Percy thought a paradigm for the modern age, human beings, and life does not exist, and no paradigm vying for supremacy (religion, scientism, new age physics and philosophies) succeeds. He sought to create a “radical anthropology” to describe human beings and life. His anthropology has existential roots and culminates in the philosophy and semeiotic of American pragmatist Charles Sanders Peirce. Unlike any other creature, humans have symbolic capacity, first manifested in a child’s naming and demonstrated in human being’s unique language ability, the ability to communicate through symbol and not just sign. Percy conveyed his anthropology in his last three novels through a number symbolism corresponding to the theme of each novel based on Peirce’s Cenopythagoreanism, viewing the world through the paradigm of number. In Lancelot, Percy uses the symbol of the inverted three to illustrate Lancelot’s inverted search for evil. In The Second Coming, he uses diamonds and squares and fours to illustrate community and authentic communication in the novel. In The Thanatos Syndrome, he uses twos and sixes to represent the search for dyadic solutions to triadic problems. Percy sees a synechistic and synchronistic interconnected “fabric of life” to the universe, enabled by human symbolic capacity, or Peirce’s concept of relations