Joe Britton

Trapped ions offer a pristine platform for quantum computation and simulation, but improving their coherence remains a crucial challenge. Here, we propose and analyze a new strategy to enhance the coherent interactions in trapped ion systems via parametric amplification of the ions' motion--by squeezing the collective motional modes (phonons), the spin-spin interactions they mediate can be significantly enhanced. We illustrate the power of this approach by showing how it can enhance collective spin states useful for quantum metrology, and how it can improve the speed and fidelity of two-qubit gates in multi-ion systems, important ingredients for scalable trapped ion quantum computation. Our results are also directly relevant to numerous other physical platforms in which spin interactions are mediated by bosons.

Building robust instruments capable of making interferometric measurements with precision beyond the standard quantum limit remains an important goal in many metrology laboratories. We describe here the basic concepts underlying spin squeezing experiments that allow one to surpass this limit. In priniciple it is possible to reach the so-called Heisenberg limit, which constitutes an improvement in precision by a factor $\sqrt{N}$, where $N$ is the number of particles on which the measurement is carried out. In particular, we focus on recent progress toward implementing spin squeezing with a cloud of beryllium ions in a Penning ion trap, via the geometric phase gate used more commonly for performing two-qubit entangling operations in quantum computing experiments.

Quantum-mechanical principles can be used to process information (QIP). In one approach, linear arrays of trapped, laser cooled ion qubits (two-level quantum systems) are confined in segmented multi-zone electrode structures. The ion trap approach to QIP requires trapping and control of numerous ions in electrode structures with many trapping zones. I investigated microfabrication of structures to trap, transport and couple large numbers of ions. Using 24Mg+ I demonstrated loading and transport between zones in microtraps made of boron doped silicon. This thesis describes the fundamentals of ion trapping, the characteristics of silicon-based traps amenable to QIP work and apparatus to trap ions and characterize traps. Microfabrication instructions appropriate for nonexperts are included. Ion motional heating was measured. <<>> Using MEMs techniques I built a Si micro-mechanical oscillator and demonstrated a method to reduce the kinetic energy of its lowest order mechanical mode via capacitive coupling to a driven radio frequency (RF) oscillator. Cooling resulted from a RF capacitive force, phase shifted relative to the cantilever motion. The technique was demonstrated by cooling the 7 kHz fundamental mode from room temperature to 45 K. <<>> I also discuss an implementation of the semiclassical quantum Fourier transform (QFT) using three beryllium ion qubits. The QFT is a crucial step in a number of quantum algorithms including Shor's algorithm, a quantum approach to integer factorization which is exponentially faster than the fastest known classical factoring algorithm. This demonstration incorporated the key elements of a scalable ion-trap architecture for QIP.

Small, controllable, highly accessible quantum systems can serve as probes at the single quantum level to study multiple physical effects, for example in quantum optics or for electric and magnetic field sensing. The applicability of trapped atomic ions as probes is highly dependent on the measurement situation at hand and thus calls for specialized traps. Previous approaches for ion traps with enhanced optical access included traps consisting of a single ring electrode or two opposing endcap electrodes. Other possibilities are planar trap geometries, which have been investigated for Penning traps and rf-trap arrays. By not having the electrodes lie in a common plane the optical access in the latter cases can be substantially increased. Here, we discuss the fabrication and experimental characterization of a novel radio-frequency (rf) ion trap geometry. It has a relatively simple structure and provides largely unrestricted optical and physical access to the ion, of up to 96% of the total 4pi solid angle in one of the three traps tested. We also discuss potential applications in quantum optics and field sensing. As a force sensor, we estimate sensitivity to forces smaller than 1 yN Hz^(-1/2).