Real-time capable individual-ion qubit measurement

Abstract

Scaling up the number of qubits for quantum simulation and quantum computation and reaching a low error rate of the involved quantum logic operations are the major challenges in the development of a fault-tolerant universal quantum computer. Trapped ions in surface-electrode traps are a promising candidate that could satisfy both criteria. These traps can be extended into a two-dimensional array, a so-called quantum charge-coupled device, in which the ions can be moved around to different zones that fulfill a specific task. In this way an architecture can be developed that could perform multiple quantum-logic operations with many ions simultaneously.

These operations were first implemented with laser beams and have evolved in the past years to a point where they are almost reaching the threshold for fault tolerance. However, this approach is fundamentally limited by spontaneous emission and is hard to scale up to a large number of qubits. An alternative approach using microwave radiation can overcome these problems. The microwave conductors can be integrated into the surface-electrode traps and therefore feature the same scalability as the trap itself. A small distance between the ions and the microwave conductors in the trap-surface is desirable to reach a strong field to drive the quantum logic operations. A downside of a reduced distance is an increased motional heating rate of the ions. To counteract this effect, the trap can be cooled down to cryogenic temperatures. Cooling the trap and its surrounding also helps to achieve excellent vacuum conditions that are required to reduce the collision rate of the ions with background gas molecules. These collisions would be fatal during a sequence of quantum logic operations.

Another important factor for the operation of a quantum simulator or quantum computer is the ability to prepare the ions in a known state and to detect the state of the ions. In this thesis, we explain how an EMCCD camera can be used as a spatially resolving detector to readout the state of each ion simultaneously and we discuss the benefits and limitations of this technique. We could show that the combined error-rate of state preparation and camera-based detection is on the order of 0.4% which is comparable with a photomultiplier-based detection for a single ion. We also demonstrated that the camera-based detection outperforms the photomultiplier when the state of two ions should be detected. Here we determined the amount of crosstalk between two ions to be so low that the error-rate is basically independent of the number of simultaneously detected qubits. We also discuss options for future improvements of the state preparation and detection system to further reduce the error rate.