Among the numerous types of architecture being explored for quantum computers are systems utilizing ion traps, in which quantum bits (qubits) are formed from the electronic states of trapped ions and coupled through the Coulomb interaction. Although the elementary requirements for quantum computation have been demonstrated in this system, there exist theoretical and technical obstacles to scaling up the approach to large numbers of qubits. Therefore, recent efforts have been concentrated on using quantum communication to link a number of small ion-trap quantum systems. Developing the array-based approach, we show how to achieve massively parallel gate operation in a large-scale quantum computer, based on techniques already demonstrated for manipulating small quantum registers. The use of decoherence-free subspaces significantly reduces decoherence during ion transport, and removes the requirement of clock synchronization between the interaction regions.

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Architecture for a large-scale ion-trap quantum computer

This paper reviews the work on cavity quantum electrodynamics of free atoms. In recent years, cavity experiments have also been conducted on a variety of solid-state systems resulting in many interesting applications, of which microlasers, photon bandgap structures and quantum dot structures in cavities are outstanding examples. Although these phenomena and systems are very interesting, discussion is limited here to free atoms and mostly single atoms because these systems exhibit clean quantum phenomena and are not disturbed by a variety of other effects. At the centre of our review is the work on the one-atom maser, but we also give a survey of the entire field, using free atoms in order to show the large variety of problems dealt with. The cavity interaction can be separated into two main regimes: the weak coupling in cavity or cavity-like structures with low quality factors Q and the strong coupling when high-Q cavities are involved. The weak coupling leads to modification of spontaneous transitions and level shifts, whereas the strong coupling enables one to observe a periodic exchange of photons between atoms and the radiation field. In this case, atoms and photons are entangled, this being the basis for a variety of phenomena observed, some of them leading to interesting applications in quantum information processing. The cavity experiments with free atoms reached a new domain with the advent of experiments in the visible spectral region. A review on recent achievements in this area is also given.

https://iopscience.iop.org/article/10.1088/0034-4885/69/5/R02/pdf

Cavity quantum electrodynamics

A vision has formed in recent years of the components necessary for a large-scale quantum network. Single trapped atoms can serve as the nodes of this network, with the links established by flying photons that are coupled to the atoms using optical resonators. This review describes progress towards the goal of multinode networks using the current generation of experiments, which have achieved unprecedented levels of atomic qubit control and light-matter coupling efficiencies.

Cavity-based quantum networks with single atoms and optical photons

We report the creation of thermal, Fock, coherent, and squeezed states of motion of a harmonically bound {sup 9}Be{sup +} ion. The last three states are coherently prepared from an ion which has been initially laser cooled to the zero point of motion. The ion is trapped in the regime where the coupling between its motional and internal states, due to applied (classical) radiation, can be described by a Jaynes-Cummings-type interaction. With this coupling, the evolution of the internal atomic state provides a signature of the number state distribution of the motion. {copyright} {ital 1996 The American Physical Society.}

Generation of Non-classical Motional States of a Trapped Atom

We give a comprehensive overview of the development of micro traps, from the first experiments on guiding atoms using current carrying wires in the early 1990's to the creation of a BEC on an atom chip.

Microscopic atom optics: from wires to an atom chip

In near-field imaging, resolution beyond the diffraction limit of optical microscopy is obtained by scanning the sampling region with a probe of subwavelength size. In recent experiments, single molecules were used as nanoscopic probes to attain a resolution of a few tens of nanometres. Positional control of the molecular probe was typically achieved by embedding it in a crystal attached to a substrate on a translation stage. However, the presence of the host crystal inevitably led to a disturbance of the light field that was to be measured. Here we report a near-field probe with atomic-scale resolution a single calcium ion in a radio-frequency trap that causes minimal perturbation of the optical field. We measure the three-dimensional spatial structure of an optical field with a spatial resolution as high as 60 nm (determined by the residual thermal motion of the trapped ion), and scan the modes of a low-loss optical cavity over a range of up to 100 m. The precise positioning we achieve implies a deterministic control of the coupling between ion and field. At the same time, the field and the internal states of the ion are not affected by the trapping potential. Our set-up is therefore an ideal system for performing cavity quantum electrodynamics experiments with a single particle.

A single ion as a nanoscopic probe of an optical field

The widely discussed applications in quantum information and quantum cryptography require radiation sources capable of producing a fixed number of photons. This paper reviews the work performed in our laboratory to produce these fields on demand. Two different methods are discussed. The first is based on the one-atom maser or micromaser operating under the conditions of the so-called trapping states. In this situation the micromaser stabilises to a photon number state. Recently, we also succeeded in determining the Wigner function of a single-photon state. The second device, recently realized in our laboratory, uses a single trapped ion in an optical cavity.

Generation of photon number states on demand

This paper describes the investigation of an optical field with atomic resolution The probe is a single trapped ion in a linear Paul trap The trap allows the longitudinal direction to be scanned by displacing the ion along the trap axis In the two transversal directions the ion is confined by the trap potential and therefore the optical field is scanned by displacement of the sample using a piezo drive The method is demonstrated by measuring particular modes of an optical cavity

Probing an Optical Field with Atomic Resolution

Recent widely discussed applications in quantum information and quantum cryptography require radiation sources able to produce a fixed number of photons. In this paper we review the work performed in our laboratory to produce such fields on demand. One setup described is based on the micromaser operated under the conditions of a trapping sate. The second device uses a single ion in an optical cavity.

Generation of Photon Number States by Cavity Quantum electrodynamics.

Radio-frequency ion-traps provide ideal conditions for storing and manipulating single particles and localizing them with a precision far below their resonance wavelength. This property is applied in our experiment to investigate the interaction between ions and the field in an optical cavity, implementing a completely deterministic dynamical evolution of the system. This is in contrast to previous cavity QED experiments using atomic beams [1] or atomic fountains [2]. In these cases, deterministic control over the motion of the atoms is not possible, because the randomly varying arrival times and trajectories result in a fluctuating atom-field interaction strength. In the ion-based cavity-QED system we have set up, a continuous interaction of a single particle with the photon field at a pre-defined position is realized. Applications include the generation of single photon pulses as well as a single-ion laser.

Gerhard R. Guthöhrlein

Papers

A single ion as a nanoscopic probe of an optical field

Generation of photon number states on demand

Probing an Optical Field with Atomic Resolution

Generation of Photon Number States by Cavity Quantum electrodynamics.

Cavity-QED with a single ion