Fast and efficient detection of single atoms is a universal requirement concerning modern experiments in atom physics, quantum optics, and precision spectroscopy. In particular for future quantum information and quantum communication technologies, the efficient readout of qubit states encoded in single atoms or ions is an elementary prerequisite. The rapid development in the eld of quantum optics and atom optics in the recent years has enabled to prepare individual atoms as quantum memories or arrays of single atoms as qubit registers. With such systems, the implementation of quantum computation or quantum communication protocols seems feasible. This thesis describes a novel detection scheme which enables fast and efficient state analysis of single neutral atoms. The detection scheme is based on photoionisation and consists of two parts: the hyperfine-state selective photoionisation of single atoms and the registration of the generated photoion-electron pairs via two channel electron multipliers (CEMs). In this work, both parts were investigated in two separate experiments. For the first step, a photoionisation probability of p_ion = 0.991 within an ionisation time of t_ion = 386 ns is achieved for a single 87Rb-atom in an optical dipole trap. For the second part, a compact detection system for the ionisation fragments was developed consisting of two opposing CEM detectors. Measurements show that single neutral atoms can be detected via their ionisation fragments with a detection efficiency of eta_atom = 0.991 within a detection time of t_det = 415.5 ns. In a future combined setup, this will allow the state-selective readout of optically trapped, single neutral 87Rb-atoms via photoionisation detection with an estimated detection efficiency eta = 0.982 and a detection time of t_tot = 802 ns. Although initially developed for single 87Rb-atoms, the concept of photoionisation detection is in principle generally applicable to any atomic or molecular species. As efficient readout unit for single atoms or even ions, it might represent a considerable alternative to conventional detection methods due to the high optical access and the large sensitive volume of the CEM detection system. Additionally, its spatial selectivity makes it particularly suited for the readout of single atomic qubit sites in arrays of neutral atoms as required in future applications such as the quantum-repeater or quantum computation with neutral atoms. The obtained high detection efficiency eta and fast detection time t_tot of the new detection method fullfill the demanding detector requirements for a future loophole-free test of Bell's inequality under strict Einstein locality conditions using two optically trapped, entangled 87Rb-atoms at remote locations. In such a configuration, the locality and the detection loophole can be simultaneously closed in one experiment.
Fast and efficient detection of single atoms is a universal requirement concerning modern experiments in atom physics, quantum optics, and precision spectroscopy. In particular for future quantum information and quantum communication technologies, the efficient readout of qubit states encoded in single atoms or ions is an elementary prerequisite. The rapid development in the eld of quantum optics and atom optics in the recent years has enabled to prepare individual atoms as quantum memories or arrays of single atoms as qubit registers. With such systems, the implementation of quantum computation or quantum communication protocols seems feasible. This thesis describes a novel detection scheme which enables fast and efficient state analysis of single neutral atoms. The detection scheme is based on photoionisation and consists of two parts: the hyperfine-state selective photoionisation of single atoms and the registration of the generated photoion-electron pairs via two channel electron multipliers (CEMs). In this work, both parts were investigated in two separate experiments. For the first step, a photoionisation probability of p_ion = 0.991 within an ionisation time of t_ion = 386 ns is achieved for a single 87Rb-atom in an optical dipole trap. For the second part, a compact detection system for the ionisation fragments was developed consisting of two opposing CEM detectors. Measurements show that single neutral atoms can be detected via their ionisation fragments with a detection efficiency of eta_atom = 0.991 within a detection time of t_det = 415.5 ns. In a future combined setup, this will allow the state-selective readout of optically trapped, single neutral 87Rb-atoms via photoionisation detection with an estimated detection efficiency eta = 0.982 and a detection time of t_tot = 802 ns. Although initially developed for single 87Rb-atoms, the concept of photoionisation detection is in principle generally applicable to any atomic or molecular species. As efficient readout unit for single atoms or even ions, it might represent a considerable alternative to conventional detection methods due to the high optical access and the large sensitive volume of the CEM detection system. Additionally, its spatial selectivity makes it particularly suited for the readout of single atomic qubit sites in arrays of neutral atoms as required in future applications such as the quantum-repeater or quantum computation with neutral atoms. The obtained high detection efficiency eta and fast detection time t_tot of the new detection method fullfill the demanding detector requirements for a future loophole-free test of Bell's inequality under strict Einstein locality conditions using two optically trapped, entangled 87Rb-atoms at remote locations. In such a configuration, the locality and the detection loophole can be simultaneously closed in one experiment.
We experimentally demonstrate a detection scheme suitable for state analysis of single optically trapped atoms in less than 1 μs with an overall detection efficiency η exceeding 98%. The method is based on hyperfine-state-selective photoionization and subsequent registration of the correlated photoion-electron pairs by coincidence counting via two opposing channel electron multipliers. The scheme enables the calibration of absolute detection efficiencies and might be a key ingredient for future quantum information applications or precision spectroscopy of ultracold atoms.
In recent years there has been increasing interest in quantum atom optics, in which the discretenature of atoms plays an important role. As such, the ability to resolve atoms at the singleatom level has become important.This thesis investigates detecting atoms using photoionisation and ion detection. Thismethod is applicable to all atoms in common use in atom optics experiments, enables theirdetection with high e±ciency, allows atom counting with sub-Poissonian resolution (¾N < pN)and is applicable for large atom numbers (up to » 105). Two approaches towards atom detectionvia photoionisation and ion detection are examined, di®ering in the rate of photoionisation. Fastphotoionisation of individual atoms can be used for detecting a series of single atoms, such asmight be produced with an atom laser. Slow ionisation is required for counting large numbersof atoms contained in a single region, due to the necessity of being able to resolve the individualions with the ion detector. The application of these methods to 87Rb is examined, althoughthey can be adapted to other atoms.The scheme for quickly detecting single atoms uses STImulated Raman Adiabatic Passage(STIRAP) to excite 87Rb atoms to the 5D5=2 level followed by ionisation from this level usinga Nd:YAG laser pulse. Simulations and experiments were carried out and used to predict thatexcitation to the 5D5=2 level can be achieved with close to 100% e±ciency, and to show theexperimental parameters required. The photoionisation cross-section of 87Rb in the 5D5=2 stateusing 1064 nm light was measured in order to predict the photoionisation e±ciency that maybe achieved, which was found to approach 100%.The scheme for atom counting uses 2£778 nm photon excitation to raise atoms to the5D5=2 state followed by ionisation from this state using a cw Nd:YAG laser. The statisticsof atom counting are examined and used to ¯nd the counting accuracy as a function of thedetection e±ciency and its uncertainty. For a precisely calibrated detector, sub-Poissoniancounting is possible for detection e±ciencies greater than 50%. The e®ect of losses and the useof the scheme for measuring number squeezing are examined. Achieving low count uncertaintiesrequires measuring the ion detection e±ciency to high accuracy. A method for doing this isproposed and analysed, and experiments were carried out to measure the ion detection e±ciencyof an ion detector in a mock experimental setup. A detailed scheme is proposed and analysed for measuring number di®erence squeezing ina RF induced double well potential using atom counting via photoionisation and ion detection.Simulations of the double well potential were performed to show how even splitting can beachieved and the degree of squeezing expected to occur. A design for an experimental setup isdescribed and analysed in detail, in order to show the accuracy with which number di®erencesqueezing can be measured.
Fast and efficient state detection of quantum systems is a crucial component of the emerging quantum technology. For example fast measurements on separated entangled atoms enable a loophole free test of Bell’s inequality, which is crucial in new applications like secure quantum key distribution and random number generation. Moreover, quantum simulation and computation experiments could potentially benefit from this measurement scheme; e.g. site-resolved detection of atoms trapped in optical lattices.
Trapped single ions provide an ideal platform for studying quantum optics, light-matter interactions, and quantum information processing. Many of these applications rely on the availability of well-defined and reproducible quantum emitters as well as precise control over the internal states of ions. To study both of these concepts, this thesis focuses on the isotope selectivity of neutral Barium (Ba) transitions as well as the state preparation of single trapped 138Ba+ ions. The first part of this thesis introduces the theoretical framework for experiments, focusing on single Ba+ ions and light-matter interactions. Therefore, the working principle of Paul traps is presented, the Ba ion is introduced and fundamental light-matter interaction tightly related to the experiments and simulations presented in this work are established. In the second part, the isotope selectivity of the neutral Ba 6s2 1S0 to 6s6p 1P1 transition at 553 nm is investigated and compared with the isotope selectivity of the 6s2 1S0 to 5d6p 3D1 transition at 413 nm. A secondary photon can be used in both cases to achieve isotope selective ionization of Ba+ ions. The isotope selectivity of the 553 nm transition was tested by carrying out spectroscopy and comparing the results with existing data. The isotope selectivity for other naturally available Ba isotopes is simulated for both transitions (553 nm and 413 nm) and for different angles between atomic beam and laser beam (90°, 85° and 80°). The simulations show, that under ideal conditions (when the atomic beam and ionization laser beam are orthogonal) both transitions perform similarly. However, the 553 nm transition exhibits greater robustness against Doppler shifts as the angle deviates from 90°. At an angle of 80° between the atomic beam and the laser beam, the maximum probability of generating 138Ba+ is 99 % using 553 nm, while the maximum probability of generating the isotope using 413 nm does not exceed 94 %. In the third part of this thesis, we focus on state preparation of Ba+ ions in the 6P1/2 state. A heralding state preparation scheme that exploits the spontaneous emission of a 389 nm photon during the decay of the excited state 6D3/2 is introduced. The detection of a photon at this wavelength projects the ion into the desired state 6P1/2 in a deterministic fashion. A simulation of this scheme shows that the preparation probability may exceed 78 %, and the heralding process ensures state preparation with 100 % fidelity, if a photon at a wavelength of 389 nm is detected. The state preparation method will be adopted in near-future experiments that are provided as an outlook at the end of this thesis.
In this paper, we describe the preparation and detection of ultracold atoms on a microchip with single-atom sensitivity. The detection scheme is based on multi-photon ionization of atoms and the subsequent guiding of the generated ions by ion optics to a channel electron multiplier. We resolve single atoms with a detection efficiency above 60%. The detector is suitable for real-time observations of static and dynamic processes in ultracold quantum gases. Although the ionization is destructive, sampling a small subset of the atomic distribution is sufficient for the determination of the desired information. We take full high-resolution spectra of ultracold atoms by ionizing only 5% of the atoms. Using an additional microwave near 6.8 GHz, the detection scheme becomes energy, position and state selective. This can be used for in situ determination of the energy distribution and temperature of atom clouds inside the trap and applied for future correlation measurements.
The interaction of single photons with single atoms is a fundamental process in quantum optics which manifests the quantum mechanical nature of light and matter. As an experimentally controlled process it enables the implementation of quantum technological applications, such as quantum information processing and quantum metrology. In the last decades, substantial progress has been attained to experimentally control and manipulate single atoms.
For example, single or multiple ions can be stored in an ion trap and their quantum states can be controlled with great precision. At the same time, experimental techniques to generate and control single photons and entangled photon pairs have been widely developed. However, only modest progress has been attained in controlling the interaction of single atoms with sin~le photons. The aim of this thesis project was to experimentally study he absorption of single photons and entangled photons pairs by single trapped DCa· ions. To this end, a source of polarization-entangled photon pairs based on the process of spontaneous parametric down-conversion (SPDC) was designed and constructed . Generating photon pairs whose interaction with an atom can be controlled imposed stringent requirements on the bandwidth, tunability, stability and brightness of the source, making its setup much more complex than usual SPDC sources. In a parallel project, a set of two separate ion traps were built, capable of storing a single or a set of a few 4DCa· ions.
he first part of the thesis documents the design, construction , operation and characterization of the entangled photon pair source. The characterization proves all the necessary requirements for the photons to interact with either of two transitions, D312 P312 and D5i2 P312 at 850 and 854 nm respectively, in 4DCa· One of the crucial requirements was to make the source bandwidth match the bandwidth of these transitions (22MHz). This was not straightforward since the typical emission bandwidths of the SPDC process is several orders of magnitude higher (1 05MHz). The ion trap setup is also briefly described. One of its crucial features is the inclusion of two high numerical-aperture objectives very close to the ion trap, allowing to enhance the interaction efficiency of the SPDC photons with the ion, and to efficiently collect the fluorescence light they emit. In a second part of the thesis, a concise description of the schemes and techniques used to detect the absorption of single photons by a single trapped ion is given, including a heoretical treatment of the methods used to analyze the data.
he core part of the thesis describes the methods used to couple the entangled photon source to one of the ion traps, and explains the results obtained with the combined setup. First it shows how the source is configured to produce heralded single photons. It then presents experiments in which the absorption by a single ion of photons produced by the source is detected, and how these absorption events are time-correlated with the detection of the heralding photon. These experiments are performed under several conditions, with a growing level of control over the interaction process. Finally, two proof-of-principle experiments are presented. In the first one, the ion is pumped to polarization-sensitive Zeeman substates and the polarization dependence of the correlated absorption rate is observed. This is a first step toward more advanced experiments in which the polarization entanglemen of the photon pairs is transferred to the entanglement of one photon with one atom or to two distant atoms. A second experiment performs heralded single photon spectroscopy on a single ion, showing the expected frequency dependence of the absorption process.
Finally, methods to extend these experiments to implement entanglement transfer from photons to atoms are outlined. In particular, it is shown how the efficiency of the source can be improved to make these experiments feasible.