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Atom detection and counting in ultracold gases using photoionisation and ion detection

T. J. Campey

Published Jan 1, 2009• 0 citations
OpenAlex

Abstract

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.

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Resonance ionization spectroscopy and one-atom detection

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Resonance ionization spectroscopy, RIS, is a multistep photon absorption process in which the final state is the ionization continuum of an atom. The RIS process can be saturated with available pulsed lasers, so that one electron can be removed from each atom of the selected type. This method was first applied to the determination of the absolute number of He($2^{1}S$) excited states produced when pulsed beams of protons interacted with helium gas. Laser schemes for RIS are classified into five basic types; with these, nearly all of the elements can be detected with commercially available lasers. A periodic table is included showing schemes applicable to all of the elements except He, Ne, F, and Ar. A compact theory of the RIS process is presented which delineates the conditions under which rate equations apply. Questions on the effects of collisional line broadening, laser coherence time, and nonresonant multiphoton ionization processes are discussed. The initial demonstration of one-atom detection of Cs is described. By using laser beams to saturate the RIS process and by using proportional counters as single-electron detectors, one-atom detection is made possible. With RIS, one-atom detection is highly selective, has the ultimate in sensitivity, and has excellent space and time resolution. Furthermore, a modification of the technique in which single electrons (or single ions) are detected with a channel electron multiplier permits single-atom detection in a vacuum. Resonance ionization spectroscopy has application to classical phenomena such as the diffusion of free atoms, the chemical reaction of free atoms with a gas, the measurement of concentration fluctuations in a dilute vapor, and a variety of other atomic fluctuation phenomena. The authors describe how RIS can be used for photophysics measurements such as far wing collisional line broadening, measurements of photoionization cross sections for excited states, and collisional redistribution among excited states. Modern applications include the detection of single atoms in individual ionization tracks, such as those created by the binary fission of a parent atom, and the extension of this technique to the detection of stable or unstable daughter atoms in time coincidence with the decay of a parent atom. These methods are beig developed for use in low-level counting to determine, for example, the flux of solar neutrinos on the earth.

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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 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 analyze photoionization and ion detection as a means of accurately counting ultracold atoms. We show that it is possible to count clouds containing many thousands of atoms with accuracies better than ${N}^{\ensuremath{-}1∕2}$ with current technology. This allows the direct probing of sub-Poissonian number statistics of atomic samples. The scheme can also be used for efficient single-atom detection with high spatiotemporal resolution. All aspects of a realistic detection scheme are considered, and we discuss experimental situations in which such a scheme could be implemented.
The authors have used laser cooling and trapping techniques to investigate photoionization and cold collisions. With laser-trapped Rb, they have measured the photoionization cross section from the first excited (5P) level by observing the photoionization-induced loss rate of neutral atoms from the trap. This technique has the advantage that it directly measures the photoionization rate per atom. Knowing the ionizing laser intensity and the excited-state fraction, the measured loss rate gives the absolute cross section. Using this technique, the Rb 5P photoionization cross section at {approximately}400 nm has been determined with an uncertainty of 9%. The authors are currently attempting to extend this method to the 5D level. Using time-ordered pulses of diode-laser light (similar to the STIRAP technique), they have performed very efficient two-photon excitation of trapped Rb atoms to 5D. Finally, they will present results from a recent collaboration which combines measurements form conventional molecular spectroscopy (single photon and double resonance) with photoassociation collisions of ultracold Na atoms to yield a precise ({le}1 ppm) value for the dissociation energy of the X {Sigma}{sub g}{sub +} ground state of the Na{sub 2} molecule.
Ion traps are widely used to contain and study charged particles. In recent years, there has been an effort to utilize trapped particles as quantum detectors or as components of quantum computers. Before trapping a particle inside an ion trap, it needs to be ionised and cooled, which is commonly achieved using fine-tuned semiconductor lasers. The main objective of this thesis is to develop PID regulator capable of stabilising a 423nm photoionising laser, which is a part of the ion trapping experiment in the Ultracold plasma laboratory. The thesis includes a basic insight into the theory of linear regulators, as well as a detailed implementation procedure with thorough description of employed hardware components.
The ability to detect single photons with a high efficiency is a crucial requirement for various quantum information applications. By combining the storage process of a quantum memory for photons with fluorescence-based quantum state measurement, it is, in principle, possible to achieve high-efficiency photon counting in large ensembles of atoms. The large number of atoms can, however, pose significant problems in terms of noise stemming from imperfect initial state preparation and off-resonant fluorescence. We identify and analyse a concrete implementation of a photon number resolving detector based on an ion Coulomb crystal inside a moderately high-finesse optical cavity. The cavity enhancement leads to an effective optical depth of 15 for a finesse of 3000 with only about 1500 ions interacting with the light field. We show that these values allow for essentially noiseless detection with an efficiency larger than 93%. Moderate experimental parameters allow for repetition rates of about 3 kHz, limited by the time needed for fluorescence collection and re-cooling of the ions between trials. Our analysis may lead to the first implementation of a photon number resolving detector in atomic ensembles.
Resonance ionization spectroscopy, RIS, is a multistep photon absorption process in which the final state is the ionization continuum of an atom. The RIS process can be saturated with available pulsed lasers, so that one electron can be removed from each atom of the selected type. This method was first applied to the determination of the absolute number of He(21S2^{1}S21S) excited states produced when pulsed beams of protons interacted with helium gas. Laser schemes for RIS are classified into five basic types; with these, nearly all of the elements can be detected with commercially available lasers. A periodic table is included showing schemes applicable to all of the elements except He, Ne, F, and Ar. A compact theory of the RIS process is presented which delineates the conditions under which rate equations apply. Questions on the effects of collisional line broadening, laser coherence time, and nonresonant multiphoton ionization processes are discussed. The initial demonstration of one-atom detection of Cs is described. By using laser beams to saturate the RIS process and by using proportional counters as single-electron detectors, one-atom detection is made possible. With RIS, one-atom detection is highly selective, has the ultimate in sensitivity, and has excellent space and time resolution. Furthermore, a modification of the technique in which single electrons (or single ions) are detected with a channel electron multiplier permits single-atom detection in a vacuum. Resonance ionization spectroscopy has application to classical phenomena such as the diffusion of free atoms, the chemical reaction of free atoms with a gas, the measurement of concentration fluctuations in a dilute vapor, and a variety of other atomic fluctuation phenomena. The authors describe how RIS can be used for photophysics measurements such as far wing collisional line broadening, measurements of photoionization cross sections for excited states, and collisional redistribution among excited states. Modern applications include the detection of single atoms in individual ionization tracks, such as those created by the binary fission of a parent atom, and the extension of this technique to the detection of stable or unstable daughter atoms in time coincidence with the decay of a parent atom. These methods are beig developed for use in low-level counting to determine, for example, the flux of solar neutrinos on the earth.