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In its current state, the CEM detection system represents a highly selective and sensitive single particle detector, allowing to determine partial pressures or concentrations of single isotopes or molecules below the detection limit of any conventional detection system. This results as the ultimate detection limit with the CEM detection system is provided by the ratio of true to accidental coincidences Nc/Nac of the correlation measurement, and not by the ratio of the photoionisation to background countsNi,e/Nib,ebof the single CEM detectors as in conventional systems (see section 5.2). It will thus be able to identify different species due to the different arrival time at the CEM detectors according to the mass-to-charge ratio of the single isotopes or molecules (see section 5.3). However, to resolve the mass of single

particle incidences in the CEM detection system according to their individual flight time, the species have to be slightly different in their mass-to-charge ratio assuming the current detector configuration (see subsection 5.3.1).

Combined with the single atom trap environment, the CEM detection system further en- ables highly-efficient precision spectroscopy on single neutral atoms with hyperfine-state selec- tivity. This will permit to measure, e.g., the photoionisation cross section of a single neutral atom free from any statistical uncertainties usually associated with atomic ensembles [245– 247], and down to single hyperfine-state resolution. The combination of the single atom trap with the CEM detection system thus resembles an ideal testing ground for ionisation-based precision spectroscopy on ultracold single atoms.

Minor amendments Better galvanic isolation from the glass cell environment or additional shielding will allow to operate the current CEM detection system at higher acceleration volt- ages ∆Uacc (see section 4.2). In accordance with the CEM efficiency model of section 2.4, 1_{Single-shot in the meaning that a definite measurement result is obtained after a single readout operation.}

6.1. Short term prospects with the current CEM detection system F=2 F=1 795 nm F’= 2 F’= 1 474 nm 2 5 P1/2 2 5 S1/2 87_Rb+ - e l2i l12 ionisation threshold 87 Rb + s + s m =-1f m =0f m =+1f

Figure 6.1:Proposed single hyperfine-state selective readout of the 87Rb-atom qubit by pola- risation dependent photoionisation via the D1-transition (λ12 = 795 nm). Using σ+-polarised light for the excitation transitionλ12, only themf =−1state is subsequently ionised while the

mf = +1state remains in a ’dark’ state.

the maximum attainable 87Rb-ion detection efficiency in such a configuration is calculated to ηi= 0.9943at an impact energy of Ekin= 10 keV (see section 5.4). For the photoelectrons, a maximum detection efficiency ofηe = 0.8940 atEkin = 2.2 keV is estimated. For such fields, the flight time of the 87Rb-ion will only be ti= 170 ns (∆Uacc= 20 kV; see section 4.4). The previous values will provide a theoretical neutral atom detection efficiency ofηatom = 0.9995. Additionally, the use of a slightly modified CEM detector configuration will yield higher de- tection efficiencies and a larger optical access than in the current system (see Appendix C). In particular the implementation of a single CEM detector unit only with an opposing conversion dynode seems to be a promising alternative as the ionisation fragments will both be detec- ted by the same CEM detector and will therefore be processed by the same pulse processing electronics. This will make a second CEM detection unit with its associated pulse processing electronics and high voltage power supplies obsolete, further simplifying the CEM detection system.

Triggered photoionisation detection In the future, pulsed photoionisation schemes with single laser pulses of high intensity will yield photoionisation times tion in the nanosecond range and with ionisation propabilities pionapproaching unity2 [171, 172, 174, 176, 248]. This will enable a heralded detection of single atoms within a nanosecond time window, triggered by the advent of the ionising laser pulse. The triggered photoionisation will allow, e.g., to explicitly determine the individual flight timesti andteof the photoionisation fragments until impact in the corresponding CEM detectors (see section 5.3). It may also be employed for a gated particle detection at the single CEM detectors within a certain time window after the photoionisation event.

Atom qubit readout protocol without STIRAP transfer The use of pulsed photoionisation and an alternative detection scheme via the D1-transition in 87Rb (fig. 6.1) will make the

2

This results as in a resonant two-step photoionisation scheme the spontaneous decay from the resonant intermediate state during the pulsed excitation of the atom can be neglected on such timescales [171, 172, 174].

adiabatic transfer of the atomic qubit state3 via a stimulated Raman adiabatic passage (STI- RAP) process [249, 250] in the current atomic qubit readout protocol obsolete [21, 23, 45]. This will be realised by polarisation dependent photoionisation of the single Zeeman suble- vels, where the corresponding circular polarisation for the excitation transition (e.g., σ+ for λ12= 795 nm) excites the ’bright’ state(|mf=−1>), but leaves the ’dark’ state(|mf= +1>) unaffected (fig. 6.1). This improved readout scheme will further shorten the detection time of the current readout protocol by 120 ns for the STIRAP transfer [18], and additionally avoid the imperfect state transfer of ηstirap= 0.97.