Resonant interaction of a single atom with single photons from a down-conversion source
C. Schuck, F. Rohde, N. Piro, M. Almendros, J. Huwer, M. W. Mitchell, M. Hennrich, A. Haase, F. Dubin, J. Eschner
RResonant interaction of a single atom with single photonsfrom a down-conversion source
C. Schuck ∗ , F. Rohde ∗ , N. Piro ∗ , M. Almendros ∗ , J. Huwer,M. W. Mitchell, M. Hennrich † , A. Haase ‡ , F. Dubin, J. Eschner ICFO - Institut de Ciencies Fotoniques, Mediterranean Technology Park, 08860 Castelldefels (Barcelona), Spain (Dated: November 30, 2018)We observe the interaction of a single trapped calcium ion with single photons produced bya narrow-band, resonant down-conversion source [A. Haase et al., Opt. Lett. , 55 (2009)],employing a quantum jump scheme. Using the temperature dependence of the down-conversionspectrum and the tunability of the narrow source, absorption of the down-conversion photons isquantitatively characterized. PACS numbers: 42.50.Ct, 42.50.Ex, 03.67.-a, 03.67.Bg
At the level of single particles, the quantum nature oflight-matter interaction becomes manifest, and the ab-sorption and emission of single photons by single atomsis one of the key physical processes on which quantumoptics is built. Seminal examples of phenomena in thisrespect are photon anti-bunching [1, 2], quantum jumps[3, 4, 5, 6], Jaynes-Cummings dynamics [7], and atom-photon entanglement [8, 9, 10].At the same time, important applications of quan-tum optics, in particular in quantum optical informa-tion technology and in quantum metrology, are based onatom-photon interaction at the single particle level. Themost precise clock is realized with a single laser-excitedtrapped ion [11], and strings of trapped ions have beenshown to be promising systems for implementing quan-tum logical algorithms [12, 13, 14, 15, 16] as well as quan-tum networks [17].A key step in converting quantum optical phenomenainto quantum technology tools is the control of the pro-cesses at all levels, i.e. of the atomic internal (electronic)and external (motional) state, and of the parameters ofthe photons, including their spatial and temporal shape,their polarization, and, ideally, their arrival times. Twomajor strategies may be distinguished how such controlis obtained: on the one hand through deterministic oper-ations, whereby typically photons are confined and con-trolled by high-finesse cavities, on the other hand throughprobabilistic operations, whereby some experimental sig-nal indicates that the desired interaction process has oc-curred. Further approaches include collective effects inatomic samples [18, 19, 20], optics with very high open-ing angles [21], or temporal and geometrical pulse shap-ing; the state-of the-art of the latter is summarized inRef. [23]. ∗ These authors contributed equally to the work. † Current address: Institut f¨ur Experimentalphysik, Universit¨atInnsbruck, Austria ‡ Current address: Dipartamento di Fisica, Universit´a di Trento,Italy
With deterministic atom-light interaction in cavities,important results have been achieved such as single-[25, 26, 27, 28] and entangled-photon [29] creation andphoton turnstile operation [30]. A prominent examplefor probabilistic operations is the experimental creationof remote atom-atom entanglement [17], and many moreideas exist including quantum repeaters [31] and all-optical quantum computing [32]. In terms of control ofatomic states, single trapped ions have produced very ad-vanced results such as quantum coherence on time scalesof seconds [33, 34], two- and three-ion quantum gates[12, 13, 35, 36], and multi-ion entanglement [14, 15]. Interms of control of individual photons, spontaneous para-metric down-conversion (SPDC) sources produce entan-gled photon pairs at high fidelities and rates [37, 38], andserve as ”heralded” single-photon sources [39].In this context we report the observation of interactionbetween a single atom and single photons from a sponta-neous parametric down-conversion source. The atom inour experiment is a single Ca + ion, trapped in a linearPaul trap and cooled by continuous laser excitation; thephotons are produced by a SPDC source, described inmore detail in Refs. [40, 41], which is tuned to provideentangled photon pairs in the wavelength range of the4D / − / transition in Ca + . In the current experi-ment we measure the interaction of the single atom withone photon of the entangled pairs by observing quan-tum jumps, very similar to Dehmelt’s proposal for spec-troscopy on highly forbidden transitions [42]. In contrastto other recent work where weak light fields interact withsingle atomic absorbers [43, 44, 45], our SPDC photonsbear the potential of transferring their non-classical prop-erties to the atoms; in the framework of quantum tech-nologies, our results form a step towards implementingphoton-to-atom entanglement transfer in quantum net-working scenarios [46].The experimental set-up is displayed schematically inFig. 1. The ion is confined by a standard linear iontrap surrounded by two diffraction-limited, high numer-ical aperture (HALO) lenses for efficient optical access[47]. Continuous laser excitation on the 4S / − / and a r X i v : . [ qu a n t - ph ] J un SPDC source ppKTP singlemodefiber PBS
Ion Trap
HALO HALO
PMT
Bandwidth filtering cavities m f l uo r e sc en c e ( c p s )
850 nm laser detuning (MHz) D P S P D m n850 m FIG. 1: Scheme of the experiment. The lasers at 397 nm,866 nm, and 854 nm enter the trap from one side while flu-orescence is collected through one of the HALO lenses. TheSPDC photons enter through the other HALO lens. Eitherthe unfiltered or the filtered arm of the source, or the 850 nmmaster laser, is used for spectroscopy of the ion. The rele-vant levels of Ca + are shown in the bottom left panel. Thebottom-right inset shows a spectrum of 397 nm fluorescencewhen the 850 nm transition is excited by the master laser ofthe SPDC source. We use this spectrum to determine the850 nm line center. A magnetic field along the optical axis ofthe HALOs removes the Zeeman degeneracies and provides aquantization axis. / − / transitions at 397 nm and 866 nm, respec-tively, provides cooling to the Lamb-Dicke regime andgenerates fluorescence which is monitored with a photoncounting detector. The type-II SPDC source producespolarisation-entangled photon pairs at 850 nm in about150 GHz bandwidth, which are split by a polarising beamsplitter, thus providing time- and frequency-correlatedphoton pairs in two output arms. In one of the armsthe photons are filtered by two actively stabilised Fabry-Perot cavities, thus producing narrow-band tunable pho-tons which are matched in frequency and bandwidth tothe 3D / − / transition of the Ca + ion at 849.802 nm[40, 41].All lasers are transfer-stabilised via optical resonatorsto a saturated-absorption signal in cesium [48], includingthe master laser of the SPDC source, which is tuned tothe 850 nm line and which in turn is the reference for the narrow-band filters in the filtered arm. The inset ofFig. 1 shows a reference spectrum taken by scanning thismaster laser across the 850 nm resonance while record-ing the ion’s fluorescence at 397 nm. The laser poweris kept low such that any light shift of the D / levelis avoided. The fluorescence exhibits a dip around the850 nm resonance frequency, which is explained by reso-nant optical pumping from D / into D / via the P / level. Since the D / level is metastable with a lifetimeof ∼ / − / transition at 854 nm, to pump the ion backinto the fluorescence cycle.Without the 854 nm laser, weak resonant excitation at850 nm produces quantum jumps, i.e. random switchingof the ion’s fluorescence between the full rate determinedby the 397 nm and 866 nm excitation, and the back-ground level (dark counts and stray light) [3, 4, 5]. Sucha ”quantum amplifier” scheme [42] is capable of detect-ing individual absorption events at extremely low rates,which has originally been proposed for observing forbid-den resonances, while in our case it is used for detect-ing an extremely low photon flux resonant with a weakdipole-allowed transition.In the first quantum jump measurement, the photonsfrom the unfiltered output arm of the SPDC source are Q uan t u m j u m p s i n m i n .
19 20
21 22
23 24 25
26 27 Cr y stal temperature (ºC) F il t e r ed pho t on s pe r s e c . FIG. 2: Top: quantum jump rate induced by photons fromthe unfiltered arm, as a function of the SPDC crystal temper-ature. Each data point corresponds to 1 hr total measurementtime, and the error bars are the calculated Poissonian ( √ n )deviations. Bottom: count rate at the output of the filteredarm, for comparison. Here the filters act as an artificial ionwith the same reson one. Points are averages of 120 measure-ments of 1 s each; error bars indicate the standard deviationof the mean. In both plots the line is a theoretical calculation[49]. sent to the ion. Like the 850 nm laser they induce opti-cal pumping into D / , but now, with the 854 nm laserswitched off, the ion remains in the metastable level foran average time of 1.2 s before returning to the groundstate by spontaneous emission. We find a rate of about0.7 on-off jumps per minute induced by the unfilteredSPDC photons, the background rate without the SPDClight being 0.09/min. Since the unfiltered arm providesphotons in about 200 GHz bandwidth, there is no nar-rowband spectral dependence of the quantum jump rate.Instead, we measured the spectral variation with thetemperature-dependent emission spectrum of the SPDCsource. The emission peak shifts by about -59 GHz/ ◦ C,such that by changing the temperature of the down-conversion crystal over ∼ ◦ C we scan the whole SPDCbandwidth over the resonance. The result is displayed inFig. 2. The observed spectral (i.e. temperature) depen-dence of the quantum jump rate, Fig. 2 (top), agrees wellwith the measured rate of photons within the absorptionbandwidth, Fig. 2 (bottom), which is recorded by usingthe tuned filters in the other arm to simulate the ion’snarrow-band absorption window.The maximum observed jump rate is also close to whatwe expect, estimated as follows. From the characteriza-tion of the SPDC source the spectral flux of photons inthe unfiltered arm is known to be around 250/(s MHz)[41] such that within the Lorentzian absorption line of22 MHz bandwidth about 10 photons/s impinge on theion. Reducing factors are (i) the population of about0.6 of the D / level, set by the excitation conditions on397 nm and 866 nm; (ii) the absorption strength on theD / − P / transition, which contributes only ∼ .
7% tothe total dipole coupling strength of the P / level; (iii)the branching ratio for decay of P / into D / , about5.9%; (iv) the polarization matching between the SPDCphotons and the transition, which contributes a reduc-tion by 1/3; and (v) the geometric overlap of the incom-ing light with the radiation pattern of the D / − P / dipole [21], about 2% [50]. From these numbers, about1 jump in 100 s is predicted.In the second quantum jump measurement, we makethe ion interact with the narrow-band output of the fil-tered arm, with a measured bandwidth of 22 MHz andstabilised to the frequency of the master laser (see Fig. 1).The narrow-band photons are tuned across the 850 nmresonance by shifting with an acousto-optical modulatorthe master laser frequency, to which the filter cavities arereferenced. The results are displayed in Fig. 3. The res-onance is clearly visible, and the linewidth correspondsto what is expected from convoluting the 22 MHz band-width of the SPDC photons with the spectroscopic res-onance width of 36 MHz, measured with a narrow-bandlaser (also shown in Fig. 3) [51]. The maximum rate is −50 −25 Detuning of filtered photons (MHz) Q u a n t u m j u m p s i n m i n . −50 0 50−25 2505101520253035 Detuning of 850 nm laser (MHz) Q u a n t u m j u m p s i n m i n . FIG. 3: Left: quantum jump rate induced by photons fromthe filtered arm, as a function of the filter frequency. Pointsare averages of 16 measurements of 5 min each, error bars in-dicate the standard deviation of the mean. Right: for compar-ison, quantum jump rate induced by the strongly attenuatedmaster laser, based on 5 min of data acquisition; error barsare the calculated Poissonian deviations. 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