Interfacing different physical systems is important in the context of building a practical quantum information network. As a first step towards interfacing different physical systems, we aim to demonstrate the indistinguishability between single photons produced via two different processes by observing the Hong-Ou-Mandel interference effect. The single photons are produced via a four-wave mixing process in a cold atomic ensemble of Rb and spontaneous emission from a single Rb atom in an optical dipole trap. This thesis reports the progress of the single Rb atom setup towards achieving this goal. We measure the temporal profile of the spontaneously emitted single photon and show that the characteristic decay time is in accordance with the values in the literature. Finally, we demonstrate that we can excite the atom with an excitation probability of 78% using a π pulse.
Interfacing different physical systems is important in the context of building a practical quantum information network. As a first step towards interfacing different physical systems, we aim to demonstrate the indistinguishability between single photons produced via two different processes by observing the Hong-Ou-Mandel interference effect. The single photons are produced via a four-wave mixing process in a cold atomic ensemble of Rb and spontaneous emission from a single Rb atom in an optical dipole trap. This thesis reports the progress of the single Rb atom setup towards achieving this goal. We measure the temporal profile of the spontaneously emitted single photon and show that the characteristic decay time is in accordance with the values in the literature. Finally, we demonstrate that we can excite the atom with an excitation probability of 78% using a π pulse.
We present Hong-Ou-Mandel interference of single photons generated via two different physical processes by two independent atomic systems: scattering by a single atom, and parametric generation via four-wave mixing in a cloud of cold atoms. Without any spectral filtering, we observe a visibility of V = 62± 4%. After correcting for accidental coincidences, we obtain V = 93± 6%. The observed interference demonstrates the compatibility of the two sources, forming the basis for an efficient quantum interface between different physical systems.
We present Hong-Ou-Mandel interference of single photons generated via two different physical processes by two independent atomic systems: scattering by a single atom, and parametric generation via four-wave mixing in a cloud of cold atoms. Without any spectral filtering, we observe a visibility of $V=62\ifmmode\pm\else\textpm\fi{}4%$. After correcting for accidental coincidences, we obtain $V=93\ifmmode\pm\else\textpm\fi{}6%$. The observed interference demonstrates the compatibility of the two sources, forming the basis for an efficient quantum interface between different physical systems.
Hong-Ou-Mandel interference between independent sources is a fundamental primitive of many quantum communication and computation protocols. We present a study of the Hong-Ou-Mandel interference of single photons generated via two different physical processes by two independent atomic systems: scattering by a single atom, and parametric generation via four-wave mixing in a cloud of cold atoms. By controlling the coherence time and central frequency of the heralded single photons generated by four-wave mixing we observe quantum beat and a varying degree of interference.
Exploring coherent photon-atom interaction at quantum level has become more and more important in modern quantum physics and quantum information science. In this PhD thesis research work, we established two cold 85Rb atoms ensemble in 2-dimensional dark-line magneto-optical trap with OD more than 100 and produced long coherence time (~1us) heralded single photons via spontaneous four-wave-mixing process with full temporal waveform control from one of the cold atomic ensemble. With this system, optical precursor of a single photon was observed for the first time, which verifies that information carried by a single photon cannot be transferred faster than light speed in vacuum even in the quantum world. This closed the longtime debate on the true speed of the information carried by a single photon. Single photon reemission timing in a two-level atomic system was the first time been controlled on-demand. With this technique, the quantum timing order of the single photon absorption and reemission was first directly observed in time domain. This technique can also be further used in the field of efficient single atom excitation that is desired for the quantum network. Finally, a series of work were conducted to improve the quantum optical memory efficiency in cold atomic ensemble. We obtained the highest single-photon storage efficiency of about 50%, which brings the quantum light-matter interface closer to practical quantum information applications. Multi-bin and multi-channel optical memory were also exploited in the following work.
Photons are good candidates for carrying quantum information because they are very stable particles: they interact weakly with the medium and barely with each other. However, this has drawbacks when you want to process the information because, in this case, it is preferable to have photon-photon interactions. For example, for applications in quantum repeaters, such interactions would allow deterministic Bell state measurements, increasing the entanglement distribution rate between two remote nodes. Getting two photons to interact with each other efficiently requires mapping them into a nonlinear medium at the single-photon level, that is, a medium that reacts differently when it interacts with a single photon than when it does with two. Such strong nonlinearity has been demonstrated with Rydberg atoms, which are atoms excited to a state with a high principal quantum number.
In this thesis we have performed nonlinear quantum optics experiments using an ensemble of cold Rydberg atoms, where we have studied the properties of the quantum light emitted by these atoms. First, we demonstrated nonlinearities at the single-photon level. To reach this stage, we made several improvements to the previous experimental setup available in the group, of which the implementation of a dipole trap was especially relevant. We evidence quantum nonlinearity by measuring photon antibunching for the transmitted light after interacting with the Rydberg state under electromagnetically induced transparency (EIT). We also showed the generation of single photons on-demand after storing weak coherent states of light pulses as collective Rydberg excitations.
Then, we studied the variation of the light statistic throughout the output pulse after propagating through the medium as Rydberg polaritons, which are superposition states of light and Rydberg excitations. We showed that the properties at the beginning and the end of the pulse were different from those of the steady state. In particular, the light detected after the input pulse was abruptly turned off gave much stronger suppression of two-photon events. Then, we investigated how to exploit this effect to generate single photons on demand. To do this, we analyzed the quality of the single photons detected at the end of the pulse as a function of the detection probability and compared the results with those obtained by storing the input pulse as collective Rydberg excitations. We showed that the photons were generated more efficiently when increasing the detection window at the cost of deteriorating the single photons statistics.
Finally, we investigated the indistinguishability of the photons emitted by our Rydberg atomic ensemble, a crucial property for using Rydberg atoms as nodes in quantum networks. We also compared the single photons generated after storage under EIT conditions with those obtained using a two-photon Raman excitation to the Rydberg state. We measured the indistinguishability by making them interfere with weak coherent states of light in a Hong-Ou-Mandel experiment. And we showed that, although we obtained better photon statistics under EIT conditions, the indistinguishability from those obtained with Raman excitation was significantly higher.
Experiments in quantum optics are motivated by fundamental questions about the quantum mechanical description of matter and light as well as by applications in quantum technologies such as quantum information processing. Exploiting the quantum character of both matter and light has turned out to be extremely rewarding for novel technological applications in the emerging fields of quantum communication and computation. Here we consider the implementation of quantum information technology with trapped ions and single photons, focusing on two key issues: the entanglement of distant atoms and the controlled interaction of a single photon with a single atom.
This thesis describes the construction of a double ion trap apparatus including the necessary frequency-stabilized laser systems, and first experiments to investigate the generation of distant atomic entanglement, the implementation of a simple quantum network and controlled single-atom single-photon interaction. The experimental setup comprises two ion traps at a distance of one meter, which are independent but identical in construction. We introduce novel realizations of a photoionization method for calcium atoms, a laser frequency stabilization scheme and a radio frequency pulse sequencer. Linear Paul traps are used to load individual or strings of calcium ions. High-numerical aperture objectives mounted on sub-micrometer positioning systems allow us to address and manipulate a single ion with strongly focused light fields and collect the emitted fluorescence photons from a large solid angle. With these bidirectional single-atom single-photon interfaces the essential building blocks of a quantum network with atoms and photons are available. One of the main objectives we pursue with the double-trap apparatus is the generation of entanglement between ions in different traps, which is one of the key tasks in a quantum network. We compare single- and two-photon schemes for probabilistic entanglement creation and analyze how the performance of the schemes depends on photon detection efficiency. As a first step towards the experimental realization of such schemes we measure the second order correlation function of fluorescence photons emitted from two distant trapped ions. We observe high-contrast quantum (Hong-Ou-Mandel) interference, proving the indistinguishability of photons emitted from different ions, which is a prerequisite for the considered entanglement schemes. To investigate the interaction of single photons and a single atom in free space we combine the ion trap apparatus with a tunable, narrowband spontaneous parametric down-conversion source. The high-numerical aperture objectives allow us to efficiently couple bandwidth-tunable photons to a single trapped ion. Such atom-photon interaction at the single-particle level is the most fundamental process in quantum optics and lies literally at the focus of many experimental implementations of quantum information processing protocols and of studies of the (quantum) limits of matter -light interaction, which were so far restricted to weak coherent light fields. Here we observe individual atomic absorption events of one photon of an entangled pair by employing a quantum jump detection scheme. To characterize and control the interaction process we modify the spectral properties of the down conversion photons and perform single-photon spectroscopy of a single ion. To go beyond experiments with cohe rent fields we study temporal correlations between the absorption of photons from the spontaneous parametric down conversion source by a single ion and the detection of the corresponding partner photon.
For the applied scheme we observe promising indications that the quantum correlations of photons produced in spontaneous parametric down conversion are preserved in the resonant interaction with a single atom. The implementation of more efficient atom-light coupling and entanglement transfer schemes with our apparatus is outlined.