Summary form only given. Achieving photon-photon interactions is one of the main objectives of the quantum information technology community, since these interactions provide the basis for deterministic quantum gates with high fidelity. In the last few years new milestones in creating phase-shifts [1] and quantum gates[2] have been achieved. Researching resource-moderate experimental realizations for the future development of this field provides a scalable framework for devices and opportunities for deployment in quantum communication networks.We highlight photon-photon interactions in unexplored physical regimes that are outside the standard approach of single atom QED or Rydberg transitions. Building on the first complete quantum optical characterization of an engineered free space phase shift triggered via atomic Kerr nonlinearities [3], we characterize novel architectures using the interaction of single photons in both a room temperature atomic vapor and cold atomic ensembles in miniaturized cavity QED setups. These different systems provide very compelling pathways to achieving photonphoton interactions.
Summary form only given. Achieving photon-photon interactions is one of the main objectives of the quantum information technology community, since these interactions provide the basis for deterministic quantum gates with high fidelity. In the last few years new milestones in creating phase-shifts [1] and quantum gates[2] have been achieved. Researching resource-moderate experimental realizations for the future development of this field provides a scalable framework for devices and opportunities for deployment in quantum communication networks.We highlight photon-photon interactions in unexplored physical regimes that are outside the standard approach of single atom QED or Rydberg transitions. Building on the first complete quantum optical characterization of an engineered free space phase shift triggered via atomic Kerr nonlinearities [3], we characterize novel architectures using the interaction of single photons in both a room temperature atomic vapor and cold atomic ensembles in miniaturized cavity QED setups. These different systems provide very compelling pathways to achieving photonphoton interactions.
A long-standing goal of quantum optics is to reach a regime where interactions between single photons can take place. This would lead to new perspectives in the domain of quantum communication and quantum simulation. This work presents the assembly and characterization of a hybrid experimental platform for the study and creation of deterministic interactions between single photons, an important milestone in quantum technologies. The experimental setup combines different technologies from cavity quantum electrodynamics and cold atoms techniques. A cold atom cloud is transported in a specially designed optical resonator, allowing the temporary conversion of intracavity photons into atomic Rydberg polaritons, creating a medium which enables strong interactions. This setup should allow us to deterministically entangle optical photons in order to generate non-classical light, to develop photonic quantum logic gates, and to emulate low-dimensional highly correlated condensed-matter systems. In a first part the setup is presented, including the manner in which the different subsystems work together. In a second part, the first EIT measurements are shown and analyzed, before describing the latest advancement made.
Photons are ideal carriers of quantum information: they are fast, low-loss, and compatible with mature photonic infrastructure.Linear optical quantum computing exploits these properties but relies on probabilistic measurements and post-selection, incurring an exponential resource overhead that limits scalability [1].Circumventing this bottleneck requires deterministic quantum logic gates, which in turn demand strong, controllable interactions between individual photons.Photons do not interact in free space or in linear optical media.The standard route to attain effective photonphoton interactions is to hybridise photons with interacting matter excitations, forming polaritons.The matter component introduces an interaction-induced anharmonic level shift U into the ladder of polariton states: when one polariton occupies a mode, the next photon sees a shifted resonance (Fig. 1a).The key figure of merit is U/γ, where γ is the total polariton decoherence rate.For U/γ ≪ 1 the nonlinearity is classical; for U/γ ≳ 1 the system enters the quantum nonlinear regime, enabling photon blockade, large conditional phase shifts, and deterministic photonic gates.Atomic Rydberg systems have provided the clearest demonstration of this regime.Atoms excited to Rydberg states -electronic states with large principal quantum numbers n ≫ 1 -possess unique properties [2].Their lifetimes scale as 1/γ ∝ n 3 and transition dipole moments as µ ∝ n 2 , leading to strong dipole-dipole (U ∝ n 4 /r 3 ) or van der Waals (U ∝ n 11 /r 6 ) interactions over long distances r.These interactions can be imprinted onto photons via electromagnetically induced transparency (EIT) [3]: photons hybridise with collective atomic excitations to form slowly propagating Rydberg polaritons that strongly interact with each other over distances of up to tens of micrometres [4].This mechanism has produced photon blockade [5]
Enhancing optical nonlinearities so that they become appreciable on the single photon level and lead to nonclassical light fields has been a central objective in quantum optics for many years. After this has been achieved in individual micro-cavities representing an effectively zero-dimensional volume, this line of research has shifted its focus towards engineering devices where such strong optical nonlinearities simultaneously occur in extended volumes of multiple nodes of a network. Recent technological progress in several experimental platforms now opens the possibility to employ the systems of strongly interacting photons, these give rise to as quantum simulators. Here we review the recent development and current status of this research direction for theory and experiment. Addressing both, optical photons interacting with atoms and microwave photons in networks of superconducting circuits, we focus on analogue quantum simulations in scenarios where effective photon-photon interactions exceed dissipative processes in the considered platforms.
Quantum information science and atom optics are among the most active fields in modem physics. In recent years, many theoretical efforts have been made to combine these two fields. Recent experimental progresses [1-3] have shown the in-principle possibility to perform scalable quantum information processing (QIP) with linear optics and atomic ensembles [4]. One of our main activities is to use atomic qubits as quantum memory and exploit photonic qubits for information transfer and processing to achieve efficient linear optics QIP. On the one hand, utilizing the interaction between laser pulses and atomic ensembles we experimentally investigate the potentials of atomic ensembles in the gas phase to build quantum repeaters for longdistance quantum communication [5], that is, to develop a new technological solution for quantum repeaters making use of the effective qubit-type entanglement of two cold atomic ensembles by a projective measurement of individual photons by spontaneous Raman processes. On the other hand, building on our long experience in research on multi-photon entanglement, we are also working on a number of experiments in the field of QIP with particular emphasis on fault-tolerant quantum computation [6], photon-loss-tolerant quantum computation [7] and cluster-state based quantum simulation [8]. In future, by combining the techniques developed in the above quantum memory and multi-photon interference experiments, we will experimentally investigate the possibility to achieve quantum teleportation between photonic and atomic qubits, quantum teleportation between remote atomic qubits and efficient entanglement generation via classical feed-forward. The techniques that are being developed will lay the basis for future large-scale realizations of linear optical QIP with atoms and photons.
Quantum information science and atom optics are among the most active fields in modern physics. One of our main activities is to use atomic qubits as quantum memory and exploit photonic qubits for information transfer and processing to achieve efficient linear optics quantum information processing (QIP). On the one hand, utilizing the interaction between laser pulses and ultra-cold atomic ensembles we experimentally investigate the potentials of atomic ensembles in the gas phase to build quantum repeaters for long-distance quantum communication, that is, to develop a new technological solution for quantum repeaters making use of the effective qubit-type entanglement of two cold atomic ensembles by a projective measurement of individual photons by spontaneous Raman processes. On the other hand, building on our long experience in research on multi-photon entanglement, The techniques that are being developed will lay the basis for future large-scale realizations of linear optical QIP with atoms and photons.
Mapping the strong interaction between Rydberg excitations in ultra-cold atomic ensembles onto single photons enables the realization of optical nonlinearities which can modify light on the level of individual photons.This novel approach forms the basis of a growing Rydberg quantum optics toolbox, which already contains photonic logic building-blocks such as single-photon sources, switches, transistors, and conditional π-phase shifts.This thesis reports on two experiments investigating strong photon-photon interactions mediated by Rydberg interactions.First, applying a ladder-type EIT scheme to Rydberg atoms in D-states with high principal quantum number results in the Rydberg interaction mediated nonlinearity accompanied by a time-dependent decay of transmission of probe light through the medium.In a joint experimental and theoretical analysis, this effect is attributed to the dephasing of propagating polaritons into stationary Rydberg excitations caused by the state mixing interaction occurring with Rydberg D-state atoms.Second, via a two-photon Raman excitation scheme Rydberg atoms are efficiently excited in a small atomic cloud.As the size of the Rydberg blockade exceeds the dimension of the medium, only a single Rydberg excitation can be present at a time.By fast engineered dephasing this Rydberg excitation is decoupled from the light field.Measurement of the transmitted light and the amount of excited Rydberg atoms gives evidence for the realization of a deterministic single-photon absorber.