Photons, the particles of light, are in most conditions very weakly interacting. Nevertheless, it is possible to make them interact by altering environmental conditions, for instance, interior certain materials or squeezing confined geometries. In this Colloquium topic photons interacting strongly when a one-dimensional geometry discussed from experimental and theoretical perspectives.

By mapping the strong interaction between Rydberg excitations in ultra-cold atomic ensembles onto single photons via electromagnetically induced transparency, it is now possible to realize a medium which exhibits optical nonlinearity at level of individual photons. We review theoretical concepts and experimental state-of-the-art this exciting new field, discuss first applications field all-optical quantum information processing.

Abstract This tutorial introduces the theoretical and experimental basics of electromagnetically induced transparency (EIT) in thermal alkali vapors. We first give a brief phenomenological description EIT simple three-level systems stationary atoms derive analytical expressions for optical absorption dispersion under conditions. Then we focus on how motion affects various parameters system. Specifically, analyze Doppler broadening transitions, ballistic versus diffusive atomic limited-volume...

We report an experiment in which optical vortex is stored a vapor of Rb atoms. Because its $2\ensuremath{\pi}$ phase twist, this mode, also known as the Laguerre-Gauss topologically stable and cannot unwind even under conditions strong diffusion. For comparison, we Gaussian beam with dark center uniform phase. Contrary to vortex, stays for over $100\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$, retrieved flat-phased image was filled light after storage time short $10\text{...

Reversible and coherent storage of light in an atomic medium is a promising method with possible applications many fields. In this work, arbitrary two-dimensional images are slowed stored warm vapor for up to $30\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$, utilizing electromagnetically induced transparency. Both the intensity phase patterns optical field maintained. The main limitation on resolution duration found be diffusion atoms. A technique analogous phase-shift lithography employed...

The strong interaction between individual Rydberg atoms provides a powerful tool exploited in an ever-growing range of applications quantum information science, simulation, and ultracold chemistry. One hallmark the is that both its strength angular dependence can be fine-tuned with great flexibility by choosing appropriate states applying external electric magnetic fields. More more experiments are probing this at short atomic distances or such high precision perturbative calculations as...

Light storage, the controlled and reversible mapping of photons onto long-lived states matter [1], enables memory capability in optical quantum networks [2-6]. Prominent storage media are warm alkali gases due to their strong coupling spin [7,8]. In a dense gas, random atomic collisions dominate lifetime coherence, limiting time few milliseconds [9,10]. Here we present experimentally demonstrate scheme that is insensitive spin-exchange collisions, thus enabling long times at high densities....

We provide a theoretical framework describing slow-light polaritons interacting via atomic Rydberg states. The method allows us to analytically derive the scattering properties of two polaritons. identify parameter regimes where polariton-polariton interactions are repulsive. Furthermore, in regime attractive interactions, we multiple two-polariton bound states, calculate their dispersion, and study resulting resonances. Finally, two-particle allow effective low-energy many-body Hamiltonian....

We implement a new, noise-free, broadband light storage scheme, opening the way to faithful multiphoton synchronization.

We show that two photons coupled to Rydberg states via electromagnetically induced transparency can interact an effective Coulomb potential. This interaction gives rise a continuum of two-body bound states. Within the continuum, metastable are distinguished in analogy with quasibound tunneling through potential barrier. find multiple branches whose energy spectrum is governed by potential, thus obtaining photonic analogue hydrogen atom. Under certain conditions, wave function resembles...

Nuclear spins of noble-gas atoms are exceptionally isolated from the environment and can maintain their quantum properties for hours at room temperature. Here we develop a mechanism entangling two such distant macroscopic ensembles by using coherent light input. The interaction between in each ensemble is mediated spin-exchange collisions with alkali-metal spins, which only virtually excited. relevant conditions experimental realizations ^{3}He or ^{129}Xe outlined.

Narrowband single photons that couple well to atomic ensembles could prove essential for future quantum networks, but the efficient generation of such remains an outstanding challenge. We realize a spatially-multiplexed heralded source are inherently compatible with commonly employed D2 line rubidium. Our is based on four-wave mixing in hot rubidium vapor, requiring no laser cooling or optical cavities, and generates high rate low noise. use Hong-Ou-Mandel interference verify...

An ensemble of noble-gas nuclear spins is a unique quantum system that could maintain coherence for many hours at room temperature and above, owing to exceptional isolation from the environment. This isolation, however, mixed blessing, limiting ability these ensembles interface with other systems coherently. Here we show spin-exchange collisions alkali-metal atoms render without impeding their long times. We formulate many-body theory hybrid reveal collective mechanism strongly couples...

Efficient synchronization of single photons that are compatible with narrow band atomic transitions is an outstanding challenge, which could prove essential for photonic quantum information processing. Here we report on the independently generated using a room-temperature memory. The photon source and memory interconnected by fibers employ same ladder-level scheme. We store retrieve heralded end-to-end efficiency η_{e2e}=25% final antibunching g_{h}^{(2)}=0.023. Our process results in over...

Vortices are topologically nontrivial defects that generally originate from nonlinear field dynamics. All-optical generation of photonic vortices-phase singularities the electromagnetic field-requires sufficiently strong nonlinearity is typically achieved in classical optics regime. We report on realization quantum vortices photons result a photon-photon interaction optical medium. The causes faster phase accumulation for copropagating photons, producing vortex-antivortex pair within...

We present a scheme for eliminating the optical diffraction of slow light in thermal atomic medium electromagnetically induced transparency. Nondiffraction is achieved an arbitrary paraxial image by manipulating susceptibility momentum space, contrast to common approach, which employs guidance specific modes real space. For negative two-photon detuning, moving atoms drag transverse components unequally, resulting Doppler trapping two dimensions.

We propose a scheme for realizing fractional quantum Hall states of light. In our scheme, photons two polarizations are coupled to different atomic Rydberg form flavors polaritons that behave as an effective spin. An array optical cavity modes overlapping with the cloud enables realization spin-$1/2$ lattice. show dipolar interaction between such polaritons, inherited from states, can be exploited create flat, topological band single spin-flip excitation. At half filling, this gives rise...

Coherent diffusion pertains to the motion of atomic dipoles experiencing frequent collisions in vapor while maintaining their coherence. Recent theoretical and experimental studies on effect coherent key Raman processes, namely, spectroscopy, slow polariton propagation, stored light, are reviewed this Colloquium.

Abstract Coherent optical memories will likely play an important role in future quantum communication networks. Among the different platforms, based on ladder-type orbital transitions atomic gasses offer high bandwidth (>100 MHz), continuous (on-demand) readout, and low-noise operation. Here we report upgraded setup of our previously-reported fast ladder memory, with improved efficiency lifetime, reduced noise. The upgrade employs a stronger control field, wider signal beam, density,...

We study quantum correlations between interacting photons realized through co-propagating Rydberg polaritons. show that the evolution of $n$-photon wavefunction is governed by a multiband dispersion featuring one massive mode and $n-1$ degenerate modes, such as two Dirac cones for $n=3$. The band structure exhibits an $n$-fold rotational symmetry, including warped light cone, in contrast to single-band, parabolic approximation often assumed For three photons, breaks symmetry photon pair...