We report an experiment in which an optical vortex is stored in a vapor of Rb atoms. Because of its 2pi phase twist, this mode, also known as the Laguerre-Gauss mode, is topologically stable and cannot unwind even under conditions of strong diffusion. For comparison, we stored a Gaussian beam with a dark center and a uniform phase. Contrary to the optical vortex, which stays stable for over 100 micros, the dark center in the retrieved flat-phased image was filled with light after a storage time as short as 10 micros. The experiment proves that higher electromagnetic modes can be converted into atomic coherences and that modes with phase singularities are robust to decoherence effects such as diffusion. This opens the possibility to more elaborate schemes for classical and quantum information storage in atomic vapors.

Reversible and coherent storage of light in atomic medium is a promising method with possible application in many fields. In this work, arbitrary two-dimensional images are slowed and stored in warm atomic vapor for up to 30 µs, utilizing electromagnetically induced transparency. Both the intensity and the phase patterns of the optical field are maintained. The main limitation on the storage resolution and duration is found to be the diffusion of atoms. A technique analogous to phase-shift lithography is employed to diminish the effect of diffusion on the visibility of the reconstructed image.PACS numbers: 42.50. Gy, 32.70.Jz When a resonant light pulse ("probe") impinges upon a gas of atoms, it is strongly absorbed, exciting the atoms to an upper state. However, if a second "pump" beam is present, which couples a second state to the same excited state, than the "probe" pulse will be able to pass the sample -a phenomenon known as electromagnetically induced transparency (EIT) [1]. The unique properties of EIT allow for a wide range of coherent light-matter phenomena, including non-linear optics [2], entanglement [3], generation of quantum pulses of light [4], and quantum communication [5]. In Ref.[6], the group velocity of the probe pulse, in a medium of ultra-cold atoms, was decreased to 17 m/s and similar results were achieved in warm vapor [7]. In Refs. [8,9,10], it was demonstrated that the probe pulse can be completely stopped, while it is contained inside the medium, by shutting off the pump. The pulse can be recovered by reopening the pump beam after a certain "storage duration". A prominent feature of this technique is the reversible and coherent storage of the information carried by the probe, in the atomic coherences.Here, we report a method for reversibly capturing complex three-dimensional light fields using EIT in atomic vapor [1,8]. The storage experiments discussed above have utilized a transverse Gaussian mode for both pump and probe beams. The current research is focused on slowing and storing information imprinted in the transverse plane of the probe beam ("images"), and on reducing the effects of atomic diffusion. In a previous work [11], the ability to slow images and delay them for several ns was demonstrated, using dispersion from fardetuned absorption lines. In the current work, we use EIT to slow images to a group velocity of several thousands m/s, achieving delays of several µs. We further use the unique properties of EIT to store the images in the atomic medium for a similar duration. The long slowing delays and storage durations are comparable with the typical diffusion time in which atoms cross the image. We demonstrate the deteriorating effect of diffusion by storing images of digits for different durations. Finally, we introduce a technique to diminish the effect of atomic diffusion by alternating the phase of neighboring features. This technique, which is the atomic analogue of the optical phase-shift lithography [12], is demonstrated by storing an image of three lines and study...

We experimentally demonstrate the manipulation of optical diffraction, utilizing the atomic thermal motion in a hot vapor medium of electromagnetically-induced transparency (EIT). By properly tuning the EIT parameters, the refraction induced by the atomic motion may completely counterbalance the paraxial free-space diffraction and by that eliminates the effect of diffraction for arbitrary images. By further manipulation, the diffraction can be doubled, biased asymmetrically to induced deflection, or even reversed. The latter allows an experimental implementation of an analogy to a negative-index lens.Any image, imprinted on a wave field and propagating in free space, undergoes a paraxial diffraction spreading and eventually blurs. In many disciplines, the possibility to reduce or manipulate the diffraction is explored, for purposes such as imaging, wave guiding, microlithography, and all-optical light processing. As was recently demonstrated, arbitrary images can be imprinted on light pulses which are dramatically slowed when traversing a medium of room-temperature atoms [ 1,2], via the process of electromagnetically induced transparency [ 3,4]. In addition to the regular free-space diffraction, the slow-light images undergo diffusion due to the thermal atomic motion [ 5,6]. Here we report an experimental demonstration of a novel technique to eliminate

We present a theoretical model for electromagnetically induced transparency (EIT) in vapor, that incorporates atomic motion and velocity-changing collisions into the dynamics of the densitymatrix distribution. Within a unified formalism we demonstrate various motional effects, known for EIT in vapor: Doppler-broadening of the absorption spectrum; Dicke-narrowing and time-offlight broadening of the transmission window for a finite-sized probe; Diffusion of atomic coherence during storage of light and diffusion of the light-matter excitation during slow-light propagation; and Ramsey-narrowing of the spectrum for a probe and pump beams of finite-size.

When performing precision measurements, the quantity being measured is often perturbed by the measurement process itself. This includes precision frequency measurements for atomic clock applications carried out with Ramsey spectroscopy. With the aim of eliminating probe-induced perturbations, a method of generalized auto-balanced Ramsey spectroscopy (GABRS) is presented and rigorously substantiated. Here, the usual local oscillator frequency control loop is augmented with a second control loop derived from secondary Ramsey sequences interspersed with the primary sequences and with a different Ramsey period. This second loop feeds back to a secondary clock variable and ultimately compensates for the perturbation of the clock frequency caused by the measurements in the first loop. We show that such a two-loop scheme can lead to perfect compensation of measurement-induced light shifts and does not suffer from the effects of relaxation, time-dependent pulse fluctuations and phase-jump modulation errors that are typical of other hyper-Ramsey schemes. Several variants of GABRS are explored based on different secondary variables including added relative phase shifts between Ramsey pulses, external frequency-step compensation, and variable second-pulse duration. We demonstrate that a universal anti-symmetric error signal, and hence perfect compensation at finite modulation amplitude, is generated only if an additional frequency-step applied during both Ramsey pulses is used as the concomitant variable parameter. This universal technique can be applied to the fields of atomic clocks, high-resolution molecular spectroscopy, magnetically induced and two-photon probing schemes, Ramsey-type mass spectrometry, and to the field of precision measurements. Some variants of GABRS can also be applied for rf atomic clocks using CPT-based Ramsey spectroscopy of the two-photon dark resonance.