Session P5: Quantum Memory and Nonlinear Optics

Storage of Multiple Images using a Gradient Echo Memory in a Vapor Cell

The development of a quantum memory (QM) that can store quantum states of light without a significant degradation is an active field of research, as QMs play a fundamental role in quantum information science. A number of different techniques have been developed for their implementation. In particular, the gradient echo memory (GEM) offers a promising technique with high recovery efficiencies and the ability of temporal multiplexing. We show that it is possible to use GEM for the simultaneous storage of multiple images, thus extending the multiplexing properties of this technique to the spatial domain. In order to implement the QM we use a 7~cm-long $^{85}$Rb vapor cell with Ne buffer gas at a pressure of 5~Torr and a linearly varying magnetic field of 15~$\mu$T/cm along the cell. We use this configuration for the storage of two different images with a temporal delay between them and show that it is possible to temporally distinguish them after the retrieval process. We have obtained recovery efficiencies up to 8~\% and storage times over 4~$\mu$s while still retaining a good spatial fidelity between the input and retrieved images. Finally, we study the effect of atomic diffusion on the storage of images and find that it limits the spatial resolution of the retrieved images.   

Characterization of a high efficiency optical memory for the storage of quantum light states

We have developed a coherent optical storage device based on a Gradient Echo Memory scheme showing efficiencies of above 65\%. The memory is realized in a warm vapor of $^{87}$Rb atoms utilizing a $\Lambda$-type energy level scheme. We use a co-propagating, co-rotating circular polarized pump beam coupled with pulsed coherent states to create an off-resonant Raman absorption line suitable for storage. Through sufficient filtration of the strong pump field, the stored light pulse is retrieved after a desired time and subjected to time-domain homodyne tomography. By repeating this sequence on an ensemble of 50 000 identical coherent states we gain enough information to completely reconstruct the quantum state of light retrieved from the memory. Furthermore, by repeating this characterization for a set of coherent states with at sufficient range of amplitudes we can completely characterize the memory process itself. This is possible by implementing a method devised by our group called coherent state Quantum Process Tomography which also has the capability to predict how well the memory will perform on any arbitrary quantum input state. We show our current storage efficiencies and what needs to be further done to demonstrate a true high efficiency, quantum optical memory.   

Nonlinear optics in atomic ytterbium

We have performed degenerate four-wave mixing experiments with cryogenically-cooled atomic ytterbium. We use buffer-gas cooling to prepare high optical density samples at a temperature of 5~K, cold enough to resolve the different isotopes and hyperfine transitions. We observe four-wave mixing. With cross-polarized pump and probe beams, we observe a conjugate beam only when the laser is closely detuned from the $^1S_0 (F=1/2) \rightarrow ^1P_1 (F=1/2)$ transition of the $^{171}$Yb ($I=1/2$) isotope. Progress towards the generation of squeezed light will be discussed.   

Multi-Spatial-Mode Noiseless Optical Amplifier

One of the most commonly-used optical linear amplifiers, the phase-insensitive amplifier (PIA), always degrades the signal-to-noise ratio of the amplified signal, and the degradation depends on the amount of gain. This problem can be avoided by the proper use of a phase-sensitive amplifier (PSA). An ideal PSA, under certain conditions, can amplify signals without degrading the signal-to-noise ratio of the input. In this sense, the PSA behaves as a noiseless amplifier. In particular, if the PSA can support multiple spatial modes it could perform noiseless amplification of images which is an important goal in imaging research. We implement a phase-sensitive optical amplifier using a four-wave mixing (4WM) process in rubidium vapor. We observe performance near the quantum limit for this type of amplifier over a range of experimental parameters. We compare the results to the ones expected for a PIA and find that our PSA behaves better than the PIA, as expected. Additionally, we observe that the amplifier supports multiple spatial modes (images) without a significant degradation of the input signal-to-noise ratio. To confirm the multi-spatial-mode character we study the behavior of the 4WM-based PSA for different spatial patterns and different spatial frequencies.   

Quantum assisted enhancement of optical magnetometer with squeezed vacuum in hot Rb vapor

We demonstrate enhancement to the sensitivity of an optical magnetometer based on the nonlinear magneto-optical Faraday effect in $^{87}$Rb vapor with the use of squeezed vacuum. We generate quantum squeezed vacuum states via the polarization self-rotation effect in hot $^{87}$Rb vapor exhibiting noise spectrum suppression ranging from frequencies of a few hundred Hz to several MHz. Injection of such squeezed states into a magneto-optical magnetometer provides broad band noise suppression of close to 2 dB. We study various parameters of the magnetometer such as Rb cell temperature, pump power, and the noise spectrum of the probe signal to identify the most favorable conditions for quantum enhanced magnetometry. Our experimental arrangement offers potential quantum improvement to the most sensitive magnetometers at frequencies down to hundreds of Hz, which can be useful for biological, geophysical, medical, or military sensing applications.   

Few-Photon Cross-Phase Modulation in Rb-Filled Photonic Bandgap Fibers

We produce cross-phase shifts (XPS) of a few milliradians on a meter beam with $<$20 signal photons, using a two-photon transition in Rb vapor confined to photonic bandgap fibers. A weak 780-nm signal beam tuned close to the 5S$_{1/2} \to $ 5P$_{3/2}$ transition of Rb-85 is used to impart a nonlinear phase shift on a strong, counter-propagating 776-nm meter beam which is tuned close to the 5P$_{3/2} \to $ 5D$_{5/2}$ transition. Using the selection rules of the relevant transitions involved, we measure the XPS as a slight polarization rotation of the meter beam. A XPS of $\sim $0.3 milliradian per signal photon is induced in our system, which, to our knowledge, represents the largest such nonlinear phase shift induced in a single-pass through a room-temperature nonlinear medium. The system response time is shown to be $<$5 ns, primarily determined by the transit-time of the atoms across the fiber core. Such a system offers the potential to explore novel quantum nonlinear effects at ultralow powers.   

Superresolution at the quantum limit with coherent light and a homodyne-based parity detection scheme

We study a simple interferometric scheme that uses coherent light and a quantum inspired detection strategy based on the measurement of the parity of photon number in one of the output modes. The scheme provides sub- Rayleigh resolution while still operating at the shot noise limit in terms of the detected photon power. Although the parity observable can be implemented using photon number resolving detectors, accurate and efficient photon number resolution at large photon numbers becomes difficult. Alternatively, we show that the super-resolving parity signal can be inferred from a simple homodyne based measurement of the quadratures of the output coherent light, also at the shot noise limit. Due to its inherent simplicity and effectiveness, the scheme can potentially be used to improve existing technologies in satellite imaging and remote sensing such as in quantum laser radar (LADAR), where atmospheric absorption forbids the use of nonclassical states of light for any quantum enhancement and renders coherent light interferometry as the optimal choice.   

Coherent Rayleigh-Brillouin Scattering in High Intensity Laser Fields

We have performed coherent Rayleigh-Brillouin scattering (CRBS) experiments on collisional gasses subject to laser intensities beyond those considered perturbative to the gasses' thermodynamic parameters. CRBS is a four wave mixing scheme traditionally used for gas diagnostic applications when utilizing low intensity laser pulses. In these experiments high intensity laser pulses are used which yield signal lineshapes inconsistent with perturbative theory. Gas heating, weak ionization, and three dimensional effects are discussed as possible nonlinear optical effects which would have to be accounted for in order to model the high intensity regime. The cause of this altered lineshape may furthermore be used to diagnose the full effect of the laser pulses on the gas.   

N-Photon Wavepackets Interacting with an Arbitrary Quantum System

Traveling nonclassical states of light are important resources for quantum metrology, secure communication, and quantum networks. Motivated by this, we derive master equations for an arbitrary quantum system (e.g. a quantum harmonic oscillator or a multi-level atom) interacting with a wavepacket of light prepared in an N-photon Fock state. We then generalize this to N-photon states with arbitrary spectral distribution functions and wavepackets in two polarization (or spatial) modes. Our method also allows the calculation of output field quantities. As an illustration of our formalism, we explore the strong coupling regime for an atom in free space and investigate the scattering of Fock states from a two-level atom.   

Single atom lensing

The lens is a fundamental optical device for redirecting the path of light. We have observed the first lensing of light by a single atom. A $^{174}$Yb$^{+}$ ion is confined in a 3D RF Paul trap, laser cooled near the Doppler limit on the $\lambda $=369.5 nm transition, and imaged at wavelength scale resolution with a large aperture phase Fresnel lens (NA=0.64). Changes to the wavefront of the illumination light are measured from background-subtracted images at different image defocusings and laser detunings. The wavefront was observed to converge for negative laser detunings (positive focal lengths), diverge for positive detunings (negative focal lengths), and agrees with an analytic microscope model of a dipole radiator. The effective focal length of the atom is on the order of lambda near resonance.