Session H2: Quantum Optics

Macroscopic optomechanical superposition via periodic qubit flipping

We propose a scheme to generate macroscopic superpositions of well distinguishable coherent states in an optomechanical system via periodic photonic qubit flipping. Our scheme does not require a single-photon strong coupling rate of an optomechanical system. The generated mechanical superposition state can be reconstructed using mechanical quantum state tomography. The proposed scheme relies on recycling of an atom, fast atomic qubit flipping, and coherent state mapping between a single-photon superposition state and an atomic superposition state. Our scheme can generate macroscopic superposition states with very large separation which is not restricted by decoherence of the atomic state or decay of the cavity photon. We discuss the experimental feasibility of our proposal under current technology.   

Single Photon Modulation by the Collective Emission of an Atomic Chain

We study the collective spontaneous emission of a linear atomic chain excited by a single photon. The interaction between the atoms and the common vacuum field can significantly change the eigenenergy and the spontaneous emission rate of the system. The system can be in the superposition of the super-radiant and sub-radiant modes which results in the non-exponential decay dynamics. The emitted photon can be either super-radiant photon or sub-radiant photon, and we can also tune their frequency and linewidth by simply changing the polarization of the incident photon or the atomic separation.   

Quantum memory of single $\gamma $-ray photon by Doppler Frequency Comb

We propose to store and retrieve a single $\gamma $-ray photon by a series of resonant M\"{o}ssbauer targets each moves with different velocities. Such velocity spectrum forms a frequency comb due to Doppler effect, which we name as Doppler frequency comb (DFC). The performance of this $\gamma $-ray photon quantum memory scheme is similar to the usual atomic frequency comb (AFC) which is used for optical quantum storage. However, instead of burning comb structure in a broad inhomogeneous broadening profile, which is not available in M\"{o}ssbauer solids, DFC utilizes very narrow resonant line-width to achieve quantum memory of energetic single $\gamma $-ray photon. Depending on the motion direction of the M\"{o}ssbauer targets, a series of input $\gamma $-ray signals can be retrieved in either the same or reversed order of the input signals.   

Heisenberg Limit Superradiant Superresolving Metrology

We propose a superradiant metrology technique to achieve the Heisenberg limit super-resolving displacement measurement by encoding multiple light momenta into a three-level atomic ensemble. We use 2N coherent pulses to prepare a single excitation superradiant state in a superposition of two timed Dicke states that are 4N light momenta apart in momentum space. The phase difference between these two states induced by a uniform displacement of the atomic ensemble has 1/4N sensitivity. Experiments are proposed in crystals and in ultracold atoms.   

Trade-off between information gain and fidelity under weak measurements

It is of general interest how a quantum measurement may disturb a quantum system while it gives information on the state of the system. We study a trade-off relation between the information gain and the output fidelity for a quantum non-demolition (QND) measurement scheme for photon numbers. To this aim, we obtain general expressions for the information gain and the output fidelity for an arbitrary initial state. We particularly investigate how the sum of these two quantities varies with the measurement strength for some general classes of states, through a single measurement or sequential measurements. We also show that the information on the photon-number distribution can always be fully retrieved for an arbitrary initial state by a large number of sequential measurements.   

Quantum amplification by superradiant emission of radiation

A laser generates light through stimulated emission of radiation and requires population inversion. Quantum interference can yield lasing without inversion. However, such phase-sensitive quantum amplification still requires some atomic population in the excited state. We present a new kind of light amplifier (called the QASER) based on collective parametric resonance which, contrary to a laser, does not need any population in the excited state and generates high frequency coherent radiation by driving an atomic ensemble with a much smaller frequency. The amplification mechanism of the QASER is governed by the difference combination parametric resonance which occurs when the driving field frequency matches the frequency difference between two normal modes of the coupled light atom system. To achieve gain one must suppress AC Stark shift caused by the driving field. The resulting superradiant amplifier holds promise for a new kind of generator of high frequency (e.g. XUV or x-ray) coherent radiation utilizing a low frequency (e.g. infrared) drive. We present an experiment which demonstrates the QASER amplification mechanism in electronic circuit in the radio frequency range.   

Resonances and super-radiance in the strong picosecond pumping of dense Rubidium

A recent series of experiments at Texas A\&M University has observed sideband emission at the effective Rabi frequency as well as super-radiant emission in both the forward and backward direction from a dense Rubidium cell pumped by a very strong pico-second pulse. The forward and backward emission spectrum was collected as the pump pulse was scanned over resonance of the D2 and D1 lines. There are a number of interesting observations that can be made through analysis of these spectrums. The most intriguing of which, is the possible observance of a resonant version of the previously proposed QASER\footnote{A. A. Svidzinsky, L. Yuan, M. O. Scully, PRX 3, 041001 (2013).} mechanism. I will briefly discuss the experiment then focus on how to theoretically understand the results.   

Towards Quantum Amplification by Superradiant Emission of Radiation

Atomic coherence effect has revealed many fascinating phenomena. Recently, our group proposed a new amplification mechanism which requires no population in excited state yet light amplification at high frequency can be achieved by the parametric resonance between the driving field and the collective superradiant oscillations of the atomic coherence, thus named Quantum Amplification by Superradiant Emission of Radiation (QASER). To get enough gain, QASER requires high atomic density and a strong driving field. In order to observe this effect, we experimentally study the optical properties of dense Rubidium vapor which is strongly pumped by nanosecond laser pulses. Experiments were done with two different setups: (1) heated rubidium cell pumped by tunable pulsed laser and (2) rubidium heatpipe pumped by 1064 nm pulses from Nd:YAG laser. We observed possible coherent emissions in these systems. We also analyze some effects that could hinder the observation of QASER.   

Towards Cooperative Resonance Effects in Rubidium Vapor

We study characteristics of fluorescence generated in rubidium vapor orthogonally to an input beam. In particular, the emissions' dependence on the detuning of the input field and the atomic number density (and therefore the collective frequency) is investigated. Our preliminary results show that there is an optimum number density to achieve maximal emission and that this optimum density depends on how far the input beam is detuned from the D1 transition of rubidium. These results are obtained using a narrow line geometry for the input beam profile. This ongoing work promises to shed light on understanding cooperative resonance effects in rubidium vapor.   

Numerical simulation of QASER in the three-level atomic system

Recently proposed QASER (quantum amplification by superradiant emission of radiation) generates light at higher frequencies than the pumping frequency $\nu_{\mathrm{d}}$ and operates at the difference combination resonance $\nu_{\mathrm{d}}=\omega_{2}$-$\omega_{1}$, which holds promise for a new kind of high-frequency radiation sources. Here we numerically simulate the QASER experiment with a near-resonance pumping pulse in a three-level atomic system as the model of Rb gas. We found the backward emissions of both transitions could have QASER-like gain. We considered possible effects of backward propagating fields to compare with the experimental data. This simulation would provide explanations for QASER experiments.   

A new type of optical parametric amplifier for coherent X-ray generation

Unlike the usual optical parametric amplification, the quantum amplification by super-radiant emission of radiation (QASER) suggests that amplification can only occur when the pump frequency is equal to the difference of the seeded frequencies. The physical mechanism behind QASER is called difference parametric resonance. The difference frequency can be potentially many order of magnitude lower than the seeded frequencies. Therefore, one could use infrared light as pump and build a difference parametric resonance based parametric amplifier for coherent X-ray generation. In this presentation I would like to discuss the mechanism of difference parametric resonance in a coupled-oscillator system, which captures the main physics of QASER as well as other physical realizations of the difference parametric resonance. A perturbation analysis is given to provide more insight into the origin of the amplification process as well as the experimental conditions necessary for realizing difference parametric resonance. We will also briefly discuss several proposals of realizing difference parametric resonance in electronic, mechanical, and acoustic systems.   

A Unified Perspective on the Nature of Bonding in Pairwise Interatomic Interactions

Different classes of ground electronic state pairwise interatomic interactions are referenced to a single canonical potential using explicit transformations. These approaches have been applied to diatomic molecules N$_2$, CO, H$_2$Ca$_2$, O$_2$; argon dimer, and one-dimensional cuts through multidimensional potentials of OC-HBr, OC-HF, OC-HCCH, OC-HCN, OC-HCl, OC-HI, OC-BrCl, and OC-Cl2 using accurate semi-empirically determined interatomic Rydberg-Klein-Rees (RKR) and morphed intermolecular potentials. These different bonding categories are represented in these systems which vary from van der Waals, halogen bonding, hydrogen bonding to strongly bound covalent molecules with binding energies covering three orders of magnitude from 84.5 cm$^{-1}$ approaches were then utilized to give a unified perspective on the nature of bonding in the whole range of diatomic and intermolecular interactions investigated to 89600.6 cm$^{-1}$ in ground state dissociation energies.