Session B6: Quantum and Coherent Control I

Interplay of classical and quantum dynamics in an ensemble of hot atoms

When a transparency window opens up for a probe light, transmitted through a resonant thermal ensemble of atomic system driven by a strong classical field, how much of it is actually due to quantum superposition of states and how it develops in the midst of classical processes like optical pumping and thermal diffusion? Here we address these questions by stroboscopically probing a closed $\Lambda$-like atomic system in a Rubidium vapor cell, driven by coherent and incoherent field, with a 100 ns probe pulse. Time evolution of transmitted probe peak shows an overshoot with turn-on of control, indicating signatures of lasing without inversion. Corresponding rise time is controlled by the driven field with a distinct signature of half cycle Rabi flop. Then optical pumping process leads to a steady state over a longer time scale, that sustain the dark state, which usually probed in EIT like spectrum. Eventual turning-off of control leads to sudden fall in transmission, which carry a unique signature to quantify close and open system, and in particular, induced coherence in the system. We use detailed numerical simulation and toy models to explain our observations and support our claims. We believe our results can provide a metric for testing competing quantum or classical hypothesis.   

Polarization control of spontaneous emission for rapid quantum state initialization

The practical implementation of quantum computers places two specific requirements on the lifetime of a qubit, namely, long relevant decoherence times, and rapid state initialization times. There is a need for protocols wherein the spontaneous emission rate of a quantum system can be selectively increased so that long state lifetimes can be maintained during operation, and upon demand, selectively decreased so that the cooling time can be drastically shortened in duration when qubit purity needs to be restored. We propose an efficient method to selectively enhance the spontaneous emission rate of a quantum system by changing the polarization of an incident control field, and exploiting the polarization dependence of the system's spontaneous emission rate. This differs from the usual Purcell enhancement of spontaneous emission rates as it can be selectively turned on and off. Using a three-level $\Lambda$ system in a quantum dot placed in between two silver nanoparticles and a linearly-polarized, monochromatic driving field, we present a protocol for rapid quantum state initialization; while maintaining long coherence times for control operations. This process increases the overall amount of time that a quantum system can be effectively utilized for quantum operations.   

Preparation of a single highly vibrationally excited quantum state using Stark induced adiabatic Raman passage

Stark induced adiabatic Raman passage (SARP) allows us to prepare an appreciable concentration of isolated molecules in a specific highly excited vibrational level. As a demonstration, we transfer nearly 100{\%} of the HD (v$=$0, J$=$0) in a supersonically expanded molecular beam of HD molecules to HD (v$=$4, J$=$0). This is achieved with a sequence of partially overlapping nanosecond pump (355 nm) and Stokes (680 nm) single-mode laser pulses of unequal intensities. The experimental spectral broadening with pump to Stokes delay and saturation against Stokes power suggest that complete population transfer has been achieved from the initial HD (v$=$0, J$=$0) to the target (v$=$4, J$=$0). By comparing our experimental data with our theoretical calculations we are able to draw two important conclusions: (1) using SARP a large population (\textgreater 10$^{\mathrm{10\thinspace }}$molecules per laser pulse) is prepared in the (v$=$4, J$=$0) level of HD, and (2) the polarizability $\alpha_{\mathrm{00,40}}$ (0.6 x 10$^{\mathrm{-41\thinspace }}$Cm$^{\mathrm{2}}$V$^{\mathrm{-1}})$ for the (v$=$0, J$=$0) to (v$=$4, J$=$0) Raman overtone transition is only about five times smaller than $\alpha_{\mathrm{00,10}}$ for the (v$=$0, J$=$0) to (v$=$1, J$=$0) fundamental Raman transition. This capability of preparing selected, highly excited vibrational quantum states of molecules opens new opportunities for fundamental scattering experiments.   

Four-Photon Stark Induced Ladder Climbing Prepares Large Ensemble of H$_{\mathrm{2\thinspace }}$in Selected High Lying Vibrational Levels

To selectively prepare highly vibrationally excited quantum states of molecules like H$_{\mathrm{2}}$, we present a novel multi-photon ladder-climbing technique where the successive rungs of the ladder are connected by Stark-induced adiabatic Raman passage (SARP). Previously, we have demonstrated that SARP achieves complete population transfer from the $v=$0 to the $v=$1 and $v=$4 levels of H$_{\mathrm{2}}$. We show here that SARP can be generalized into a continuously coupled, multiphoton adiabatic passage which uses one or more intermediate states having strong Raman coupling to access highly vibrationally excited states weakly coupled to the ground state. As an example, we consider the case of four-photon coherent excitation to high vibrational levels of H$_{\mathrm{2}}$ via an intermediate level coupled to both the initial and target levels by two-photon SARP. Using a sequence of commercially available single mode, nanosecond lasers, a pump pulse partially overlapping with two Stokes pulses, we show that the complete population of $v=$0 can be selectively transferred to the most weakly coupled $v=$6 and $v=$9 vibrational levels of H$_{\mathrm{2}}$, without leaving any population stranded in the intermediate level. The present method provides a practical way of generating an entangled pair of fragments without resorting to an ultracold system.   

Coherent and simultaneous addressing of individual atoms in a 1D Optical Lattice

Coherent addressing and independent control of individual atoms are key elements for the lattice-based quantum computing. While recent approaches using a focused addressing laser beam enables a fast and high-fidelity addressing of individual atoms, the process is inevitably sequential for independent control of multiple qubits. On the other hand, when individual atoms are addressed by a position-dependent Zeeman shift, a simultaneous addressing is possible because each atom has a distinct identity. In previous experiments, however, use of large B-gradient to overcome an inhomogeneous broadening due to differential ac-Stark shift complicated the noise control and hindered the coherent addressing. Instead of using a large B-gradient, we reduce the linewidth down to the Fourier limit by using ``magic polarization'' that removes the trap-induced differential shift. In our demonstration, single 7Li atoms in a 1D lattice with 532-nm spacing are resolved in RF domain with the nearest-site resolution preserving long coherence. Moreover, two adjacent atoms are simultaneously addressed and controlled independently, paving the way for a more generalized parallel processing of multiple qubits.   

Fast adiabatic control near the quantum speed limit

The design of fast, high fidelity adiabatic waveforms is an important open challenge in quantum control. We propose a simple optimization scheme for designing fast, accurate adiabatic waveforms that maximize adiabaticity in Berry’s superadiabatic interaction frames and can be applied to both single and multi-qubit control. For single qubit control, the optimized pulses are compared against both non-adiabatic optimal control GRAPE pulses and pulses derived from Slepian window functions, and they are shown to achieve high fidelities at pulse lengths near the quantum speed limit. We also demonstrate the design of a two-qubit entangling operation and implement the scheme using NMR.   

Coherent Control About a Conical Intersection

Conical intersections (CIs) are degeneracies between molecular potential energy surfaces that occur in essentially all molecules with more than three atoms. Many studies have established that CIs allow for non-Born-Oppenheimer (non-adiabatic) molecular dynamics. In addition, CIs have many useful attributes for coherent control that have not been fully studied. Here we demonstrate two modes of control around a CI that make use of these properties. The first method uses a continuous light field, resonant absorption, and stimulated emission to control the population on two intersecting electronic states. The second method uses a pulsed light field and the geometric phase accumulated by a wavepacket traversing a CI to control the shape of the wavepacket.   

Doppler-free spectroscopy of the atomic rubidium fine structure using ultrafast spatial coherent control method

Spectral programming solutions for the ultrafast spatial coherent control (USCC) method to resolve the fine-structure energy levels of atomic rubidium are reported. In USCC, a pair of counter-propagating ultrashort laser pulses are programmed to make a two-photon excitation pattern specific to particular transition pathways and atom species, thus allowing the involved transitions resolvable in space simultaneously. With a proper spectral phase and amplitude modulation, USCC has been also demonstrated for the systems with many intermediate energy levels. Pushing the limit of system complexity even further, we show here an experimental demonstration of the rubidium fine-structure excitation pattern resolvable by USCC. The spectral programming solution for the given USCC is achieved by combining a double-V-shape spectral phase function and a set of phase steps, where the former distinguishes the fine structure and the latter prevents resonant transitions. The experimental results will be presented along with its application in conjunction with the Doppler-free frequency-comb spectroscopy for rubidium hyperfine structure measurements.