Session W40: Quantum Information in AMO Physics

Nonlinear Optics Quantum Computing and Quantum Simulation with Circuit-QED

One approach to quantum information processing is to use photons as quantum bits and rely on linear optical elements for most operations. However, some optical nonlinearity is necessary to enable universal quantum computing. Here, we suggest a circuit-QED approach to photon-based quantum processors and quantum simulators in the microwave regime, including a deterministic two-photon interaction. Our specific example uses a hybrid quantum system comprising a LC resonator coupled to a superconducting flux qubit to implement a nonlinear coupling. Compared to the self-Kerr nonlinearity, we find that our approach has improved tolerance to noise in the qubit while maintaining fast operation. We also envision using a similar resonator and fluxonium qubit system to create higher order photon nonlinearities, which is a generalization of effective two-photon interactions and opens the range of potential Hamiltonians that can be efficiently simulated.   

Two Electromagnetically Induced Transparency Windows and Cross-Phase Modulation with Four-Level Superconducting Artificial Atoms

Superconducting circuit quantum electrodynamics (SCQED) employs microwave transmission lines coupled to artificial atoms, which are typical two-level and recently three-level for electromagnetically induced transparency (EIT). We propose SCQED with a four-level tripod-configuration artificial atom to enable cross-phase modulation between two traveling-wave microwave fields. Our master-equation analysis for three driving fields (``signal,'' ``probe'' and ``coupling'') demonstrates the existence of two distinct EIT transparency windows in the spectral-response profile as a function of coupling and weak fields strength. We provide the first theoretical analysis of this unexpected second window and show its advantages over the known first EIT window. Specifically we show that this second EIT window provides both the signal and probe fields with identical response functions provided that their Rabi frequencies and detunings are the same. Exploiting the second window with judiciously chosen external flux and energy detuning result in low absorption, excellent group velocity matching, and high nonlinearity, thereby enabling strong cross-phase modulation for SCQED.   

Nonlocal Interferometry Using Macroscopic Coherent States and Weak Nonlinearities

Bell's inequality has been violated numerous times in microscopic systems with the use of nonlocal interferometry. Described here will be an extension of the Franson interferometer to the macroscopic case of coherent states entangled in phase. The entanglement is generated using weak nonlinearities, and the entanglement is probed using single photons and homodyne detection. Without loss the predicted nonlocal interference visibility of the interferometer is unity, and the inclusion of atomic absorption allows for a large number of photons to be absorbed with only a small reduction in the visibility. This interferometer can be extended in a straightforward manner to a quantum key distribution scheme using the Ekert protocol to insure security. A method for the extension of the entanglement distance using entanglement swapping is described. This nonlocal interferometer may therefore be of practical use in quantum communications in addition to being of fundamental interest.   

Efficacy of weak measurement reversal for stochastic amplitude damping

A recent experiment demonstrated the restoration of entanglement in a photonic system using weak measurement reversal [S. Kim et al., Nature Physics 8, 117 (2012)]. Here, we analyze the statistical properties of entanglement for pairs and triples of entangled qubits subject to stochastic amplitude damping followed by restoration. After the random disturbance, the state is restored by applying a static weak measurement reversal. We then show that the fidelity of the restored state, and therefore its entanglement, can be restored with high success, despite the statistical fluctuations of the disturbance. In particular, we show that the variance of the entanglement of the restored states is substantially reduced, independent of the strength of the disturbance. We conclude with a proposed experimental implementation.   

Nonlinear waveguide arrays and disorder

Waveguide arrays with quadratic nonlinearity has been studied recently. We investigated the waveguide arrays with quadratic nonlinearity and explored the possibility of generating broadband continuous-variable entanglement in such structures. We propose an integrated approach toward continuous-variable entanglement based on integrated waveguide quantum circuits, which are compact and relatively more stable. We further continued our work on waveguide arrays by studying a hybrid system which contains a combination of linear and nonlinear waveguides. We assume that all the waveguides except the central one are assumed to be linear. The central waveguide is assumed to have $\chi^{(2)}$ nonlinearity. We assume that the central waveguide is pumped through a coherent light. The coupling between the waveguide is achieved by the evanescent overlap of the guided modes. For all the other waveguides in the array the light propagates in the linear regime. We also study the effect of disorder which can be introduced by varying the distance between the waveguides. We are particularly interested in investigating the effect of disorder and quadratic non-linearity in the waveguide array system.   

Squeezing of Spin Waves in a Three-Dimensional Atomic Ensemble

Spin squeezed states (SSS) have generated considerable interest for their potential applications in quantum metrology and quantum information processing. Many protocols for generating SSS in atomic gases rely on the Faraday interaction that creates entanglement between atoms through the coupling of the collective spin of the ensemble to polarization modes of an optical field. Most descriptions of this process rely on an idealized one-dimensional plane wave model of light-matter interactions that is not appropriate for describing a real system consisting of a cigar-shaped cold atomic cloud in dipole trap interacting with a probe laser beam. We provide a first principles three-dimensional model of squeezing via a quantum nondemolition measurement of the collective magnetization for an ensemble of atoms with hyperfine spin $f$. The model includes spin waves, diffraction, paraxial modes, and optical pumping, derived by a full master equation description. Including dissipative dynamics, we find the optimal ensemble geometry and input Gaussian beam parameters for generating spin squeezing. We also study the effect of enhancing the atom-light interface using internal hyperfine control of atoms with large spin $f$.   

Selection of semiconductor quantum dots for multi-qubit encoding using an optical microcavity

Controlling the quantum states of excitons or spin-polarized carriers in semiconductor quantum dots (QDs) has been the focus of a considerable research effort in recent years due to the promise of using this approach to develop a solid state quantum computing architecture. In such experiments, the need to isolate the optical response of a single QD represents a formidable challenge, one that is greatest for QDs with emission wavelengths compatible with existing telecommunications infrastructure due to the lower quantum efficiency of the associated detectors. Encoding qubits in ensembles of QDs would greatly facilitate quantum state readout due to the larger optical signals involved, however the spread of optical transition energies limits the fidelity of the control process. Here we report time-resolved differential transmission experiments on QDs in a dielectric Bragg stack optical microcavity. Our results indicate that the angle dependent transmission resonance of the cavity allows for the separate excitation and detection of distinct subsets of QDs in the ensemble differentiated by their optical transition energies. These findings demonstrate the feasibility of developing a scalable computing architecture based on multi-qubit encoding using semiconductor QDs.   

Quantum plasmonics of a metal nanoparticle array for on-chip nanophotonic network

With the advancement of nanofabrication techniques, metallic nanoparticles have been attracting significant attention due to their novel capabilities offering the prospects of miniaturization, scalability, and strong coherent coupling to single-emitters that conventional photonics cannot achieve. In this work, we investigate an array of metal nanoparticles for on-chip quantum networking, quantum computation and communication on scales far below the diffraction limit. For this purpose, we first consider the transfer of quantum states, including single qubits as plasmonic wave packets, and explore the interference of single plasmons associated with the quantum properties of the plasmon excitation. In addition, we study dipole induced reflection effects in the plasmonic setting. The results seem promising for quantum control applications such as single-photon switching and slow light in the nanoscale. We also propose a scheme of entanglement generation between distant emitters embedded in the array of metal nanoparticles. The techniques introduced in this work may assist in the further theoretical and experimental studies of plasmonic nanostructures for quantum control applications and probing nanoscale optical phenomena.   

Dynamic Hole Trapping Effect in an InAs/AlGaAs quantum dot molecule

It is well established that the charge and spin configurations of single electrons or holes are promising candidates for next generation computational and logic devices. Quantum Dots Molecules (QDMs) are attractive components for confining and manipulating single charges because the discrete energy levels, charge interactions and spin properties can be tailored with size and composition. The strong confinement QDMs causes overlap of wavefunctions and results in different Coulomb interactions and unique energy levels for different numbers of charges and even for distinct spatial distributions of the same total charge. Quantitative measurements of the Coulomb interactions are important in order to understand charge and spin interactions and design structures for device applications. We present a new phenomenon discovered during optical spectroscopy of a QDM with an AlGaAs barrier between two QDs. AlGaAs barrier allows an extra hole to be trapped in a metastable state of the higher energy QD due to the higher barrier potential. This ?dynamic trapped hole? occurs only under certain electric field conditions and perturbs the Coulomb interactions of the other charges present in the QDM. We propose a model of the kinetic pathways that leads to this dynamic hole trapping effect. We compare the energy of states with and without the extra hole in order to understand many body Coulomb interactions that perturb states energies. We then discuss the challenges and opportunities this effect provides for future devices.   

Ideal Multipole Ion Traps from Planar Ring Electrodes

We present designs for multipole ion traps based on a set of planar, annular, concentric electrodes which require only rf potentials to confine ions. We illustrate the desirable properties of the traps by considering a few simple cases of confined ions. We predict that mm-scale surface traps may have trap depths as high as tens of electron volts, or micromotion amplitudes in a 2-D ion crystal as low as tens of nanometers, given realistic experimental parameters. We also discuss applications to quantum information science, frequency metrology, and cold ion-atom collisions.   

Resolved sideband spectra of calcium ions in a Penning trap

I report on recent work at Imperial College London, with laser cooled calcium-40 ion Coulomb crystals in Penning traps. Penning traps provide a number of advantages over the more common radiofrequency (RF) trap; namely the ability to trap 3-dimensional, micromotion-free ion Coulomb crystals, and the ability to produce deep traps while maintaining a large ion-electrode surface distance. While these factors should permit lower heating rates than in typical RF traps, very little research has been conducted into the behavior and control of small Coulomb crystals in Penning traps due to the experimental challenges involved. We have spent several years developing techniques to overcome these obstacles, and are now making rapid progress towards the sub-Doppler cooling and coherent control of small ion crystals. We have already observed high resolution optical spectra showing sidebands due to radial and axial motions, giving estimated temperatures close to the Doppler limit.   

Rapid ion cooling by controlled collision

I propose a method to cool trapped ions by controlled collisions. Motional excitation of a hot ion is transferred to a coolant ion due to Coulomb interaction when they are brought to proximity. The whole process can be conducted diabatically, involving only a few oscillation periods of the harmonic trap. Our proposal is useful for rapid recooling of ion qubits during quantum computation and fast cooling of an ion whose mass is significantly different from the coolant ion.