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QCCQI 2008 Quantum/Classical Control in Quantum Information QUROPE WORKSHOP QUANTUM/CLASSICAL CONTROL IN QUANTUM INFORMATION: THEORY AND EXPERIMENTS 13-20, September 2008, Otranto (Italy) Abstracts CONTRIBUTED PAPERS A solid-state light-matter interface at the single photon level Mikael Afzelius, Hugues de Riedmatten, Matthias Staudt, Christoph Simon, Nicolas Gisin Group of Applied Physics, University of Geneva Coherent and reversible mapping of quantum information between light and matter is an important experimental challenge in quantum information science. In particular, it is a decisive milestone for the implementation of quantum networks and quantum repeaters. So far, quantum interfaces between light and atoms have been demonstrated with atomic gase, and with single trapped atoms in cavities. Here we will present experimental results of coherent and reversible mapping of a light field with less than one photon per pulse onto an ensemble of 10^7 atoms naturally trapped in a solid. This is achieved by coherently absorbing the light field in a solid-state atomic medium, which has been prepared by spectrally shaping the optical inhomogeneous transition into an Atomic Frequency Comb (AFC) [1]. The state of the light is mapped onto collective atomic excitations on an optical transition and stored for a pre-programmed time up of to 1μs before being released in a well defined spatio-temporal mode as a result of a collective interference due to the AFC. The coherence of the process is verified by performing an interference experiment with two stored weak pulses with a variable phase relation. Visibilities of more than 95% are obtained, which demonstrates the high coherence of the mapping process at the single photon level. In addition, we show experimentally that our interface allows one to store and retrieve light fields in multiple temporal modes. [1] M. Afzelius, C. Simon, H. de Riedmatten, N. Gisin, Multi-Mode Quantum Memory based on Atomic Frequency Combs, arXiv:0805.4164 State selective microwave potentials on atom chips - towards a controlled phase gate Pascal Böhi(1,2), Max Riedel(1,2), Johannes Hoffrogge(2), Theodor W. Hänsch(1,2) and Philipp Treutlein(1,2) (1)Max-Planck-Institut für Quantenoptik, Garching, Germany (2)Fakultät für Physik, Ludwig-Maximilians-Universität München, München, Germany We present the status of our experiment with microwave near-fields on atom chips. Microwave near-fields are a key ingredient for atom chip applications such as quantum information processing, entanglement of Bose-Einstein condensates, atom interferometry, the study of Josephson effects and chipbased atomic clocks. We have integrated miniaturized microwave guiding structures on our atom chip. The micrometersized structures allow to generate microwave near-fields with unusually strong gradients. Through microwave dressing of hyperfine states, these can be used to create state-selective double-well potentials, which are the basic building block for a collisional quantum phase gate [1] on the atom chip. [1] P. Treutlein et al., Phys. Rev. A 74, 022312 (2006) Spatial and Spectral Phase Control in Quantum Interferometry Cristian Bonato (1, 2), Olga Minaeva (1, 3), Alexander V. Sergienko (1), Bahaa E. A. Saleh (1), Stefano Bonora (2), Paolo Villoresi (2) (1) Department of Electrical and Computer Engineering, Boston University, 8 Saint Mary's Street, Boston (MA) 02215 (2) CNR-INFM LUXOR, Department of Information Engineering, Via Gradenigo 6/B35131, Padova (Italy) (3) Department of Physics, Moscow State Pedagogical University 119992 Moscow (Russia) The study of quantum entanglement has lead to important applications in the field of quantum information and quantum metrology. Nonclassical states concurrently entangled in wave- vector, frequency and polarization can be generated by means of the nonlinear optical process of spontaneous parametric downconversion (SPDC). Frequency entanglement is at the heart of the even-order dispersion cancellation effect: only the odd-order dispersion terms contribute to the intererference pattern in the coincidence rate when a sample is placed in one arm of a HOM interferometer. The cancellation of group-velocity-dispersion leads to a reduction in the broadening of a white-light interference pattern thus fostering superior accuracy in position and trip-time measurements. More tools dealing with spectral (dispersion) and spatial (aberration) phase control may prove to be useful in other applications. Here we introduce a new spectral-domain technique that allows to separate the contribution of even-order and odd-order dispersion terms in different subregions of a global quantum interference pattern. This effect is based on the manipulation of the quantum probability amplitudes of the entangled-photon pairs produced by SPDC. Selection of specific parameters of our coincidence interferometer enables us to separate the detection of two non- classical dispersion cancellation effects in one experimental setup. In addition, we experimentally demonstrate a spatial counterpart of even-order dispersion cancellation, based on the entanglement of the transverse components of the wave-vectors emitted in SPDC. In particular, we modulate the spatial phase of entangled photons in the far-field by a Fourier-domain controller comprising a deformable mirror. We then feed the photons in a type-II quantum interferometer, using it as an analysis tool: due to the correlations between wave-vector and frequencies the interference pattern in the polarization-temporal domain will be affected by spatial distortions imparted by the adaptive mirror. We show that even-order aberrations are cancelled and therefore do not affect the shape of the dip. For example, astigmatism, defocus and spherical aberration are cancelled, while coma and trefoil are not. In conclusion, we introduce new quantum optical tools for spatial and spectral dispersion menagement and control. We believe that these new physical effects will be useful in quantum metrological and quantum imaging applications. Ultrafast manipulation of a tunable flux qubit by pulses: observation of coherent oscillations, RSFQ control and emerging strategies Fabio Chiarello (1), M.G. Castellano (1), P. Carelli (2), C. Cosmelli (3), J. Lisenfelf (4), A. Lukashenko (4), S. Poletto (4), G. Torrioli (1) and A.V. Ustinov (4) (1) Istituto di Fotonica e Nanotecnologie - CNR, 00156 ROMA, Italy (2) Dip. Ingegneria Elettrica, Università dell’Aquila, 67040 Monteluco di Roio, Italy (3) Dip. Fisica, Università di Roma “La Sapienza”, 00185 Roma, Italy (4) Physikalisches Institut, Universitaet at karlsruhe (TH), D- 76131 Karlsruhe, Germany We present a particular superconducting flux qubit, the double SQUID qubit, manipulated with a technique based on the fast modification of the qubit potential with pulses, in the absence of microwaves. This technique has been experimentally tested, and we observed coherent oscillations with frequencies that can be tuned from about 6GHz to 25GHz, which are very high values with respect to similar systems. The capability to tune the oscillation frequency, the simple “digital-like” manipulation of the qubit, the good tolerance to external noise and, in particular, the very short time required for a single operation make this system one of the most promising for quantum computing applications. We discuss also some development, such as the RSFQ control of the system (which is particularly suitable for this kind of manipulation), and the controllable coupling of many qubits. Numerical optimisation applied to control problems Pierre Becq de Fouquieres, Sonia G. Schirmer Centre for Mathematical Sciences, Wilberforce Road, Cambridge CB3 0WA, United Kingdom We consider the problem of finding good control pulses for fully characterised quantum systems. Although pulses motivated by geometric decomposition are widely used within the experimental community, applying numerical optimisation techniques to the problem often leads to better pulses (eg: of shorter total time, lower total energy, lower peak amplitude). We study a local update scheme, constrained so as to impose a high level of smoothness on the optimised pulse. This minimises the amount of information content in the final pulse, so as to make it more readily implementable; In particular, its spectrum can be expected to decay rapidly for large frequencies, so that it can realistically be implemented by standard frequency domain optical pulse shaping equipment. Although, the focus of this work has been unitary gate engineering, it is immediately applicable to quantum state preparation, and can be readily generalised to apply to dissipative systems. Quantum simulations on a few-qubit system Miroslav Dobsicek, Goran Johansson, Vitaly Shumeiko, and Goran Wendin Chalmers Univ. of Technology, Dept. of Microtechnology and Nanoscience, MC2 Applied Quantum Physics Laboratory, SE-412 96 Goteborg, Sweden The design and study of robustness of small testbed applications currently represents one of the short-term chief goals in the quantum computing field. We focus on sample quantum simulations which can be performed with as much as three to four qubits, e.g. in the next generation of superconducting qubit systems. Recently, we discussed how to perform quantum phase estimation algorithm in an iterative manner [1] and herewith reduce the number of required qubits. Our ongoing work deals with compact mapping from fermionic systems to qubits in order to reduce the number of qubits even more. Additionally, we perform classical simulations of designed quantum circuits, study the effects of noise and discuss some fine tunning for supercoducting qubits with ZZ-coupling. [1] Miroslav Dobsicek, Goran Johansson, Vitaly Shumeiko, and Goran Wendin. Arbitrary accuracy iterative quantum phase estimation algorithm using a single ancillary qubit: A two-qubit benchmark. Physical Review A, 76:030306(R), 2007. Controlling many-body quantum systems via time-periodic forcing Andre Eckardt ICFO-The Institute of Photonic Sciences Av. Canal Olimpic, s/n, E-08860 Castelldefels, Barcelona, Spain It will be pointed out that time-periodic potential modulations can be a robust and powerful tool for the manipulation of many- body systems as they are realized experimentally with ultracold (bosonic) atoms in optical lattice potentials. Such control schemes are reminiscent of manipulating internal atomic or molecular degrees of freedom by means of coherent radiation, and we describe them theoretically by an approach similar to the dressed atom picture. In their simplest form, off-resonant forcing is used to effectively modify the tight-binding tunneling matrix element $-J$ describing the kinetics of these systems. This effect (including $J\\approx0$ and $J<0$) has recently been measured from the coherent expansion of a Bose-Einstein condensate in a shaken lattice [PRL 99, 220403 (2007)]. We predict the tunnel modification to survive in the strongly correlated regime, allowing to induce the transition from a superfluid to a Mott insulator and back by smoothly switching on and off a kHz drive [PRL 95, 260404 (2005)]. These phenomena as well as further control schemes based on time-periodic forcing will be discussed. Local temperature in quantum thermal states Artur Garcia-Saez ICFO-The Institute of Photonic Sciences We consider parts of quantum spin chains at thermal equilibrium, focusing on their properties from a thermodynamical perspective. Under some conditions, it is expected that the description of blocks of the chain as thermal states with the same temperature as the whole chain will fail. Specifically, we analyze when the temperature ceases to be an intensive magnitude by employing the quantum fidelity as a particularly sensitive figure of merit. Then we show that the blocks can be considered indeed as thermal states with a high fidelity, provided an effective local temperature is properly identified. Such a result originates from typical properties of reduced sub-systems of energy-constrained Hilbert spaces. Finally, the relation between local and global temperature is analyzed as a function of the size of the block and the system parameters. This allows to single out in details the departure from the classical behavior in these quantum systems. Maximizing Noisy Quantum Memory Channels Capacity by Dynamical Modulation Goren Gordon and Gershon Kurizki Department of Chemical Physics, Weizmann Institute of Science, Rehovot 76100, Israel Applying selective modulations on transmitted qubits, encoding classical information, through a quantum noisy memory channel is shown to be able to drastically increase the channel capacity. The memory channel, whereby transmitted qubits affect each other's decoherence through the channel, can be characterized by four independent parameters, namely the channel decoherence rate magnitude, asymmetry, cross-decoherence(memory) and effective temperature. We analyze the entire parameter space to reveal a non-trivial interplay between the parameters and their effects on the channel capacity. We show that decreased magnitude and effective temperature, together with increased cross-decoherence and decoherence asymmetry maximize the channel capacity. Furthermore, a sharp transition from an optimal factorized to optimal fully entangled basis for encoding the classical information is demonstrated as a function of the channel parameters. A parameter manifold whereby above it, it is beneficial to encode the information in a fully entangled (Bell) basis is presented, and may suggest an important first step in selecting an experimentally optimal protocol for a dynamically controlled memory channel. Cold Ytterbium atoms in High-finesse optical cavities: Cavity Cooling and Collective Interactions H. Gothe, M. Cristiani, T. Valenzuela, J. Eschner ICFO – The Institute of Photonic Sciences, Mediterranean Technology Park, 08860 Castelldefels (Barcelona), Spain The quantum behavior of cold atoms interacting with photons confined in high-finesse cavities has been a subject of rising interest during the last decade. In particular, new schemes for laser cooling based on cavity feedback have been extensively studied both theoretically and experimentally. Furthermore, such systems are potential building blocks in quantum information processing, serving for the interconversion between photonic and atomic quantum states. Here we present the status of the experimental setup we are developing at ICFO. This apparatus will be suitable for studying collective excitation of a cold atomic cloud interacting with the standing wave of a resonator, with the perspective of using this system for investigating new cooling mechanisms based on atom- cavity interaction, as well as cavity-QED-based atom-photon interfaces. We recently observed cooling and confinement of 174Yb atoms in a Magneto-Optical Trap operating on the 1S0→1P1 transition (λ=399nm, Γ=2π·28 MHz), and we observed the 1S0→3P1 inter- combination transition (λ=556nm, Γ=2π·182 kHz) for various isotopes. At the moment we are stabilising the 556nm laser source, in order to use this transition for improved cooling and trapping. At the same time a high-finesse cavity at 556nm is being designed Spectral Characterisation of SPDC Entangled Photons Sources Marco Gramegna(1), Giorgio Brida(1), Valentina Caricato(1), Maria V. Chekhova(2), Mikhail V. Fedorov(3) Marco Genovese(1), Leonid A. Krivitsky(4), Sergej P. Kulik(2) (1) INRIM - Istituto Nazionale di Ricerca Metrologica, Strada delle Cacce 91, 10135 Turin, Italy (2) Department of Physics, M. V. Lomonosov Moscow State University Leninskie Gory, 119992 Moscow, Russia (3) A. M. Prokhorov General Physics Institute, Russian Academy of Sciences, Russia (4) Institut fur Optik, Information und Photonik Max-Planck Forschungsgruppe, Universitaet Erlangen-Nurnberg, Guenther-Scharowsky-Str. 1/Bau 24, 91058 Erlangen, Germany Entangled biphoton states generated via Parametric Down Conversion (SPDC) stand at the heart of quantum optics and quantum information,and the two-photon correlations can be investigated with respect to several variables like polarization, momentum or frequency, being these both discrete or continuous variables states. To explore deeper the entanglement properties in continuous variables and perform a characterization of SPDC sources in terms of frequency variables, we report experimental evidence for creation of biphoton states with high spectral entanglement, under condition when a femtosecond pulsed pump beam is well shaped to provide biphoton coincidence spectrum much narrower and single- particle one much wider than the pump spectrum[1-2], and the evaluation of the ratio R between the FWHM of the two distributions, as a measure of the achievable entanglement degree, being temporal walk-off the physical key factor providing a large contrast between single- to coincidence distributions. The design of our experiment considered frequency entangled biphoton states by e -> o + o type-I SPDC decay, in collinear degenerate regime, obtained by a 397.5 nm doubled mode-locked laser. With a beam splitter generating signal and idler channels, the photons were addressed to two SPADs, with spectral selection resolution of 0.2 nm for channel. Two-photon correlations have been investigated by fixing one monochromator at the maximal transmission wavelength on signal gate and scanning the one placed in the idler gate to observe the spectral distribution of single counts and coincidences, showing experimental evidence for a large contrast between these distributions, in comparison also with the spectral properties of the pump pulse. The operational method relates the degree of entanglement to R, approximately equal to the Schmidt number, that corresponds to experimentally measurable ratio between single particle and coincidence widths of the relative photon wave packets: the greater R, the higher entanglement between two photons. Preliminary measurements valued R=153, more larger than 1 (separable states), showing good agreement with theory. It will be shown how to increase this value compensating for spatial-frequency chirp of the pump pulse and a study of the the behavior of entanglement degree as a linear function of the crystals length. [1] Yu.M. Mikhailova, P.A. Volkov, M.V.Fedorov, arxiv:quant- ph/0801.0689v1 (2008) [2] M.V. Fedorov, et al., P.R.L., 99, 063901 (2007) Perfet state transfer in quantum spin chains Giulia Gualdi Dipartimento di Fisica, Università di Camerino We investigate the most general conditions under which a finite long-range interacting spin chain achieves unitary fidelity and the shortest transfer time in transmitting an unknown input qubit. At the same time we gain a deeper insight into system dynamics, that allows us to identify an ideal system involving sender and receiver only. However, this two-spin ideal chain is unpractical due to the rapid decrease of the coupling strength with the distance. Therefore, we propose an optimization scheme for approaching the ideal behaviour, while keeping the interaction strength still reasonably high. The procedure is scalable with the size of the system and straightforward to implement. Spin Squeezing on the Cesium Clock Transition N. Kjaergaard, P. Windpassinger, J. Appel, D. Oblak, U. Hoff, and E. Polzik QUANTOP, Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark When an ensemble of N independent particles is prepared in a coherent superposition of two internal quantum states, a projective measurement of the population difference will have a variance of N. This so-called projection noise is a current limitation to the precision of atomic clocks. In recent experiments on dipole trapped ensembles of Cs atoms in an equal supersition of the clock states we have observed the quantum projection noise by interrogations using off-resonant probe laser light. The population difference between the clock states is measured via the state dependent phase shift of probe light as recorded in a Mach Zehnder interferometer. Since the dispersive light-atom interaction has a quantum nondemolition measurement character we can use the information gained when applying a probe pulse of light to predict the outcome of a subsequent measurement beyond the standard quantum limit. Hence a reduction or "squeezing" of the population difference is encountered. Since a two-level quantum system is equivalent to a spin 1/2 particle this is referred to a pseudo-spin squeezing. The observation of squeezing implies that the particles in the ensemble are non- classically correlated (entangled). When taking into account decoherence resulting from spontaneously scattered probe photons our experiments show about 3 dB of spectroscopically relevant squeezing (noise reduction) . Continous Variable Entanglement Distribution Natalia Korolkova1 , Tomas Tyc1,2, Ladislav Mista1,3, David Menzies1, Gary Sinclair1 1 University of St. Andrews, Scotland, UK 2 Masaryk University, Brno, Czech Republic 3 Palacky University, Olomouc, Czech Republic e-mail:nvk@st-andrews.ac.uk We address different aspects of long-distance quantum communication using infinitely – dimensional, continuous-variable quantum systems. With examples of several schemes, we discuss the physics underlying entanglement distribution over large distances and tailoring of quantum systems for this purpose. This work also responds to the quest for experimentally feasible building elements for optical quantum repeater. First, we discuss entanglement concentration scheme for infinite-dimensional quantum systems based on non-linear cross- Kerr coupling of the one part of two-mode squeezed vacuum and an ancillary coherent state. We then show how the whole family of such entanglement concentration protocols can be derived using the framework and concept of weak quantum measurements. Next, we modify this scheme for the quantum state engineering purposes. Both schemes can be implemented in the same experimental setting. We propose an experiment that employs the cross-Kerr effect to create highly non-classical non-Gaussian states of light via interaction of two coherent beams in an atomic medium exhibiting electromagnetically-induced transparency, subsequent measurement on one beam and feed-forward on the other. The resultant states are highly non-classical states of electromagnetic field an d exhibit negativity of their Wigner function has a distinctly pronounced “crescent” shape specific for the Kerr-type interactions, which so far was not demonstrated experimentally. We show that creating and detecting such states should be possible with the present technology using electromagnetically induced transparency in a four-level atomic system in N-configuration. Finally, we address the question of quantum information distribution in general. As a development from the earlier work of Cubitt et al for qubits [1], we demonstrate the possibility to distribute entanglement without sending entanglement in infinite- dimensional systems. Remarkably, for mixed quantum states one can entangle two distant modes by sending a separable mode. This can be done using experimentally feasible Gaussian states and operations involving single-mode squeezed states, correlated displacements and beam splitters, dispensing with the CNOT gates of the qubit case. The distributed entanglement is distillable and therefore can be used for quantum communication. The proposed schemes prepare the ground for better understanding and engineering of optical quantum networks, continous-variable cryptography and other entanglement-based communication protocols using light modes and/or atomic ensembles. References [1] T.S. Cubitt et al, Phys. Rev. Lett. 91, 037902 (2003). Sideband Transitions for Quantum Information Processing in Circuit Quantum Electrodynamics P. J. Leek (1), P. Maurer (1), S. Filipp (1), M. G\"oppl (1), M. Baur (1), L. Steffen (1), R. Bianchetti (1), J. Fink (1), A. Blais (2), A. Wallraff (1) (1) Department of Physics, ETH Z\"urich, CH-8093, Z\"urich, Switzerland. (2) D\'partement de Physique, Universit\'e de Sherbrooke, Sherbrooke, Qu\'ebec, J1K 2R1 Canada. The preparation of multiple qubit entangled states and implementation of universal two-qubit quantum gates are important milestones in the development of a quantum information processor. Such tasks may be carried out by making use of sideband transitions with a harmonic mode that is dispersively coupled to multiple qubits. Such a scheme has been employed successfully in ion traps, in which quanta of information are interchanged between individual ions and collective vibrational modes of the ions in the trap [1]. In circuit QED, in which superconducting qubits are strongly coupled to the harmonic modes of a microwave resonator [2], such sideband transitions are also possible [3]. In superconducting qubits, the use of sidebands may have advantages over direct coupling methods since qubits and resonator can remain at all times decoupled and at fixed frequencies chosen for optimal coherence. In this talk I will report on recent spectroscopic and time resolved experiments involving driving of sidebands between transmon style qubits [4] and a resonator in circuit QED, and discuss potential use of such transitions for computational tasks. [1] H\"affner et al., Nature 438, 643 (2005). [2] Wallraff et al., Nature 431, 162 (2004). [3] Wallraff et al., Phys. Rev. Lett. 99, 50501 (2007). [4] Koch et al., Phys. Rev. A, 76, 42319 (2007). QLib - A Matlab Package for Quantum Information Theory Calculations with Applications Shai Machnes Tel-Aviv University, Israel Developing intuition about quantum information theory problems is difficult, as is verifying or ruling-out of hypothesis We present a Matlab package intended to provide the QIT community with a new and powerful tool-set for quantum information theory calculations. The package covers most of the "QI textbook" and includes novel parametrization of quantum objects and a robust optimization mechanism. New ways of re-examining well-known results is demonstrated. QLib is designed to be further developed and enhanced by the community and is available for download at www.qlib.info Photonic crystal defect cavities coupled to N-V centres in diamond Luca Marseglia (1), A. C. Stanley-Clarke(2), J.P. Harrison(1), R. Gibson(1), Y.-L. D. Ho(1), Jeremy O’Brien(2), J.G. Rarity(1) (1)Department of Electrical & Electronic Engineering, University of Bristol, Merchant Ventures Building, Woodland Road, Bristol BS8 1UB, UK (2)H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK The nitrogen-vacancy (N-V) defect in ultra-pure diamond shows great promise for the implementation of qubits for quantum computing. The N-V defect is a three level system which could behave as an efficient room temperature source of single photons at a wavelength of 637 nm. The defect has a ground state spin that can be addressed optically, hence an efficient coupling of light to this transition required. Here we aim to place the N-V centre into a cavity at the centre of a suspended slab photonic crystal structure made from hexagonal array of cylindrical air holes. The light is confined by distributed Bragg reflection in the plane of periodicity (xy) and by total internal reflection in the perpendicular plane (z). The work consists of both modelling the photonic crystal to optimise the parameters and fabricating the structures using focussed ion beam milling. We calculated the photonic band gap of the photonic crystal structure and then modelled it using finite difference domain methods. The photonic crystal structure considered was a modified defect cavity (M3). Starting with a hexagonal array of holes, the central three are filled and the diameters and positions of the holes surrounding the cavity are modified, as shown in Figure 1. To design high quality factor cavities (Q>10,000) we simulate the photonic crystal structure and vary the parameters to maximise Q. The parameters calculated were the correct lattice constant which allows the photonic crystal slab to be resonant with the 637 nm wavelength emission, the sizes of the holes (both “normal” and modified), and the shifts in position. Having simulated photonic crystal structure cavities M3 we have begun to investigate fabrication using focused ion beam lithography (FIB), obtaining triangular lattice structures as seen in figure 1. We will be assessing the damage due to the etching process using a variety of optical diagnostics. The eventual aim then will be to locate the position of a suitable single N-V centre using a scanning confocal microscope and etch a cavity around it. Modification of the emission properties due to the cavity will then be investigated, again making use of the scanning confocal microscope. Quantum spin models with trapped electrons Irene Marzoli Università di Camerino, Dipartimento di Fisica 62032 Camerino, Italy I will discuss a scheme to design and control an effective spin- spin interaction starting from a system of trapped electrons. The insertion of a magnetic field gradient, combined to the Coulomb interaction, enables an effective J-coupling between the particles in the array. The resulting system may be regarded as an artificial molecule, suitable for nuclear magnetic resonance (NMR) quantum computation. This analogy suggests to use techniques, similar to the refocusing schemes of NMR, to better design and control the effective spin-spin interaction. Our proposal relies on the application of appropriate sequences of electromagnetic pulses, alternated to periods of free evolution, to engineer an effective spin Hamiltonian. The final goal is to reproduce notable quantum spin systems, such as Ising and XY models. Static and Dynamic Controls in Virtual Photonic Quantum Circuits Hideaki Matsueda Kochi University, Emeritus Professor, 230-28 Nagatani-cho, Iwakura, Sakyo-ku, Kyoto 606-0026, Japan The quantum information controling may have two aspects, i.e. satatic and dynamic. Control by some built-in guiding structures is an example of the former, and by abrupt operations such as nonadiabatic bang-bang pulses is the latter. According to the Hermiticity of quantum formalism, it is natural to regard the basic flow of the quantum computing as the spontaneous evolution of wave functions through a quantum circuit (QC), entangling and disentangling themselves on the way. The entangling may be thought of as the manifestation of multipole-multipole interactions (MMIs) assisted by virtual photons (VPHs), and the disentangling may be triggered by an external operations that abruptly put the set of basis toward which the wave functions should collapse. Another example is the dynamic suppression of decoherence by sharp and strong bang-bang pulses. The VPHs are guided along the pre-fabricated nano-structured paths having features finer than their range determined by the time-energy uncertainty principle, where the separations between devices, polarization axies of the alignments, and the energy gaps should be designed for purposeful and may be one-way manner guiding. The MMI is assumed to be started by pumping photons of which number is just sufficient to excite only limited number of sites, e.g. only one site out of two sites, and no electron transfer will occur while the energy is transferred, so that it goes spontaneously or adiabatically not only throughout the range, but also over temporal differences, even realizing retrospective interconnections. In this paper, the coherent MMIs of various origines are graphically compared with respect to the energy, lifetime, and range, on the basis of our spectroscopic data, and a Minkowski type space-time diagram is illustrated for some major MMIs, visualizing the difference due to materials. Finally, speculative prospects are discussed including the retrospective entangling action which may bring a nanotech version of a time machine working within the uncertainty time slot, and a relatively adiabatical dynamic control for the near future. Spin chain quantum state transfer at the quantum speed limit M. Murphy, S. Montangero, V. Giovannetti, T. Calarco (1) Institute für QIV, Universität Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Deutschland (2) Scuola Normale Superiore, P.zza dei Cavalieri 7, 56126 Pisa, Italy Quantum state population transfer through spin chains may provide a viable means of propagating information within a quantum computer. We study here the controlled propagation of a quantum state along a spin chain by the application of a moving parabolic magnetic potential. Using optimal control techniques we find that optimisation of the control parameters for both the magnetic field strength and the moving speed of the parabolic potential yields very high fidelity of population transfer along the spin chain for short times and large numbers of spin sites. Furthermore, we show that the optimal control allows us to achieve the maximum speed of state transfer as allowed by the quantum speed limit. Two-photon nonlinearity in one atom cavity QED I. Schuster, A. Kubanek, A. Fuhrmanek, T. Puppe, P.W.H. Pinkse, K. Murr and G. Rempe Max-Planck-Institut for Quantum Optics, Hans-Kopfermann-Str. 1, D-85748 Garching, Germany Cavity quantum electrodynamics explores fundamental processes of light-matter interaction at the level of single atomic and photonic quanta and could provide a wealth of new applications. While microwave experiments have long explored effects with several intracavity photons, optical experiments have focussed on the weak excitation regime, with typically not more than one photon in the cavity. Here we report on a laser spectroscopy experiment which probes the energy-level structure of a strongly coupled atom-cavity system at higher excitation, with a single atom dipole-trapped inside a high-finesse optical cavity. A new resonance is observed at a frequency distinctively different from those studied in previous cavity QED experiments. We show that the new resonance results from a multi-photon transition between the anharmonically spaced energy levels. This is direct spectroscopic proof of the quantum nature of the combined atom-cavity system. It is also found that the response of the system is nonlinear in the laser intensity. This occurs in a regime where conventional optical nonlinearities are highly suppressed. The investigation of nonlinear quantum optics with just a single atom opens up new avenues for the controlled generation of novel multi-photon states. [1] I. Schuster, A. Kubanek, A. Fuhrmanek, T. Puppe, P.W.H. Pinkse, K. Murr and G. Rempe, Nature Physics 4, 382 (2008) Quantum State Preparation in Micro-traps Brian o'Sullivan, Thomas Busch Ultra Cold Quantum Gases Group, Physics Department, University College Cork, Cork - Ireland Reaching high fidelities while not compromising fast time scales is one of the most important criteria any realistic technique for quantum information processing must deliver. While this usually requires good control over many experimental parameters, alternative adiabatic techniques are often known that can make life easier in the laboratory. Here we investigate techniques for state preparation of single atoms in systems of spatially separated micro-traps. For such systems it was recently shown that an analogue to the celebrated three-level STIRAP technique in optics can be constructed, allowing for high fidelity atomic transport, EIT and CPT. As spatial atom-optical systems contain various additional degrees of freedom as compared to optical systems, they hold a large promise for developing new and exciting techniques. In our system we consider a four trap diamond arrangement in two dimensions and show that an atom trapped initially in a single trap can be transferred into an arbitrary, but well defined, spatial superposition state. This process requires only control over individual trapping frequencies and a STIRAP type positioning sequence of the traps. We will show that that process does not only allow for large fidelities when carried out perfectly, but is also robust against many experimental uncertainties. Conservation Laws Limit Quantum Control Accuracy Masanao Ozawa Graduate School of Information Science, Nagoya University, chikusa-ku, Nagoya 464-8601, JAPAN The threshold theorem of quantum error correction ensures fault- tolerant quantum computing, if every component can be controlled within a constant threshold accuracy. However, the threshold is so demanding that the realizability problem of scalable quantum computers is reduced to the problem of controllability under such a stringent accuracy requirement rather than solved in principle. In order to figure out how fundamental laws set a limit for the elementary gate operations, we consider here the accuracy limit induced by conservation laws. The idea that conservation laws limit quantum control goes back to the works of Wigner, Araki, and Yanase (the WAY theorem) stating that observables not-commuting with additively conserved quantity cannot be measured precisely [1]. Recently, the WAY theorem has been reformulated in the modern framework of measurement theory to obtain various quantitative generalizations [2, 3] derived from the universally valid reformulation of the uncertainty principle on the noise- disturbance trade-off [4, 5, 6], and applied them to quantum limits of the accuracy of elementary gate operations under the angular momentum conservation law obeyed by the interaction between the computational qubits and the controller, including the atom-field interaction described by a Jaynes-Cummings model. The inevitable error probability has been shown to be inversely proportional to the variance of the controller\'s conserved quantity for the CNOT gate [7, 8], the Hadamard gate [3], and the NOT gate [9], while the SWAP gate obeys no constraint. In this talk, these considerations will be extended to multiqubit gates such as the Toffoli gate, the Fredkin gate, and general controlled unitary gates. This work was supported in part by the SCOPE project of MIC and the CREST project of JST. [1] H. Araki and M. M. Yanase, Phys. Rev. 120, 622 (1960). [2] M. Ozawa, Phys. Rev. Lett. 88, 050402 (2002). [3] M. Ozawa, Int. J. Quant. Inf. 1, 569 (2003). [4] M. Ozawa, Phys. Rev. A 67, 042105 (2003). [5] M. Ozawa, Ann. Phys. (N.Y.) 311, 350 (2004). [6] M. Ozawa, J. Opt. B: Quantum Semiclass. Opt. 7, S672 (2005). [7] M. Ozawa, Phys. Rev. Lett. 89, 057902 (2002). [8] M. Ozawa, Phys. Rev. Lett. 91, 089802 (2003). [9] T. Karasawa and M. Ozawa, Phys. Rev. A 75, 032324 (2007). Spin chains as quantum wires: maximising information transfer speed and fidelity though minimal control Peter J. Pemberton-Ross (1), Sonia G. Schirmer (1) (1) Department of Applied Maths and Theoretical Physics, University of Cambridge, Wilberforce Road, Cambridge, CB3 0WA, United Kingdom Spin chains have been proposed as quantum wires for information transfer in solid state quantum architectures. We show that huge gains in both transfer speed and fidelity are possible using a minimalist control approach that relies only a single, local, on-off switch actuator. We show that effective switching time sequences can be determined using a simple optimization technique that lends itself to closed-loop direct laboratory optimization for both ideal chains and disordered chains. MaxLik-based photon statistics reconstruction by on/off detection Giorgio Brida(1), Marco Genovese(1), Marco Gramegna(1), Alice Meda(1), Stefano Olivares(2), Matteo G. A. Paris(2), Fabrizio Piacentini(1), Paolo Traina(1) (1)Istituto Nazionale di Ricerca Metrologica (INRIM), Strada delle Cacce 91, 10135 Torino, Italy (2) Dipartimento di Fisica dell'Università di Milano, Via Celoria 16, 20133 Milano (MI), Italy The reconstruction of the photon distribution of one or more modes of radiation plays a crucial role in fundamental quantum optics, and finds relevant applications in quantum communication, imaging and spectroscopy. Few years ago, a new method for reconstructing the photon statistics of optical states was developed and demonstrated. It is based on the maximum-likelihood (MaxLik) applied to an on/off detection with variable quantum efficiency scheme. Here we present the results obtained at the INRIM labs by applying this method (and its derivations) to several optical fields, both monopartite[1] and bipartite[2], all generated via spontaneous or stimulated Parametric Down-Conversion (PDC): from the heralded photon regime to the multi-photon one, all the results are in good agreement with the theoretical predictions, showing fidelity values above 99%. [1] G. Zambra et al., Phys. Rev. Lett. 95, 063602 (2005); [2] G. Brida, M. Genovese, M.G.A. Paris, F. Piacentini, Opt. Lett., Vol. 31, Issue 23, (2006); Quantum simulation in Ion traps and BECs A. Retzker, R.C. Thompson, D.M. Segal, M.B. Plenio, J. I. Cirac, B. Reznik Imperial College London,SW7 2PE, UK. Max-Planck-Institut f¨ur Quantenoptik, Hans-Kopfermann-Str. 1, 85748 Garching, Germany. Department of Physics and Astronomy, Tel-Aviv University, Tel Aviv 69978, Israel The field of quantum information processing has dramatically evolved in the past decade. Various systems have been proposed for the realization of a quantum computer. Despite an enormous experimental progress, due to the high degree of precision that is required, the realization of a large scale quantum computer is still not within reach. Recently it was recognized that quantum simulators are less demanding than quantum computers. In this talk I will describe proposals for quantum simulations in Ion traps and BECs. It will be shown that the radial degree of freedom of strings of trapped ions in the quantum regime may be prepared and controlled accurately through the variation of the external trapping potential while at the same time its properties are measurable with high spatial and temporal resolution. This provides a new test-bed giving access to static and dynamical properties of the physics of quantum-many-body systems and quantum phase transitions that are hard to simulate on classical computers. Furthermore, it allows for the creation of double well potentials with experimentally accessible tunnelling rates and with applications in testing the foundations of quantum physics and precision sensing. A scheme for the study of methods for detecting Unruh-like acceleration radiation effects in a Bose-Einstein condensate in a 1+1 dimensional setup will be descried. In particular, the dispersive effects of the Bogoliubov spectrum on the ideal case of exact thermalization will be explained. Single-ion single-photon interaction F. Rohde, C. Schuck, N. Piro, M. Almendros, A. Haase, M. Hennrich, F. Dubin, M. Mitchell, J. Eschner Institut de Ciencies Fotoniques, Mediterranean Technology Park, 08860 Castelldefels, Barcelona, Spain The controlled interaction of individual atoms and photons is an important building block for transferring quantum information between distant nodes of a quantum network. We study the absorption of single photons by a single Calcium ion, using heralded photons generated by a spontaneous parametric down- conversion source whose emission is tailored to coincide with the 20MHz bandwidth of the atomic resonance. Measurement of high fidelity single-qubit gates in circuit-QED using gate randomization L. Tornberg (1), J. M. Chow (2), Jens Koch (2), Jay Gambetta (3), Lev S. Bishop (2), M. H. Devoret (2), S. M. Girvin (2), and R. J. Schoelkopf (2) (1) Microtechnology and Nanoscience, MC2, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden (2) Departments of Physics and Applied Physics, Yale University, New Haven, Connecticut 06520, USA (3) Institute for Quantum Computing and Department of Physics and Astronomy, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada The realization of quantum computers relies upon the ability to perform elementary qubit gates with sufficiently low error probability. Standard methods for determining such gate fidelities include both state and process tomography, which are sensitive to errors in the state initialization and measurement. In addition, the cost in terms of experimental resources scales poorly with the number of qubits for both these methods. In this work, we measure the fidelity of single-qubit gates in circuit-QED using a randomization procedure suggested in [1], previously implemented in trapped ions [2]. This scheme avoids the difficulties associated with tomography by applying long sequences of randomly chosen gates, which effectively averages the fidelity of an operation over all initial states. Because of this, exact state preparation and measurement are no longer critical issues. Implementing this procedure on a superconducting transmon qubit, we demonstrate single-qubit gate fidelities of 0.98 only limited by relaxation and higher level excitations. To validate the results of the randomization procedure, we compare with the gate fidelity obtained using process tomography and with the fidelity measure proposed by Lucero et. al.[3]. [1] J. Emerson et. al., J. Opt. B: Quantum Semiclass. Opt. 7 (2005) S347-S352 [2] E. Knill, et. al., Phys. Rev. A., 77, 012307 (2008) [3] E. Lucero et. al., arXiv:0802.0903 Enhance the performance of decoy-state quantum key distribution with parametric down-conversion source Qin Wang and Anders Karlsson Department of Microelectronics and Applied Physics, KTH-The Royal Institute of Technology, KTH, Electrum 229, SE-164 40 Kista, Sweden We study the behavior of decoy state quantum key distribution systems using conditionally prepared down-conversion source, analyzing such a source running in a mode of operation with either a thermal photon number distribution or a Poisson photon number distribution. By comparing both modes of operations to the three- intensity proposal of Wang et al's, to the one-intensity proposal of Adachi et al's, and to currently demonstrated faint-laser pulse system, we demonstrate that a down-conversion source in a Poissonian mode of operation can largely enhance the performance of decoy state quantum key distribution system. This makes down- conversion based decoy state system an interesting alternative to current technological implementations based on faint laser pulses. Spin Squeezing on the Cs Clock Transition P. Windpassinger, D. Oblak, U. Busk Hoff, J. Appel, N. Kjærgaard, and E. S. Polzik QUANTOP, Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark Quantum nondemolition probing of a collective atomic (pseudo)-spin is a powerful instrument in quantum information processing and control. We present a method for non-destructive probing on the clock transition of laser-cooled, dipole trapped Cs atoms. The phase shift imposed by the atomic sample on an off-resonant probe laser beam is determined with a Mach-Zehnder interferometer. In the setup, the measurement accuracy of the population difference of the two clock states (the pseudo-spin component) has reached an accuracy which is limited only by the quantum noise of light (shot noise) and of the atoms (projection noise)[1]. The observation of correlations between two consecutive non-destructive measurements on the same ensemble allow us to infer the degree of pseudo-spin squeezing of the clock state populations. Due to the non-destructive probing we can follow the evolution of the population difference of the Cs-atom clock states online when subjected to microwave fields. This allows us to observe Rabi oscillations on the clock transition over an extended period of time, which should yield a significant improvement of the signal-to-noise ratio compared to the traditional fluorescence-based destructive probing. Further, the results of detailed studies of the effect of probe- induced inhomogeneous light-shift and of the destructive probe- induced spontaneous photon scattering and its influence on spin squeezed state are discussed [2]. [1] P. Windpassinger et. al, Phys. Rev. Lett. 100, 103601 (2008) [2] P. Windpassinger et. al, New J. Phys. 10, 053032 (2008)

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