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) .