A brief overview of the history, purpose, structure and performance of the EMALI network will be given. Furthermore, some thoughts about how to change and improve the structure and efficiency of Initial-Training-Network calls will be presented.

Hide Abstract.

Quantum technologies promise to enhance the capabilities of transmission and processing of information beyond what is possible using classical physics. Applications are envisaged to communications, cryptography, metrology, imaging and computation. Photonics provides a promising route to implementing such quantum-enhanced technologies, and all-optical designs for these technologies exist, based on uniquely quantum features such as reduced noise, increased correlations and measurement back-action that are fundamentally different that those used in the design of classical optical versions of these processing devices. Underpinning photonic implementations of this technology lies the generation, manipulation and stabilization of non-classical states of light. The distribution of photonic entangled quantum states and their application to real-world processing tasks is therefore a central element of quantum information science. The key capabilities that enable this technology are the preparation of pure single and multi-photon states, the manipulation of these by means of external controls, the storage of the outputs of the sources and devices and the registration of the outcomes of appropriate measurement operations.

Guided waves in optical microstructures provide important physical features that enable these goals. The key features of waveguides that are critical for such development include control of dispersion by means of the guide size [1], controllable birefringence [2], stability of multiple-nested interferometers [3], and flexibility of configuration [4]. Particularly useful examples can be found in both photonic-crystal fibers as well as standard polarization preserving single mode fibers. The relative merits of these sources are predicated on the quality of the structure fabrication and the ease of coupling multiple sources together. Further, integrated photonic circuits enable concatenation of multiphoton interferometers that can be used both to prepare states, and as sensing devices. When combined with efficient and simple quantum memories, this provides a scalable platform for distributing high-quality entanglement.

The certification of quantum performance requires careful assessment of the resources used in delivering the measurement results. For some technologies, such as sensing, assessment criteria for quantifying resources are not yet fully established. The situation is further complicated when taking into account decoherence and loss, since this can dramatically increase the resources consumed [6]. The goal of a palpable improvement over classical devices therefore demands a framework that can be universally applied [7]. Photonics provides a flexible platform by which criteria can be tested, and enables designs for true quantum operation to be framed.

References
[1] O. Cohen et al., Phys. Rev. Lett. 102, 123603 (2009).
[2] B. J. Smith et al., Opt. Exp. 17, 23589 (2009).
[3] B. J. Smith et al., Opt. Exp. 17, 13516 (2009).
[4] G. Puentes et al., Phys. Rev. Lett. 102, 080404 (2009).
[5] K. Reim et al., Nat. Phot. 4, 218 (2010).
[6] U. Dorner et al., Phys. Rev. Lett. 102, 040403 (2009).
[7] N. Thomas-Peter et al., http://arxiv.org/abs/1007.0870.

Hide Abstract.

Quantum memories, capable of temporarily 'freezing' a pulse of light, are a crucial component for quantum computers and quantum communications. So far, quantum memories - either ensemble based or single absorbers - have operated with bandwidths of kHz or MHz and have been very sensitive to the experimental environment. Robust, higher bandwidth (faster) quantum memories operating with very short laser pulses are a prerequisite for reliable and broadband quantum technology devices that allow for high-speed quantum processing and high data transfer rates in completely secure quantum networks. Here we report the storage and retrieval of sub-nanosecond low intensity light pulses consisting of several thousand photons with spectral bandwidths exceeding 1 GHz in cesium vapor. The novel memory interaction takes place via a far off-resonant two-photon transition in which the memory bandwidth is dynamically generated by the strong control field [1]. This makes the memory robust to environmental noise and allows an increase of speed by a factor of more than 100 compared to existing quantum memories. The memory works with a total efficiency of 15% and its coherence is demonstrated by directly interfering the stored and retrieved pulses [2].

References
[1] Nunn J. et al. Mapping broadband single-photon wave packets into an atomic memory, Phys. Rev. A 75, 011401 (2007).
[2] Reim K. F. et al. Towards high-speed optical quantum memories, Nature photonics (DOI 10.1038/NPHOTON.2010.30).

Hide Abstract.

T.B.A.

Hide Abstract.

Christiane P. Koch, Matthias M. Müller, Daniel Reich, Jiri Vala, Haidong Yuan, Birgitta K. Whaley and Tommaso Calarco
Institut für Theoretische Physik, FU Berlin, Arnimallee 14, 14195 Berlin, Germany.
ckoch@physik.fu-berlin.de

Optimal control is a promising tool for quantum information processing since it allows to implement a desired operation with extremely high fidelity. The implementation of a two-qubit operation such as a CNOT gate typically poses the main difficulty in order to realize a universal set of gates for quantum computing. In an optimal control approach, implementation of a CNOT gate can be achieved by maximizing the projection of the actual evolution onto CNOT as a functional of a control such as a laser field [1].

However, for a given encoding of qubits in a physical system, it is a priori not clear whether CNOT is the two-qubit gate that can best be implemented or whether a gate that is equivalent to CNOT up to local, i.e. single-qubit, operations would be a more suitable choice. Based on the Cartan decomposition [2] and the geometric theory of two-qubit operations [3], we develop a functional to obtain the entangling power of a desired two-qubit gate, i.e. to optimize for a gate that is locally equivalent to the desired two-qubit operation. This functional turns out to be non-convex and we employ a second-order Krotov method to derive a monotonically convergent optimization algorithm. We apply this new functional to the implementation of a fast Rydberg gate [4].

References
[1] J.P. Palao and R.Kosloff, Phys. Rev. A 68, 062308 (2003).
[2] H.Yuan and N.Khaneja, Phys. Rev. A 72, 040301 (2005).
[3] J.Zhang, J.Vala, S.Sastry, and K.B. Whaley, Phys. Rev. A 67, 042313 (2003).
[4] D.Jaksch, J.I. Cirac, P.Zoller, S.L. Rolston, R.Côté, and M.D. Lukin, Phys. Rev. Lett. 85, 2208 (2000).

Hide Abstract.

A key task in quantum networks is the distribution of entanglement between distant atomic qubits that serve as nodes. Recently, this has been demonstrated experimentally [1] through an approach that uses the projective measurement of the photons emitted by the two distant atoms to entangle them. Another strategy to implement this efficiently is to generate entangled photon pairs by spontaneous parametric down-conversion (SPDC), and make them interact with the distant atoms, transferring the entanglement from photon to atomic qubits [2, 3]. We present a step towards such entanglement transfer: the absorption of a single down-conversion photon by a single 40Ca+ ion, heralded by the partner photon.

Using type-II spontaneous parametric down-conversion in a ppKTP crystal, we generate time-correlated, frequency and polarization-entangled photon pairs [4]. One photon mode is coupled into a filtering line consisting of two cascaded Fabry-Perot cavities, which are tuned to the 854 nm transition in 40Ca+. The light transmitted through these cavities matches the atomic transition linewidth of 22 MHz and is monitored by a single-photon detector. The second, correlated, photon mode is focused onto a single trapped 40Ca+ ion through a high-numerical-aperture lens. The ion trap setup is described in [5].

A photon absorption event induces a quantum jump, detected as a sudden change in the fluorescence rate of the ion. The photons detected behind the filter cavities herald the presence of a resonant partner photon at the ion. The correlation function of the arrival times of the filtered photons and the quantum jumps introduced by the partner photons reveals coincidences between the two events. Additionally, the polarization dependence of this process is demonstrated after suitable preparation of the ion in Zeeman substates. Finally, we propose a possible scheme for transferring the quantum state of a photon to the internal state of the ion.

References
[1] D. L. Moehring et al., Nature 449, 68 (2007).
[2] J. I. Cirac et al., Phys. Rev. Lett. 78, 3221 (1997).
[3] S. Lloyd et al., Phys. Rev. Lett. 87, 167903 (2001).
[4] A. Haase et al., Opt. Lett. 34, 55 (2009); N. Piro et al., J. Phys. B 42, 114002 (2009).
[5] S. Gerber et al., New J. Phys. 11, 013032 (2009).

Hide Abstract.

Spins confined in semiconductor quantum dots offer new possibilities for realizing quantum optical systems with unique properties. In this talk, I will describe all-optical measurements that reveal rich spin physics in single self-assembled InAs/GaAs quantum dots. To study electron and nuclear spin dynamics, we used resonant optical excitation of the quantum dot charged-exciton transitions and observed many striking phenomena such as breakdown of nuclear spin temperature approach in demagnetization, anomalous Hanle effect, and nuclear spin polarization that is completely stable against dipolar diffusion. We also observed that optically mediated coupling between electron and nuclear spins could lead to a bi-directional nuclear spin polarization, which in turn allows the electron+nuclear spin system to track the changes in laser frequency dynamically on both sides of the quantum dot resonance. Our measurements reveal that the confluence of the laser excitation and nuclear spin polarization suppresses the fluctuations in the nuclear Overhauser field.

Hide Abstract.

Single-photon non-linear optics relies on strong enough non-linear interactions between photons. For a single quantum emitter strongly coupled to a cavity, this non-linear interaction is typically provided through the non-linearity of the emitter itself, as predicted by the Jaynes-Cummings model. The single-photon non-linearity manifests itself in the so-called photon blockade effect, where the absorption of one photon prevents the absorption of a second photon of the same colour.

A single self-assembled quantum dot strongly coupled to a photonic crystal cavity constitutes a solid-state-based realizaton of such an emitter-cavity system. While the mesoscopic nature of the system leads to unusal effects like cavity feeding [1], the system still exhibits the expected Jaynes-Cummings non-linearity. While photon blockade has been demonstrated previously in off-resonant photoluminescence measurements [2], resonant scattering experiments so far have suffered from fairly weak coupling constants and low signal-to noise ratio [3]. In the first part of my talk, I will present our recent efforts aimed at a clear demonstration of photon blockade in resonant scattering.

In the remaining part of my talk, I will then discuss the status of our new experimental effort on the realization of polariton blockade in quantum wells strongly coupled to DBR cavities. Unlike in the Jaynes-Cummings case, here, a single polariton, i.e. a quantum-well exciton strongly coupled to a cavity, prevents the creation of a second polariton through direct exciton-exciton repulsion [4]. Such a photonic dot would constitute the basic building block for the creation of arrays of non-linear cavities, which in turn can be used to study strongly correlated photonic systems in the solid-state [5].

References
[2] K. Hennessy et al., Nature 445, 896 (2007).
[3] A. Faraon et al., Nature Physics 4, 859 (2008).
[4] A. Verger et al., PRB 73, 193306 (2006).
[5] I. Carusotto et al., PRL 103, 033601 (2009).

Hide Abstract.

We present two characteristic examples of analogues of well established quantum coherent phenomena in classical electromagnetism. The first is a classical analogue of electromagnetically-induced transparency. Namely, we will examine the case of a two-dimensional lattice of metallic nanoparticles on top of a guiding substrate. If the substrate supports guided modes which fall within the surface-plasmon absorption band of the lattice of nanoparticles then one can quench light absorption via a mechanism which resembles electromagnetically-induced transparency in atomic gases [1]. Alternatively, the substrate can make metal absorb light in spectral regions well above the surface-plasmon band where metal is essentially transparent. The second analogue is about the coherent control of the optical near-field in nanostructures. Namely, we will show that one can achieve spatio-temporal control over the excitation of surface exciton-polaritons in semiconductor nanoparticles by a proper choice of a chirped light pulse [2]. This is achieved in nanoparticles made from semiconductors with strong excitonic oscillator strength and small absorption linewidth such as copper chloride or oxide.

References
[1] V. Yannopapas, E. Paspalakis, and N. V. Vitanov, Phys. Rev. B 80, 035104 (2009).
[2] V. Yannopapas and N. V. Vitanov, Phot. Nanostr., submitted.

Hide Abstract.

Optical excitations of an electron spin confined in self-assembled quantum dots (QD) have favorable selection rules that allow for recycling trion transitions where the scattered photon polarization is strongly correlated with the electron spin-state [1]. Realization of a spin-photon interface in the spirit of what has been recently realized for trapped ions [2], however, suffers from the fact that the background laser scattering overwhelms the QD resonance fluorescence (RF). While single QD RF has been recently observed by several groups [3,4], the reported experiments did not demonstrate a complete suppression of the background in a charge-controlled QD that is essential for the realization of a spin-photon interface. We demonstrate background-free detection of single QD RF with an efficiency of 0.1% [5]. The bunching of the resonantly scattered photons reveals information about electron spin dynamics. High-fidelity fast spin-state initialization heralded by a single photon enables the realization of quantum information processing tasks such as nondeterministic distant spin entanglement. The realization of a spin-photon interface constitutes a key step towards implementation of quantum information processing protocols such as nondeterministic spin entanglement between distant spins. Given that the collection efficiency could be increased by a factor of 10, our scheme would constitute a single-shot QND measurement of the electron spin. Collection efficiencies from micro-cavities have been predicted to be as high as 35% [6] which would in turn give up to a factor of 15 improvement in the overall collection efficiency, implying that a QND measurement is within reach using our scheme.

References
[1] A. Imamoglu et al., Phys. Rev. Lett. 83, 4204 (1999).
[2] B. B. Blinov et al., Nature 428, 153 (2004).
[3] A. N. Vamivakas et al., Nature Phys. 5, 198 (2009).
[4] E. B. Flagg et al., Nature Phys. 5, 203 (2009).
[5] S.T.Yilmaz et al., Phys. Rev. Lett. (to be published).
[6] M. Larque´ et al., New J. Phys. 11, 033022 (2009).

The interplay between coherent and incoherent processes in quantum gas dynamics is still a challenging topic with important implications both for accurate descriptions of current experiments on BECs [1], and for fundamental understanding of the microphysical origin of irreversibility. Traditional treatments of this situation have often relied on an ad hoc splitting of the system into coherent and incoherent subsystems [2], which interact; but in fact dynamical depletion of a condensate, as in experimentally observed 's-wave haloes', provides matter wave decoherence even in the absence of a thermal population. We therefore construct a more unified description which replaces the fully coherent Gross-Pitaevskii non-linear Schrödinger equation with a nonlinear master equation. As an application we analyze the possible suppression of matter wave tunneling due to local incoherent scattering within the condensate.

References
[1] G. B. Jo et al, Phys. Rev. Lett. 99, 240406 (2007).
[2] E. Zaremba, A. Griffin, T. Nikuni, Phys. Rev. A 57(6), 4695 (1998)

Hide Abstract.

Recent developments in cooling and trapping of atoms – in particular optical lattices – make it nowadays possible to implement, with unprecedented accuracy, models that can simulate the behaviour of electrons in crystals. Moreover, not only can optical lattices be used for spin-1/2 fermions (like electrons), but also for higher-spin fermions, bosons, Bose-Fermi mixtures, etc. In fact, optical lattices can be used to probe few-body effects which cannot be observed in traditional Solid-State setups due to crystal imperfections and fast energy dissipation mechanisms.

In this talk, I will present our main results on two- and three-body physics on one-dimensional lattices [1]. After building up the tight-binding theory explaining the existence of repulsively-bound atom pairs in an optical lattice – as experimentally observed in [2] – I will show how the two-body problem with arbitrary but finite-range interactions can always be exactly solved, even in the most general case of distinguishable particles with different effective masses. Then, I will explain how novel, rather exotic three-body bound states, with no analog in free space, show up as a zero-range interaction between the particles becomes large, that is, when the strong-coupling regime is entered. These exotic states will be shown to be a consequence of an off-resonant, effective atom-pair exchange operator, which yields universal, asymptotically exact binding energies of the atom-pair bound states with respect to its continuum.

References
[1] JPB 41, 161002 (2008); PRA 81, 011601 (2010); PRA 81, 042102 (2010).
[2] Nature 441, 853 (2006).

Hide Abstract.

In many implementations of quantum information processing schemes, the control of individual qubits relies on the ability to arbitrarily manipulate, address and couple individual information carriers, like single atoms or single photons. Here, we report on a novel dipole-trapping experiment that will ultimately allow to trap single neutral atoms in separate dipole traps and to displace them individually.

In order to reach a high degree of control on single atoms, we are implementing a scheme that enables us to trap 87Rb atoms in an array of individual optical dipole-traps. These dipole-traps are created by imaging the surface of a digital light-modulator. The light-modulator is a digital micro-mirror device (DMD) whose surface consists of 1024 x 768 micro-mirrors. The micro-mirrors can be individually switched. By switching the micro-mirrors, the dipole-trap array can be dynamically rearranged. The DMD is imaged by an isoplanatic optical system [1], which is diffraction limited with a numerical aperture of NA=0.5 and thus is able to focus the light to a submicron spot size.

Recently we have observed trapping of atoms in separate dipole traps. This is the first step towards an array of trapped individual atoms.

References
[1] E. Brainis et. al., Optics Communication 282

Hide Abstract.

There has been great interest in recent years in quantum control landscapes. Given an objective J that depends on a control field ε the dynamical landscape is defined by the properties of the Hessian dλ²J/d λε² at the critical points dλJ/d λε=0. Traps are critical points which are suboptimal maxima of J, and second order traps are critical points at which Hessian is negative semi-definite. In the talk we will show that the dynamical control landscapes can have second order traps for a general class of quantum systems and illustrate this statement with an example of a 3-level Λ-system.

Hide Abstract.

In the programme that consists in simulating condensed matter problems with quantum gases, the generation of gauge fields plays a special role. Orbital magnetism of electrons leads to crucial phenomena of condensed matter, like the Quantum Hall effect. However, because atoms in quantum gases have no charge, orbital magnetism is absent in these systems. Therefore, in order to design a proper "simulator", one must look for substitutes to the Lorentz force.

This talk will present two possible routes for the generation of artificial gauge fields acting on atomic gases. The first one uses the analogy between Coriolis and Lorentz forces, and consists in rotating the trapped gas. The second one takes advantage of the Berry's phase that appears when an atom moves slowly in a light field, following adiabatically one of the dressed states. Pros and cons of each route will be discussed, together with the latest experimental achievements.

Hide Abstract.

T.B.A.

Hide Abstract.

The interaction of light with atomic ensembles has become increasingly interesting as a way of storing information encoded and transported by light into long-lived coherences of the atomic ground state [1]. The efficiency of this transfer is increased, the higher the optical depth of the ensemble. Also the storage of several spatial modes becomes then achievable.

We are investigating the feasibility of a quantum memory in a Bose-Einstein condensate. As a measure of the interaction strength we performed spatially resolved Faraday rotation experiments. We both investigate Bose condensed and cold thermal samples. While condensed samples have higher optical depths, thermal clouds give rise to less diffraction, easing the interpretation of the data.

References
[1] Julsgaard et al., Nature 43, 482 (2004).
[2] M. Kubasik et al., PRA 79, 043815 (2009).

Hide Abstract.

R.J. Sewell, M. Koschorreck, M. Napolitano, B. Dubost, N. Behbood and M.W. Mitchell
ICFO-Institut de Ciencies Fotoniques, Mediterranean Technology Park, 08860 Castelldefels (Barcelona), Spain.

I will present results of experimental work in our group studying quantum metrology of ultracold atoms. The experiments are preformed with a dipole-trapped sample of rubidium 87 atoms, which provides a long-lived spin system, probed non-destructively by paramagnetic Faraday rotation, a form of quantum non-demolition measurement (QND) [1]. In the first set of results, we demonstrate sub-projection-noise sensitivity of the measurement [2,3], and spin squeezing of a magnetically sensitive coherent spin state prepared in the F=1 hyperfine manifold, with a spin noise reduction by 2.9 dB compared to the initial projection noise level and a metrological improvement of the sensitivity of the magnetometer of 2 dB. In the second, we experimentally demonstrate better than Heisenberg scaling in a nonlinear measurement of the atomic spin. We use short intense probe pulses to exploit nonlinear terms in the interaction Hamiltonian due to atom-mediated photon-photon interactions [4] to measure the atomic spin. The measurement is independently verified with a (linear) QND measurement. Results show a scaling of the sensitivity ∝ N^-3/2 over two orders of magnitude in the photon number, in full agreement with theory.

References
[1] M. Koschorreck, M. Kubasik, M. Napolitano, S.R. de Echaniz, H. Crepaz, J. Eschner, E.S. Polzik and M.W. Mitchell, Polarization-based Atom-Light Quantum Interface with an All-optical Trap, Phys. Rev. A 79, 043815 (2009).
[2] M. Koschorreck, M. Napolitano, B. Dubost and M.W. Mitchell, Sub-projection-noise sensitivity in broadband atomic magnetometry, Phys. Rev. Lett. 104, 093602 (2010).
[3] M. Koschorreck, M. Napolitano, B. Dubost and M. W. Mitchell QND Measurement of Large-Spin Ensembles by Dynamical Decoupling, Phys. Rev. Lett. 105, 093602 (2010) (2010).
[4] M. Napolitano, N. Behbood, B. Dubost, M. Koschorreck, R. J. Sewell and M. W. Mitchell, Nonlinear quantum metrology of atomic magnetisation with super-Heisenberg scaling. Submitted. (2010).

Hide Abstract.

We present a scheme which allows for measuring the mean value of the atomic field operator similar to the experimental setup of [1,2]. This corresponds in optics to measuring the quadrature of the field by means of a balanced homodyne detection, where the field to detect is superposed at a beam splitter with a local oscillator, namely, a laser field. In our case atoms outcoupled from the system to determine are coupled to a Bose-Einstein condensate, which acts as local oscillator. The outcoupling is performed by two Raman lasers and the light intensity I(t) of one of the Raman beams is calculated. It is shown that I(t) shows an oscillatory behaviour in time if the atoms in the system to determine are in a symmetry broken state such that ⟨ψ⟩ ≠ 0, which can be interpreted as Josephson oscillations of atoms between the system and the condensate mediated by the Raman lasers [2].

The light intensity I(t) can be used to measure ⟨ψ⟩ of the atomic system and we show that our scheme allows to measure the temperature of a Bose-Einstein condensate or to monitor in situ the quantum phase transition from Mott-insulator to superfluid state for ultracold atoms in an optical lattice.

References
[1] M.Saba, T.A.Pasquini, C.Sanner, Y.Shin, W.Ketterle and D.E.Pritchard, Science 307, 1945 (2005).
[2] Y.Shin, G.B.Jo, M.Saba, T.A.Pasquini, W.Ketterle and D.E.Pritchard, Phys. Rev. Lett. 95, 170402 (2005).
[3] D.L.Luxat and A.Griffin, Phys. Rev. A 65, 043618 (2002).

Hide Abstract.

Ultracold atoms and Bose-Einstein condensates in optical lattices are an important tool for investigating quantum phenomena and for quantum simulation, for instance of solid-state systems owing to excellent quantum control over a large numbers of atoms. Single ultra-cold neutral atoms within an optical lattice may act as qubits for quantum information storage and processing. Within the Mott insulator phase, atoms are loaded into the optical lattice with exactly one per site.

The Mott insulator regime is characterized by the balance between the tunneling energy and the onsite interaction energy. Flexible control of those parameters is required for making good use of this highly parallel quantum system. We have achieved quantum control of the tunneling rate between lattice sites independently of the on-site interaction by exploiting the peculiar characteristics of periodically forced optical lattices. The periodic driving created by a frequency modulation allows the creation of a dressed Mott insulator and of many-body states with modified fundamental properties, impossible to produce with static potentials. Systems with a negative value and/or anisotropy of the tunneling energy may produce new quantum phases.

Ultra-cold Rydberg atoms having long-range dipole-dipole interactions produce quantum gases with very strong interactions. The dipole blockade effect in Rydberg excitation is an amazing result of such strong interactions. We have recently characterized Rydberg excitation of atoms in a BEC, in particular the excitation spectra and the superfluid coherence.

In order to perform efficient quantum computation with ultracold atoms in optical lattices, single site initialization and readout operations should be performed with high fidelity. Because the charge amplification associated with existing detectors increases the single ion signal to a measurable size, we have explored the detection of ions produced by the atomic ionization. The ion collection efficiency allowed us to monitor routinely the ionization of as few as three ultracold atoms.

Hide Abstract.

When light interacts with coherently driven three-level quantum systems under conditions of electromagnetically induced transparency (EIT) [1], composite particles, called dark-state polaritons (DSP) [2] are formed which behave as massive objects with variable mass. They are the basis of slow, stored and stationary light. I will introduce the concept of dark-state and stationary-light polaritons and explain how to create scalar and gauge potentials (effective magnetic fields). I will remark on generalizations to multi-component polaritons which behave as spinor-like objects obeying a Dirac dynamics. In the latter case the possibility to manipulate the mass including its sign allows for the experimental study of unusual localization phenomena in models such as the random-mass Dirac model [3]. Photon-photon interactions mediated either by resonant nonlinear optical processes or atom-atom collissions in 1D set-ups can lead to strongly correlated polariton gases.

Schroedinger (left) vs. Dirac dynamics (right) of stationary light polaritons

References
[1] M. Fleischhauer, A. Imamoglu, J.P. Marangos, Rev. Mod. Phys. 77, 633 (2005).
[2] M. Fleischhauer, M.D. Lukin, Phys. Rev. Lett. 84, 5094 (2000).
[3] R.G. Unanyan, et al. arxiv 1005.3429.

Hide Abstract.

Reliable quantum channels, based on, for example, spin chains, are indispensable for achieving scalable and efficient quantum information processing in solid-state systems with fixed qubit positions and finite-range interqubit interactions. In this talk, I will discuss several protocols for complete state or excitation transfer in static and dynamic spin chains and examine their robustness with respect to diagonal and off-diagonal disorder. In particular, I will show that, for a given chain length and maximal achievable interspin exchange (XY) coupling strength, the optimal static spin-coupling protocol, implementing the fastest state transfer between the two ends of the chain, is more susceptible to off-diagonal (XY coupling) disorder, as compared to a much slower but robust adiabatic transfer protocol with time-dependent coupling strengths. These results have important implications for attaining scalability in quantum information processing systems based on arrays of coupled quantum dots, superconducting qubits, or atoms in optical lattices.

Hide Abstract.

ICFO-Institute of Photonic Sciences, Barcelona, Spain.
ICREA- Catalan Institute for Research and Advanced Studies, Barcelona, Spain.

Entanglement is the fundamental characteristic of quantum physics. Large experimental efforts are devoted to harness entanglement between various physical systems. In particular, entanglement between light and material systems is interesting due to their prospective roles as flying and stationary qubits in future quantum information technologies, such as quantum repeaters and quantum networks. In this talk, I will present the first demonstration of entanglement between a photon at telecommunication wavelength and a single collective atomic excitation stored in a solid state device. One photon from an energy-time entangled pair created by spontaneous down conversion is mapped onto a crystal doped with Neodymium ions and then released into a well-defined spatial mode after a predetermined storage time up to 200 ns. The other photon is at telecommunication wavelength and is sent directly through a 50 m fiber link to an analyzer. Successful transfer of entanglement to the crystal and back is proven by a violation of the Clauser-Horne-Shimony-Holt (CHSH) inequality by almost three standard deviations (S = 2.64 ±0.23). These results represent an important step towards quantum communication technologies based on solid-state devices. In particular, our resources are promising for building efficient multiplexed quantum repeaters for long-distance quantum networks.

This work was performed at the Group of Applied Physics, University of Geneva, Switzerland with Christoph Clausen, Imam Usmani, Félix Bussières, Nicolas Sangouard, Mikael Afzelius and Nicolas Gisin.

References
[1] C. Clausen, I. Usmani, F. Bussières, N. Sangouard, M. Afzelius, H. de Riedmatten and N. Gisin, arXiv:1009.0489.

A similar independent work with a Tm doped crystal has been reported in:

[2] E.Saglamyurek, N. Sinclair, J. Jin, J. A. Slater, D. Oblak, F. Bussières, M. George, R. Ricken, W. Sohler, and W. Tittel, arXiv:1009.0490.

Hide Abstract.

Aim of our work is to access and explore higher dimensional photonic states. In terms of stability and complexity usual bulk-optic setups greatly limit the capabilities of reaching higher dimensional systems. However, the rapid development in integrated photonic circuits in recent years, especially in the telecommunication industry, opens new possibilities. Our approach is to use integrated photonic circuits on-chip as well as in-fiber to reach photonic states of higher dimension.

We are working on a fully integrated realization of a device called a multiport capable of applying any unitary transformation depending on its internal (tunable) parameters. Also, we are currently building an integrated in-fiber source for creating higher dimensional entangled photons. Combining the source and the multiport will result in a very general tool for experiments in higher dimensional spaces by setting the device to a variety of different incoming entangled states followed by applying any desired unitary transformation.

The first goal will be to demonstrate the principle functioning as well as the scalability of such integrated devices for creating and manipulating higher dimensional states.

Hide Abstract.

Radosław Łapkiewicz, Peizhe Li, Christoph Schäff, Nathan Langford, Sven Ramelow, Marcin Wieśniak and Anton Zeilinger
Institute for Experimental Physics, University of Vienna, Boltzmanngasse 5, A-1090 Vienna, Austria.
Institute for Quantum Optics and Quantum Information (IQOQI), Austrian Academy of Sciences, Boltzmanngasse 3, A-1090 Vienna, Austria.

In Quantum Mechanics, in contrast to other physical theories, not all properties can be measured simultaneously (the Heisenberg Uncertainty Principle is a manifestation of this fact). An interesting question is whether a joint probability distribution can describe the outcomes of all possible measurements, allowing a quantum system to be mimicked by classical means. We show the first experimental evidence that even for a single three-level quantum system no such classical model can exist that correctly describes the results of a simple set of measurements as suggested by Klyachko et al. [1]. This is the simplest system where such a contradiction is possible. It is also indivisible and as such cannot contain entanglement. Our result sheds new light on the conflict between quantum and classical physics and provides insight into the limitations of simulating quantum systems using, hidden or not, classical information.

References
[1] A. Klyachko, M. Ali Can, S. Binicioğlu, A. Shumovsky, Phys. Rev. Lett. 101, 020403 (2008).

Hide Abstract.

Coercing quantum systems into performing useful computational tasks requires a high degree of control. By 'useful', we really mean three things: firstly, the computation should be efficient, so we get the right balance between what we put in and what we get out; secondly, it should be relatively error-free, meaning that we can perform tasks with a very high degree of accuracy; and thirdly it should be fast, ideally as fast as possible. In this talk, I will show some examples of how control theory applied to a one-dimensional spin chain can address all of these issues.

References
[1] M. Murphy, S. Montangero, V. Giovannetti, and T. Calarco, Phys. Rev. A 82, 022381 (2010).
[2] D. Burgarth, K. Maruyama, M. Murphy, S. Montangero, T. Calarco, F. Nori, and M. B. Plenio, Phys. Rev. A 81, 040303(R) (2010).
[3] T. Caneva, M. Murphy, T. Calarco, R. Fazio, S. Montangero, V. Giovannetti, and G. E. Santoro, Phys. Rev. Lett. 103, 240501 (2009).

Hide Abstract.

ETH Zurich, Zurich, Switzerland.
www.qudev.ethz.ch

T.B.A.

Hide Abstract.

A string of trapped interacting ions at zero temperature exhibits a structural phase transition to a zigzag structure, tuned by reducing the transverse trap potential or increasing the particle density. The transition is driven by transverse, short wavelength vibrational modes. We propose a quantum field--theoretical description of this transition by the one dimensional Ising model in a transverse field. Based on the mapping to this model, we estimate the quantum critical point in terms of the system parameters, and find a finite, measurable deviation from the critical point predicted by the classical theory. A measurement procedure is suggested which can probe the effects of quantum fluctuations at criticality. These results can be extended to describe the transverse instability of ultracold polar molecules in a one dimensional optical lattice.

The non-equilibrium dynamics is then studied when the transverse trap frequency is quenched across the value at which the chain undergoes a continuous phase transition from a linear to a zigzag structure. An equation for the order parameter, corresponding to the transverse size of the zigzag structure, is determined when the vibrational motion is damped via laser cooling. The number of structural defects produced during a linear quench of the transverse trapping frequency is predicted and verified numerically. It is shown to obey the scaling predicted by the Kibble-Zurek mechanism, when extended to take into account the spatial inhomogeneities of the ion chain in a linear Paul trap.

Hide Abstract.

Department of Physics, Sofia University, 5 James Bourchier blvd, 1164 Sofia, Bulgaria.

Quantum information processing with sequences of one- and two-qubit gates has been established as the standard model of the future universal quantum computer. The number of physical operations required in this model, however, appears too large even for elementary demonstrations of quantum algorithms and conditional quantum gates. Viewing the quantum computer as a quantum simulator, we have developed a pool of alternative approaches that use the intrinsic symmetries of an ensemble of trapped ions; these have allowed us to design much faster implementations, with much fewer steps than in the standard model. For example, a linear chain of ions in a Paul trap is ideally suited for running the Grover’s quantum search algorithm because the collective propagator of such a system, under certain conditions, is the reflection about the mean, which is the key element of Grover’s algorithm. In another example, the Hamiltonian of a ring of ions possesses a circulant symmetry, which implies that the respective propagator is directly related to the quantum Fourier transform, which is the essential part of Shor’s factorization and other algorithms. Hence the individual mathematical steps in Grover’s and Shor’s algorithms can be realized in such a single-purpose quantum computer with a single laser pulse, rather than by a sequence of a great number of pulses as in the standard model. We have used similar arguments for the design of techniques for single- shot generation of various highly-entangled many-particle states, such as Dicke states and cluster states.

Another approach along this line is based upon the so-called composite pulses, which have been used for a long time in nuclear magnetic resonance and, since recently, in single-qubit rotations of trapped ions. A composite pulse is represented by a sequence of pulses, each with a specific area and a well-defined phase. These phases are used as free parameters, which are determined from the conditions for a specific unitary operation. The result is a transition profile, which combines very high fidelity of gate operations with robustness vs parameter variations. Motivated by the objective to optimize the manipulation of the ion qubits we have developed a pool of composite pulses for single- and multi-qubit gates by using a novel approach based on SU(2) transformations instead of the more cumbersome traditional Bloch sphere O(3) rotations. We have used these to design new realizations of multiply-conditional gates, e.g. C-NOT, Toffoli, and generally C^N-NOT gates, which require much fewer physical interactions than in the standard approaches, essentially a single composite pulse; moreover, these implementations are robust against variations in the pulse area and the Lamb-Dicke parameter.

Hide Abstract.

T.B.A.

Hide Abstract.