Showing papers by "Rainer Blatt published in 2013"

A quantum information processor with trapped ions

Abstract: Quantum computers hold the promise to solve certain problems exponentially faster than their classical counterparts. Trapped atomic ions are among the physical systems in which building such a computing device seems viable. In this work we present a small-scale quantum information processor based on a string of 40Ca+ ions confined in a macroscopic linear Paul trap. We review our set of operations which includes non-coherent operations allowing us to realize arbitrary Markovian processes. In order to build a larger quantum information processor it is mandatory to reduce the error rate of the available operations which is only possible if the physics of the noise processes is well understood. We identify the dominant noise sources in our system and discuss their effects on different algorithms. Finally we demonstrate how our entire set of operations can be used to facilitate the implementation of algorithms by examples of the quantum Fourier transform and the quantum order finding algorithm.

Quantum simulation of dynamical maps with trapped ions

Abstract: Dynamical maps are well known in the context of classical nonlinear dynamics and chaos theory. A trapped-ion quantum simulator can be used to study the generalized version of dynamical maps for many-body dissipative quantum systems.

Integrated fiber-mirror ion trap for strong ion-cavity coupling

Abstract: We present and characterize fiber mirrors and a miniaturized ion-trap design developed to integrate a fiber-based Fabry-Perot cavity (FFPC) with a linear Paul trap for use in cavity-QED experiments with trapped ions. Our fiber-mirror fabrication process not only enables the construction of FFPCs with small mode volumes, but also allows us to minimize the influence of the dielectric fiber mirrors on the trapped-ion pseudopotential. We discuss the effect of clipping losses for long FFPCs and the effect of angular and lateral displacements on the coupling efficiencies between cavity and fiber. Optical profilometry allows us to determine the radii of curvature and ellipticities of the fiber mirrors. From finesse measurements, we infer a single-atom cooperativity of up to 12 for FFPCs longer than 200 μm in length; comparison to cavities constructed with reference substrate mirrors produced in the same coating run indicates that our FFPCs have similar scattering losses. We characterize the birefringence of our fiber mirrors, finding that careful fiber-mirror selection enables us to construct FFPCs with degenerate polarization modes. As FFPCs are novel devices, we describe procedures developed for handling, aligning, and cleaning them. We discuss experiments to anneal fiber mirrors and explore the influence of the atmosphere under which annealing occurs on coating losses, finding that annealing under vacuum increases the losses for our reference substrate mirrors. X-ray photoelectron spectroscopy measurements indicate that these losses may be attributable to oxygen depletion in the mirror coating. Special design considerations enable us to introduce a FFPC into a trapped ion setup. Our unique linear Paul trap design provides clearance for such a cavity and is miniaturized to shield trapped ions from the dielectric fiber mirrors. We numerically calculate the trap potential in the absence of fibers. In the experiment additional electrodes can be used to compensate distortions of the potential due to the fibers. Home-built fiber feedthroughs connect the FFPC to external optics, and an integrated nanopositioning system affords the possibility of retracting or realigning the cavity without breaking vacuum.

Measurement-based quantum computation with trapped ions.

Abstract: Measurement-based quantum computation represents a powerful and flexible framework for quantum information processing, based on the notion of entangled quantum states as computational resources. The most prominent application is the one-way quantum computer, with the cluster state as its universal resource. Here we demonstrate the principles of measurement-based quantum computation using deterministically generated cluster states, in a system of trapped calcium ions. First we implement a universal set of operations for quantum computing. Second we demonstrate a family of measurement-based quantum error correction codes and show their improved performance as the code length is increased. The methods presented can be directly scaled up to generate graph states of several tens of qubits.

Quantum-state transfer from an ion to a photon

Abstract: Researchers demonstrate deterministic quantum-state transfer from a 40Ca+ ion to a photon in an optical cavity by controlling the transition probabilities and the frequency difference of two simultaneous Raman fields. They used process tomography to characterize the quantum-state transfer, providing a process fidelity of 92% and a state-transfer efficiency of 16%.

Atom-atom entanglement by single-photon detection.

Abstract: A scheme for entangling distant atoms is realized, as proposed in the seminal paper by [C. Cabrillo et al., Phys. Rev. A 59, 1025 (1999)]. The protocol is based on quantum interference and detection of a single photon scattered from two effectively one meter distant laser cooled and trapped atomic ions. The detection of a single photon heralds entanglement of two internal states of the trapped ions with high rate and with a fidelity limited mostly by atomic motion. Control of the entangled state phase is demonstrated by changing the path length of the single-photon interferometer.

Heralded entanglement of two ions in an optical cavity.

Abstract: We demonstrate precise control of the coupling of each of two trapped ions to the mode of an optical resonator When both ions are coupled with near-maximum strength, we generate ion-ion entanglement heralded by the detection of two orthogonally polarized cavity photons The entanglement fidelity with respect to the Bell state Ψ+ reaches F≥(919±25)% This result represents an important step toward distributed quantum computing with cavities linking remote atom-based registers

Entanglement-enhanced detection of single-photon scattering events

Abstract: A highly efficient method is demonstrated for detecting individual photons scattering from short-lived transitions in single trapped ions. An entangled state is used to amplify the tiny momentum kick an ion receives on scattering a photon. Cat-state spectroscopy has an 18-fold higher measurement sensitivity than the direct detection method.

Demonstration of genuine multipartite entanglement with device-independent witnesses

Abstract: Experimentally verifying that quantum states are indeed entangled is not always straightforward. With the recently proposed device-independent entanglement witnesses, genuine multiparticle entanglement of six ions has now been demonstrated.

Experimental generation of quantum discord via noisy processes.

Abstract: Quantum systems in mixed states can be unentangled and yet still nonclassically correlated. These correlations can be quantified by the quantum discord and might provide a resource for quantum information processing tasks. By precisely controlling the interaction of two ionic qubits with their environment, we investigate the capability of noise to generate discord. Firstly, we show that noise acting on only one quantum system can generate discord between two. States generated in this way are restricted in terms of the rank of their correlation matrix. Secondly, we show that classically correlated noise processes are capable of generating a much broader range of discordant states with correlation matrices of any rank. Our results show that noise processes prevalent in many physical systems can automatically generate nonclassical correlations and highlight fundamental differences between discord and entanglement.

A quantum information processor with trapped ions

Abstract: Quantum computers hold the promise to solve certain problems exponentially faster than their classical counterparts. Trapped atomic ions are among the physical systems in which building such a computing device seems viable. In this work we present a small-scale quantum information processor based on a string of $^{40}$Ca${^+}$ ions confined in a macroscopic linear Paul trap. We review our set of operations which includes non-coherent operations allowing us to realize arbitrary Markovian processes. In order to build a larger quantum information processor it is mandatory to reduce the error rate of the available operations which is only possible if the physics of the noise processes is well understood. We identify the dominant noise sources in our system and discuss their effects on different algorithms. Finally we demonstrate how our entire set of operations can be used to facilitate the implementation of algorithms by examples of the quantum Fourier transform and the quantum order finding algorithm.

Certifying Systematic Errors in Quantum Experiments

Abstract: When experimental errors are ignored in an experiment, the subsequent analysis of its results becomes questionable. We develop tests to detect systematic errors in quantum experiments where only a finite amount of data is recorded and apply these tests to tomographic data taken in an ion trap experiment. We put particular emphasis on quantum state tomography and present three detection methods: the first two employ linear inequalities while the third is based on the generalized likelihood ratio.

A quantum information processor with trapped ions

Abstract: Quantum computers hold the promise to solve certain problems exponentially faster than their classical counterparts. Trapped atomic ions are among the physical systems in which building such a computing device seems viable. In this work we present a small-scale quantum information processor based on a string of 40 Ca + ions confined in a macroscopic linear Paul trap. We review our set of operations which includes non-coherent operations allowing us to realize arbitrary Markovian processes. In order to build a larger quantum information processor it is mandatory to reduce the error rate of the available operations which is only possible if the physics of the noise processes is well understood. We identify the dominant noise sources in our system and discuss their effects on different algorithms. Finally we demonstrate how our entire

Heralded entanglement of two ions in an optical cavity

Abstract: We demonstrate precise control of the coupling of each of two trapped ions to the mode of an optical resonator. When both ions are coupled with near-maximum strength, we generate ion-ion entanglement heralded by the detection of two orthogonally polarized cavity photons. The entanglement fidelity with respect to the Bell state Ψ+ reaches F≥(91.9±2.5)%. This result represents an important step toward distributed quantum computing with cavities linking remote atom-based registers.

Undoing a quantum measurement.

Abstract: In general, a quantum measurement yields an undetermined answer and alters the system to be consistent with the measurement result. This process maps multiple initial states into a single state and thus cannot be reversed. This has important implications in quantum information processing, where errors can be interpreted as measurements. Therefore, it seems that it is impossible to correct errors in a quantum information processor, but protocols exist that are capable of eliminating them if they affect only part of the system. In this work we present the deterministic reversal of a fully projective measurement on a single particle, enabled by a quantum error-correction protocol in a trapped ion quantum information processor. We further introduce an in-sequence, single-species recooling procedure to counteract the motional heating of the ion string due to the measurement.

Shot-noise-limited monitoring and phase locking of the motion of a single trapped ion.

Abstract: We perform a high-resolution real-time readout of the motion of a single trapped and laser-cooled ${\mathrm{Ba}}^{+}$ ion. By using an interferometric setup, we demonstrate a shot-noise-limited measurement of thermal oscillations with a resolution of 4 times the standard quantum limit. We apply the real-time monitoring for phase control of the ion motion through a feedback loop, suppressing the photon recoil-induced phase diffusion. Because of the spectral narrowing in the phase-locked mode, the coherent ion oscillation is measured with a resolution of about 0.3 times the standard quantum limit.

Device-independent demonstration of genuine multipartite entanglement

Abstract: Julio T. Barreiro∗a,1 Jean-Daniel Bancal∗b,2 Philipp Schindler, Daniel Nigg, Markus Hennrich, Thomas Monz, Nicolas Gisin, and Rainer Blatt 3 Institut für Experimentalphysik, Universität Innsbruck, Technikerstrasse 25, 6020 Innsbruck, Austria Group of Applied Physics, University of Geneva, Geneva, Switzerland Institut für Quantenoptik und Quanteninformation, Österreichische Akademie der Wissenschaften,Technikerstrasse 21A, 6020 Innsbruck, Austria ∗ These authors contributed equally to this work.

Free-space read-out and control of single-ion dispersion using quantum interference

Abstract: We perform a free-space measurement and control of the refractive index of a single trapped ion in the presence of quantum interference effects. The single atom refractive index is characterized by the Faraday rotation of a laser field tightly focused onto a trapped and laser-cooled barium ion. It is tuned using the internal ion state that is optically controlled via a $V$ or a $\ensuremath{\Lambda}$ scheme. Measurements of the phase shift associated with an electromagnetically induced transparency are then performed and the internal state on the qubit transition is read-out with a detection fidelity of ($98\ifmmode\pm\else\textpm\fi{}1$)%.

The 20th anniversary of quantum state engineering

Abstract: This special issue is dedicated to the 20th anniversary of quantum state engineering, a field which studies techniques of preparation, manipulation and characterization of arbitrary quantum states within a Hilbert space associated with a particular physical system. The development of the field became possible due to technological achievements that enabled active control over the coherent dynamics of various quantum-mechanical systems at the level of their individual components. The issue features 12 articles on quantum engineering of various systems, therefore presenting a comprehensive account of the field’s current state...

Experimental characterization of quantum dynamics through many-body interactions.

Abstract: We report on the implementation of a quantum process tomography technique known as direct characterization of quantum dynamics applied on coherent and incoherent single-qubit processes in a system of trapped (40)Ca(+) ions. Using quantum correlations with an ancilla qubit, direct characterization of quantum dynamics reduces substantially the number of experimental configurations required for a full quantum process tomography and all diagonal elements of the process matrix can be estimated with a single setting. With this technique, the system's relaxation times T(1) and T(2) were measured with a single experimental configuration. We further show the first, complete characterization of single-qubit processes using a single generalized measurement realized through multibody correlations with three ancilla qubits.

Quantum simulation of open-system dynamical maps with trapped ions

Abstract: P. Schindler∗,1 M. Müller∗,2 D. Nigg, J. T. Barreiro†,1 E. A. Martinez, M. Hennrich, T. Monz, S. Diehl, 4 P. Zoller, 4 and R. Blatt 4 Institut für Experimentalphysik, Universität Innsbruck, Technikerstrasse 25, 6020 Innsbruck, Austria Departamento de F́ısica Teórica I, Universidad Complutense, Avenida Complutense s/n, 28040 Madrid, Spain Institut für Theoretische Physik, Universität Innsbruck, Technikerstrasse 25, 6020 Innsbruck, Austria Institut für Quantenoptik und Quanteninformation, Österreichische Akademie der Wissenschaften,Technikerstrasse 21A, 6020 Innsbruck, Austria ∗ These authors contributed equally to this work. † Present address: Fakultät für Physik, Ludwig-Maximilians-Universität München & Max-Planck Institute of Quantum Optics, Germany

Quantum Information Processing with Trapped Ions

Abstract: Entangling operations and quantum simulations are performed using an extended toolbox for a trapped-ion quantum computer. Decoherence of large entangled states is measured and open-system simulations are carried out, scalability is investigated with a CQED setup.

Experimental Characterization of Quantum Dynamics Through Many-Body Interactions

Abstract: Daniel Nigg, Julio T. Barreiro1,∗ Philipp Schindler, Masoud Mohseni3,† Thomas Monz, Michael Chwalla, Markus Hennrich, and Rainer Blatt Institut fur Experimentalphysik, Universitat Innsbruck, Technikerstrasse 25, A–6020 Innsbruck, Austria Institut fur Quantenoptik und Quanteninformation der Osterreichischen Akademie der Wissenschaften, Technikerstrasse 21a, A–6020 Innsbruck, Austria Research Laboratory of Electronics, Massachusetts Institute of Technology, Massachusetts 02139, USA (Dated: Monday 14 May, 2012)

Quantum simulation ‐ an exciting adventure

Abstract: Quantum many-body systems, as they appear in different branches of physics and chemistry, are very hard to simulate. The number of parameters defining their quantum states grows exponentially with the number of constituents. This prevents us from exploring and investigating relevant problems involving relatively small numbers of subsystems even with the help of the most powerful supercomputers. Richard Feynman already noticed this difficulty back in 1981 when he wrote his visionary paper “Simulating Physics with Computers” [Int. J. Theor. Phys. 21:6/7, 467 (1982)]. There, he proposed to use quantum systems (as opposed to classical computers) to perform the simulation and in this way to circumvent the need to store and compute a colossal number of parameters, necessary to describe the pertaining superpositions. What was a visionary idea at that time has become a very active field of research in the last few years. The extraordinary progress of both theoretical and experimental research in quantum physics allows us now to tame, control, and manipulate various quantum systems with unprecedented precision. Laser, magnetic, electronic and other technologies enable us to arrange quantum subsystems in different geometries, modify their interactions, and detect them. This allows us to engineer Hamiltonians and emulate systems displaying some of the most intriguing quantum phenomena. This tremendous progress is, to a large extent, due to the pioneering work of many researchers during the last twenty to thirty years. Here, the investigations of basic phenomena with one or few atoms or photons were of particular impact. The degree of control achieved in the experiments has enabled the observation of mind-boggling quantum effects, related to the superposition principle or to entangled states, and their disappearance due to decoherence. In fact, the 2012 Nobel Prize has been awarded to two scientists, S. Haroche and D. Wineland, for such ground-breaking experiments. Such experiments, often in combination with laser cooling of atoms or ions and Bose-Einstein condensation, have formed one of the pillars on which the field of quantum simulations is being established. Quantum simulations with cold atoms, either in magnetic traps or in optical lattices, with trapped ions, with photons, with superconducting devices, with quantum dots, etc., have been theoretically proposed and experimentally considered by many researchers. Most of the proposals so far deal with problems in condensed matter physics that are either difficult to tackle with modern supercomputers or are difficult to observe in solid state systems. This has established a very close link between different disciplines, like atomic, molecular, and optical (AMO) physics, and condensed matter physics. Although some of the experiments are still in their infancy, and a lot of research is still needed, the extraordinary progress made in all these research areas during the last few years makes us feel confident that in the not too distant future quantum simulators will provide a key tool to investigate many-body quantum systems. Furthermore, we also expect that quantum simulators will help quantum physicists to establish new and fruitful links to other scientific communities, like highenergy physics or quantum chemistry. The present special issue contains various review papers as well as research papers on quantum simulations. It starts out with two papers written by last year’s Nobel Prize winners describing their foundational work (Wineland, p. 739 and Haroche, p. 763). The issue continues with three thorough, invited review papers covering different topics on quantum simulations. The first one reviews recent theoretical proposals to use cold atoms in optical lattices to simulate lattice gauge theories of the sort that appear in highenergy physics (U.-J. Wiese, p. 783). The second covers both theoretical proposals and experimental demonstrations of cold-atom systems to simulate the physics of matter in the presence of gauge fields, displaying so-called topological phenomena, which so far have only been observed in solid-state systems (I. Spielman, p. 794). The third one contains an extensive overview and new proposals to perform a broad range of quantum simulations using

Trapped ions for simulating interacting spins

Abstract: Strings of laser-cooled trapped ions can be precisely controlled and manipulated with coherent narrow-band laser light. The use of entangling laser-ion interactions opens up the prospect of simulating the physics of interacting spins. I will present experiments that we have carried out with small ion crystals [1] and discuss the prospects of doing experiments with long ion strings.