Showing papers by "Rainer Blatt published in 2005"

Scalable multiparticle entanglement of trapped ions

Abstract: The generation, manipulation and fundamental understanding of entanglement lies at the very heart of quantum mechanics. Entangled particles are non-interacting but are described by a common wavefunction; consequently, individual particles are not independent of each other and their quantum properties are inextricably interwoven. The intriguing features of entanglement become particularly evident if the particles can be individually controlled and physically separated. However, both the experimental realization and characterization of entanglement become exceedingly difficult for systems with many particles. The main difficulty is to manipulate and detect the quantum state of individual particles as well as to control the interaction between them. So far, entanglement of four ions or five photons has been demonstrated experimentally. The creation of scalable multiparticle entanglement demands a non-exponential scaling of resources with particle number. Among the various kinds of entangled states, the 'W state' plays an important role as its entanglement is maximally persistent and robust even under particle loss. Such states are central as a resource in quantum information processing and multiparty quantum communication. Here we report the scalable and deterministic generation of four-, five-, six-, seven- and eight-particle entangled states of the W type with trapped ions. We obtain the maximum possible information on these states by performing full characterization via state tomography, using individual control and detection of the ions. A detailed analysis proves that the entanglement is genuine. The availability of such multiparticle entangled states, together with full information in the form of their density matrices, creates a test-bed for theoretical studies of multiparticle entanglement. Independently, 'Greenberger-Horne-Zeilinger' entangled states with up to six ions have been created and analysed in Boulder.

Quantum information processing and communication: strategic report on current status, visions and goals for research in Europe

Abstract: We present an excerpt of the document "Quantum Information Processing and Communication: Strategic report on current status, visions and goals for research in Europe", which has been recently published in electronic form at the website of FET (the Future and Emerging Technologies Unit of the Directorate General Information Society of the European Commission, http://www.cordis.lu/ist/fet/qipc-sr.htm). This document has been elaborated, following a former suggestion by FET, by a committee of QIPC scientists to provide input towards the European Commission for the preparation of the Seventh Framework Program. Besides being a document addressed to policy makers and funding agencies (both at the European and national level), the document contains a detailed scientific assessment of the state-of-the-art, main research goals, challenges, strengths, weaknesses, visions and perspectives of all the most relevant QIPC sub-fields, that we report here.

Robust entanglement

Abstract: It is common belief among physicists that entangled states of quantum systems lose their coherence rather quickly The reason is that any interaction with the environment which distinguishes between the entangled sub-systems collapses the quantum state Here we investigate entangled states of two trapped Ca+ ions and observe robust entanglement lasting for more than 20 s

Experimental and theoretical study of the 3d 2 D-level lifetimes of 40 Ca +

Abstract: We report measurements of the lifetimes of the 3d {sup 2}D{sub 5/2} and 3d {sup 2}D{sub 3/2} metastable states of a single laser-cooled {sup 40}Ca{sup +} ion in a linear Paul trap. We introduce a measurement technique based on high-efficiency quantum state detection after coherent excitation to the D{sub 5/2} state or incoherent shelving in the D{sub 3/2} state, and subsequent free, unperturbed spontaneous decay. The result for the natural lifetime of the D{sub 5/2} state of 1168(9) ms agrees excellently with the most precise published value. The lifetime of the D{sub 3/2} state is measured with a single ion and yields 1176(11) ms which improves the statistical uncertainty of previous results by a factor of four. We compare these experimental lifetimes to high-precision ab initio all order calculations [D{sub 3/2} state: 1196(11) ms; D{sub 5/2} state: 1165(11) ms] and find a very good agreement. These calculations represent an excellent test of high-precision atomic theory and will serve as a benchmark for the study of parity nonconservation in Ba{sup +} which has similar atomic structure.

Robust state preparation of a single trapped ion by adiabatic passage

Abstract: We report adiabatic passage experiments with a single trapped $^{40}$Ca$^+$ ion. By applying a frequency chirped laser pulse with a Gaussian amplitude envelope we reach a transfer efficiency of 0.990(10) on an optical transition from the electronic ground state S$_{1/2}$ to the metastable state D$_{5/2}$. This transfer method is shown to be insensitive to the accurate setting of laser parameters, and therefore is suitable as a robust tool for ion based quantum computing.

Scalable ion trap quantum computing without moving ions

Abstract: A hybrid quantum computing scheme is studied where the hybrid qubit is made of an ion trap qubit serving as the information storage and a solid-state charge qubit serving as the quantum processor, connected by a superconducting cavity. In this paper, we extend our previous work [CITE] and study the decoherence, coupling and scalability of the hybrid system. We present our calculations of the decoherence of the coupled ion-charge system due to the charge fluctuations in the solid-state system and the dissipation of the superconducting cavity under laser radiation. A gate scheme that exploits rapid state flips of the charge qubit to reduce decoherence by the charge noise is designed. We also study a superconducting switch that is inserted between the cavity and the charge qubit and provides tunable coupling between the qubits. The scalability of the hybrid scheme is discussed together with several potential experimental obstacles in realizing this scheme.

Quantum computing with trapped ions

Abstract: The paper reports on the investigation of single Ca/sup +/ ions and crystals of Ca/sup +/ ions confined in a linear Paul trap. These are also investigated for quantum information processing. Recent experimental advancements towards a quantum computer with such a system are also discussed.

Entanglement of trapped ions

Abstract: We report on the scalable and deterministic generation and tomographic characterization of entangled states of up to 8 trapped ions and experiments towards entangling ions and photons.

Teleportation with atoms

Abstract: The teleportation of an atomic state accomplishes the complete transfer of information from one particle to another, employing the non‐local properties of quantum mechanics Recently, two groups have achieved the deterministic teleportation of a quantum state between a pair of trapped ions Following closely the original proposal of Bennett et al, a highly entangled pair of ions is created, a complete Bell‐state projective measurement involving the source ion and one of the entangled pair is carried out, and state reconstruction conditioned on this measurement is performed on the other half of the entangled pair

Application of process tomography to quantum teleportation

Abstract: Three Ca-40 ions trapped in a linear Paul trap serve as a three-qubit register that can be manipulated by series of suitably tailored laser pulses. We implement deterministic quantum teleportation in the following way: First, we create a maximally entangled pair of ions that will be shared between the sending and the receiving party. Next, the third quantum bit is prepared in an arbitrary quantum state that will be teleported to the quantum bit of the receiver (the target). This is done by measuring the quantum bits of the sender in a basis of maximally entangled states and depending on the measurement result - applying one out of four unitary transformations to the target qubit. This teleportation protocol can be completely characterized by preparing six different input states and tomographically reconstructing the corresponding output states. The information obtained in this way is used for reconstructing the quantum process with the help of a maximum-likelihood technique.

Precision measurement and calculation of the 3d /sup 2/D-level lifetimes of /sup 40/Ca+

Abstract: Int this paper, a measurement technique based on deterministic coherent excitation to the D/sub 5/2/ state or incoherent shelving in the D/sub 3/2/ state is introduced, followed by a waiting period with free spontaneous decay and finally a measurement of the remaining excitation by high-efficiency quantum state detection.

Electronic feedback control of single ion motion

Abstract: The paper demonstrates an alternative method to cool the ion's motion below its Doppler-limit by using "cold damping" or homodyne feedback control. For cold damping, an additional friction force proportional to the instantaneous velocity of the ion is created, by supplying additional electric fields along the direction of the motion of the ion, which are -90/spl deg/ phase-shifted with respect to the measured amplitude of the motion.

Observation of very long-lived entanglement

Abstract: We report on the preservation of bi-partite entanglement in an ion trap for over 10 seconds.