S. Gulde

University of Innsbruck, Innsbruck, Tyrol, Austria

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Publications (23)154.28 Total impact

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    ABSTRACT: Cavity quantum electrodynamics (QED) studies the interaction between a quantum emitter and a single radiation-field mode. When an atom is strongly coupled to a cavity mode, it is possible to realize important quantum information processing tasks, such as controlled coherent coupling and entanglement of distinguishable quantum systems. Realizing these tasks in the solid state is clearly desirable, and coupling semiconductor self-assembled quantum dots to monolithic optical cavities is a promising route to this end. However, validating the efficacy of quantum dots in quantum information applications requires confirmation of the quantum nature of the quantum-dot-cavity system in the strong-coupling regime. Here we find such confirmation by observing quantum correlations in photoluminescence from a photonic crystal nanocavity interacting with one, and only one, quantum dot located precisely at the cavity electric field maximum. When off-resonance, photon emission from the cavity mode and quantum-dot excitons is anticorrelated at the level of single quanta, proving that the mode is driven solely by the quantum dot despite an energy mismatch between cavity and excitons. When tuned to resonance, the exciton and cavity enter the strong-coupling regime of cavity QED and the quantum-dot exciton lifetime reduces by a factor of 145. The generated photon stream becomes antibunched, proving that the strongly coupled exciton/photon system is in the quantum regime. Our observations unequivocally show that quantum information tasks are achievable in solid-state cavity QED.
    Nature 03/2007; 445(7130):896-9. DOI:10.1038/nature05586 · 42.35 Impact Factor
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    ABSTRACT: We investigate experimentally quantum information processing using a string of trapped ions where each ion represents a bit of quantum information (qubit). We report on the realization of the Cirac Zoller universal control-NOT gate with a two-ion crystal. Quantum gates between the ions are realized by coupling them through their collective quantized motion. We show that the control ion's qubit state determines the evolution of the target ion's qubit state. Additionally, we report on a tomographic method to fully characterize the quantum state of a qubit register. Using this tomographic method we investigate Bell states which we generate in a two-qubit system and quantify their entanglement.
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    ABSTRACT: Arbitrary atomic Bell states with two trapped ions are generated in a deterministic and preprogrammed way. The resulting entanglement is quantitatively analyzed using various measures of entanglement. For this, we reconstruct the density matrix using single qubit rotations and subsequent measurements with near-unity detection efficiency. This procedure represents the basic building block for future process tomography of quantum computations. As a first application, the temporal decay of entanglement is investigated in detail. We observe ultralong lifetimes for the Bell states Psi(+/-), close to the fundamental limit set by the spontaneous emission from the metastable upper qubit level and longer than all reported values by 3 orders of magnitude.
    Physical Review Letters 07/2004; 92(22):220402. DOI:10.1103/PhysRevLett.92.220402 · 7.73 Impact Factor
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    ABSTRACT: A single $\chem{{}^{40}Ca^{+}}$-ion is trapped and laser-cooled to its motional ground state. Laser radiation which couples off-resonantly to a motional sideband of the ion's $S_{1/2}$ to $D_{5/2}$ transition causes a phase shift proportional to the ion's motional quantum state $|n\rangle$. As the phase shift is conditional upon the ion's motion, we are able to demonstrate a universal 2-qubit quantum gate operation where the electronic target state $\{S,D\}$ is flipped depending on the motional qubit state $|n\rangle=\{|0\rangle, |1\rangle\}$. Finally, we discuss scaling properties of this universal quantum gate for linear ion crystals and present numerical simulations for the generation of a maximally entangled state of five ions.
    EPL (Europhysics Letters) 03/2004; 65(5). DOI:10.1209/epl/i2003-10174-3 · 2.27 Impact Factor
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    ABSTRACT: We report on tomographic means to study the stability of a qubit register based on a string of trapped ions. In our experiment, two ions are held in a linear Paul trap and are entangled deterministically by laser pulses that couple their electronic and motional states. We reconstruct the density matrix using single qubit rotations and subsequent measurements with near-unity detection efficiency. This way, we characterize the created Bell states, the states into which they subsequently decay, and we derive their entanglement, applying different entanglement measures.
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    ABSTRACT: A single 40Ca+ ion is trapped and laser cooled to its motional ground state. Laser radiation which couples off-resonantly to a motional sideband of the ion's S1/2 to D5/2 transition causes a phase shift proportional to the ion's motional quantum state |n>. As the phase shift is conditional upon the ion's motion, we are able to demonstrate a universal 2-qubit quantum gate operation where the electronic target state {S,D} is flipped depending on the motional qubit state |n>={|0>,|1>}. Finally, we discuss scaling properties of this universal quantum gate for linear ion crystals and present numerical simulations for the generation of a maximally entangled state of five ions.
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    ABSTRACT: Quantum information processing is performed with single trapped Ca(+) ions, stored in a linear Paul trap and laser-cooled to the ground state of their harmonic quantum motion. Composite laser-pulse sequences were used to implement SWAP gate, phase gate and controlled-NOT gate operations. Stark shifts on the quantum-bit transitions were precisely measured and compensated. For a demonstration of quantum information processing, a Deutsch-Jozsa algorithm has been implemented using two quantum bits encoded on a single ion.
    Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 08/2003; 361(1808):1363-74. DOI:10.1098/rsta.2003.1206 · 2.86 Impact Factor
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    ABSTRACT: Using optical Ramsey interferometry, we precisely measure the laser-induced ac-Stark shift on the S(1/2)-D(5/2) "quantum bit" transition near 729 nm in a single trapped 40Ca+ ion. We cancel this shift using an additional laser field. This technique is of particular importance for the implementation of quantum information processing with cold trapped ions. As a simple application we measure the atomic phase evolution during a n x 2 pi rotation of the quantum bit.
    Physical Review Letters 05/2003; 90(14):143602. DOI:10.1103/PhysRevLett.90.143602 · 7.73 Impact Factor
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    ABSTRACT: Abstract Single trapped Ca, ions we realize the quantum,bit (qubit) by operating on a narrow optical S‐D (quadrupole) transition. The S $ P $ D dipole transitions are driven for optical cooling, state preparation, and state detection. S1/2
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    ABSTRACT: Quantum computers have the potential to perform certain computational tasks more efficiently than their classical counterparts. The Cirac-Zoller proposal for a scalable quantum computer is based on a string of trapped ions whose electronic states represent the quantum bits of information (or qubits). In this scheme, quantum logical gates involving any subset of ions are realized by coupling the ions through their collective quantized motion. The main experimental step towards realizing the scheme is to implement the controlled-NOT (CNOT) gate operation between two individual ions. The CNOT quantum logical gate corresponds to the XOR gate operation of classical logic that flips the state of a target bit conditioned on the state of a control bit. Here we implement a CNOT quantum gate according to the Cirac-Zoller proposal. In our experiment, two 40Ca+ ions are held in a linear Paul trap and are individually addressed using focused laser beams; the qubits are represented by superpositions of two long-lived electronic states. Our work relies on recently developed precise control of atomic phases and the application of composite pulse sequences adapted from nuclear magnetic resonance techniques.
    Nature 04/2003; 422(6930):408-11. DOI:10.1038/nature01494 · 42.35 Impact Factor
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    ABSTRACT: Determining classically whether a coin is fair (head on one side, tail on the other) or fake (heads or tails on both sides) requires an examination of each side. However, the analogous quantum procedure (the Deutsch-Jozsa algorithm) requires just one examination step. The Deutsch-Jozsa algorithm has been realized experimentally using bulk nuclear magnetic resonance techniques, employing nuclear spins as quantum bits (qubits). In contrast, the ion trap processor utilises motional and electronic quantum states of individual atoms as qubits, and in principle is easier to scale to many qubits. Experimental advances in the latter area include the realization of a two-qubit quantum gate, the entanglement of four ions, quantum state engineering and entanglement-enhanced phase estimation. Here we exploit techniques developed for nuclear magnetic resonance to implement the Deutsch-Jozsa algorithm on an ion-trap quantum processor, using as qubits the electronic and motional states of a single calcium ion. Our ion-based implementation of a full quantum algorithm serves to demonstrate experimental procedures with the quality and precision required for complex computations, confirming the potential of trapped ions for quantum computation.
    Nature 02/2003; 421(6918):48-50. DOI:10.1038/nature01336 · 42.35 Impact Factor
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    ABSTRACT: We present the experimental implementation of the Deutsch-Josza algorithm using the electronic and motional states of a single, cooled and trapped 40Ca+ ion as qubits.
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    ABSTRACT: Realisation of controlled-NOT gate (CNOT) operation between two individual ions as a universal two-bit quantum gate based from the proposals of Cirac and Zoller, is presented. Two single 40Ca+ ions are held in a linear Paul trap and are individually addressed with focused laser beams. Superpositions of the S12 /and the D52/ long lived electronic states represent a qubit. A sequence of well suited laser pulses flips the state of the target ion conditional to the state of the control ion.
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    ABSTRACT: Summary form only given. Stark shifts occurring in driven multi level systems are a major problem for ion trap quantum computers. We show how to characterise the shifts, how to compensate for them and how to use them for a novel dispersive gate.
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    ABSTRACT: Two-level ionic systems, where quantum information is encoded in long-lived states (quantum bits, qubits), are discussed extensively for quantum information processing. We present a collection of measurements which characterize the stability of a qubit based on the S1/2 - D5/2 transition of single 40Ca+ions in a linear Paul trap. We find coherence times of 1 ms, discuss the main technical limitations and outline possible improvements.
    Journal of Physics B Atomic Molecular and Optical Physics 01/2003; 36(3). · 1.92 Impact Factor
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    ABSTRACT: Two-level ionic systems, where quantum information is encoded in long lived states (qubits), are discussed extensively for quantum information processing. We present a collection of measurements which characterize the stability of a qubit based on the $S_{1/2}$--$D_{5/2}$ transition of single $^{40}$Ca$^+$ ions in a linear Paul trap. We find coherence times of $\simeq$1 ms, discuss the main technical limitations and outline possible improvements. Comment: Proceedings of "Trapped charged particles and fundamental interactions" submitted to Journal of Physics B (IoP)
    Journal of Physics B Atomic Molecular and Optical Physics 11/2002; DOI:10.1088/0953-4075/36/3/319 · 1.92 Impact Factor
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    ABSTRACT: Single trapped Ca ions, stored in a linear Paul trap and laser--cooled to the ground state of their harmonic quantum motion are used for quantum information processing. As a demonstration, composite laser pulse sequences were used to implement phase gate and CNOT gate operation. For this, Stark shifts on the qubit transitions were precisely measured and compensated. With a single ion stored inside a high-finesse optical cavity, a cavity mode can be coherently coupled to the qubit transition.
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    ABSTRACT: We report a simple and efficient method to load a Paul trap with Ca+ ions. A beam of neutral atomic calcium is ionized in a two-step photo-ionization process using uv-diode lasers near 423nm and 390nm. Photo-ionization of a calcium beam for loading a Paul trap has first been demonstrated by Kjaergaard et al. The advantages of our method are the use of cheap and easily handled diode-laser systems and the large cross section for field ionization when exciting high-lying Rydberg states. Finally, we discuss the advantages of photo-ionization for ion generation compared to loading by electron bombardment.
    Applied Physics B 11/2001; 73(8):861-863. DOI:10.1007/s003400100749 · 1.63 Impact Factor
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    ABSTRACT: form only given. A novel type of ground state laser cooling of a single trapped ion is achieved using a technique which tailors the absorption profile for the cooling laser by exploiting electromagnetically induced transparency in the Zeeman structure of the S/sub 1/2/-P/sub 1/2/ dipole transition. This new method is robust, easy to implement and proves particularly useful for cooling several motional degrees of freedom simultaneously, which is of great practical importance for the implementation of quantum logic schemes with trapped ions.
    Quantum Electronics and Laser Science Conference; 05/2001
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    ABSTRACT: Ground state cooling and coherent manipulation of ions in an rf-(Paul) trap is the prerequisite for quantum information experiments with trapped ions. With resolved sideband cooling on the optical S 1/2 -D 5/2 quadrupole transition we have cooled one and two 40 Ca + ions to the ground state of vibration with up to 99.9% probability. With a novel cooling scheme utilizing electromagnetically induced transparency on the S 1/2 -P 1/2 manifold we have achieved simultaneous ground state cooling of two motional sidebands 1.7 MHz apart. Starting from the motional ground state we have demonstrated coherent quantum state manipulation on the S 1/2 -D 5/2 quadrupole transition at 729 nm. Up to 30 Rabi oscillations within 1.4 ms have been observed in the motional ground state and in the n = 1 Fock state. In the linear quadrupole rf-trap with 700 kHz trap frequency along the symmetry axis (2 MHz in radial direction) the minimum ion spacing is more than 5 m for up to 4 ions. We are able to cool two ions to the ground state in the trap and individually address the ions with laser pulses through a special optical addressing channel.
    10/2000; DOI:10.1109/QELS.2000.901945