Publications (23)191.9 Total impact
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ABSTRACT: Cavity quantum electrodynamics (QED) studies the interaction between a quantum emitter and a single radiationfield 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 selfassembled 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 quantumdotcavity system in the strongcoupling 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 offresonance, photon emission from the cavity mode and quantumdot 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 strongcoupling regime of cavity QED and the quantumdot 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 solidstate cavity QED.  [Show abstract] [Hide abstract]
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 controlNOT gate with a twoion 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 twoqubit system and quantify their entanglement.  [Show abstract] [Hide abstract]
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 nearunity 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.  [Show abstract] [Hide abstract]
ABSTRACT: A single $\chem{{}^{40}Ca^{+}}$ion is trapped and lasercooled to its motional ground state. Laser radiation which couples offresonantly 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 2qubit 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.  [Show abstract] [Hide abstract]
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 nearunity 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.  [Show abstract] [Hide abstract]
ABSTRACT: A single 40Ca+ ion is trapped and laser cooled to its motional ground state. Laser radiation which couples offresonantly 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 2qubit 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.  [Show abstract] [Hide abstract]
ABSTRACT: Quantum information processing is performed with single trapped Ca(+) ions, stored in a linear Paul trap and lasercooled to the ground state of their harmonic quantum motion. Composite laserpulse sequences were used to implement SWAP gate, phase gate and controlledNOT gate operations. Stark shifts on the quantumbit transitions were precisely measured and compensated. For a demonstration of quantum information processing, a DeutschJozsa algorithm has been implemented using two quantum bits encoded on a single ion. 
Article: Precision Measurement and Compensation of Optical Stark Shifts for an IonTrap Quantum Processor
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ABSTRACT: Using optical Ramsey interferometry, we precisely measure the laserinduced acStark 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.  [Show abstract] [Hide abstract]
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  [Show abstract] [Hide abstract]
ABSTRACT: Quantum computers have the potential to perform certain computational tasks more efficiently than their classical counterparts. The CiracZoller 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 controlledNOT (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 CiracZoller 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 longlived 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.  [Show abstract] [Hide abstract]
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 DeutschJozsa algorithm) requires just one examination step. The DeutschJozsa 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 twoqubit quantum gate, the entanglement of four ions, quantum state engineering and entanglementenhanced phase estimation. Here we exploit techniques developed for nuclear magnetic resonance to implement the DeutschJozsa algorithm on an iontrap quantum processor, using as qubits the electronic and motional states of a single calcium ion. Our ionbased 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.  [Show abstract] [Hide abstract]
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.  [Show abstract] [Hide abstract]
ABSTRACT: We present the experimental implementation of the DeutschJosza algorithm using the electronic and motional states of a single, cooled and trapped 40Ca+ ion as qubits.  [Show abstract] [Hide abstract]
ABSTRACT: Realisation of controlledNOT gate (CNOT) operation between two individual ions as a universal twobit 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.  [Show abstract] [Hide abstract]
ABSTRACT: Twolevel ionic systems, where quantum information is encoded in longlived 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.  [Show abstract] [Hide abstract]
ABSTRACT: Twolevel ionic systems, where quantum information is encoded in longlived 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.  [Show abstract] [Hide abstract]
ABSTRACT: Single trapped Ca ions, stored in a linear Paul trap and lasercooled 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 highfinesse optical cavity, a cavity mode can be coherently coupled to the qubit transition. 
Article: Simple and Efficient Photoionization Loading of Ions for Precision Ion Trapping Experiments
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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 twostep photoionization process using uvdiode lasers near 423nm and 390nm. Photoionization 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 diodelaser systems and the large cross section for field ionization when exciting highlying Rydberg states. Finally, we discuss the advantages of photoionization for ion generation compared to loading by electron bombardment. 
Conference Paper: Quantum information processing with trapped Ca+ ions
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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.  [Show abstract] [Hide abstract]
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 rftrap 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.
Publication Stats
2k  Citations  
191.90  Total Impact Points  
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Institutions

20002004

University of Innsbruck
 Institute for Experimental Physics
Innsbruck, Tyrol, Austria
