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.

We report the realization of an elementary quantum processor based on a linear crystal of trapped ions. Each ion serves as a quantum bit (qubit) to store the quantum information in long lived electronic states. We present the realization of single-qubit and of universal two-qubit logic gates. The qwo-qubit operation relies on the coupling of the ions through their collective quantized motion. A detailed description of the setup and the methods is included.Comment: More detailed information as compared to Schmidt-Kaler et al., nature 422,408 (2003

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 ≃1 ms, discuss the main technical limitations and outline possible improvements.

We describe the construction and operation of a segmented linear Paul trap, fabricated in printed-circuit-board technology with an electrode segment width of 500 µm. We prove the applicability of this technology to reliable ion trapping and report the observation of Doppler cooled ion crystals of 40 Ca + with this kind of traps. Measured trap frequencies agree with numerical simulations at the level of a few percent from which we infer a high fabrication accuracy of the segmented trap. To demonstrate its usefulness and versatility for trapped ion experiments we study the fast transport of a single ion. Our experimental results show a success rate of 99.0(1) % for a transport distance of 2 × 2 mm in a round-trip time of T = 20 µs, which corresponds to 4 axial oscillations only. We theoretically and experimentally investigate the excitation of oscillations caused by fast ion transports with error-function voltage ramps: For a slightly slower transport (a round-trip shuttle within T = 30 µs) we observe nonadiabatic motional excitation of 0.89(15) meV.

We report heteronuclear photoassociation spectroscopy in a mixture of magneto-optically trapped 6 Li and 7 Li. The laser-induced decrease in the 7 Li steady-state particle number, only appearing in the presence of 6 Li, gives clear evidence of photoassociation to form 6 Li 7 Li. Hyperfine resolved spectra of the vibrational level v = 83 of the singlet state A 1 Σ + u have been taken up to intensities of 1000 W/cm 2 . The absolute resonance frequencies and the rotational constant have been measured. Saturation of the photoassociation rate has been observed for two hyperfine transitions, which can be shown to be due to saturation of the rate coefficient near the unitarity limit. Saturation intensities on the order of 40 W/cm 2 can be determined.