L. Deslauriers

Scaling and suppression of anomalous heating in ion traps

We measure and characterize anomalous motional heating of an atomic ion confined in the lowest quantum levels of a novel rf ion trap that features moveable electrodes. The scaling of heating with electrode proximity is measured, and when the electrodes are cooled from 300 to 150 K, the heating rate is suppressed by an order of magnitude. This provides direct evidence that anomalous motional heating of trapped ions stems from microscopic noisy potentials on the electrodes that are thermally driven. These observations are relevant to decoherence in quantum information processing schemes based on trapped ions and perhaps other charge-based quantum systems.

T-junction ion trap array for two-dimensional ion shuttling, storage, and manipulation

We demonstrate a two-dimensional 11-zone ion trap array, where individual laser-cooled atomic ions are stored, separated, shuttled, and swapped. The trap geometry consists of two linear rf-ion trap sections that are joined at a 90° angle to form a T-shaped structure. We shuttle a single ion around the corners of the T-junction and swap the positions of two crystallized ions using voltage sequences designed to accommodate the nontrivial electrical potential near the junction. Full two-dimensional control of multiple ions demonstrated in this system may be crucial for the realization of scalable ion trap quantum computation and the implementation of quantum networks.

Implementation of Grover's quantum search algorithm in a scalable system

We report the implementation of Grovers quantum search algorithm in the scalable system of trapped atomic ion quantum bits. Any one of four possible states of a two-qubit memory is marked, and following a single query of the search space, the marked element is successfully recovered with an average probability of 60(2)%. This exceeds the performance of any possible classical search algorithm, which can only succeed with a maximum average probability of 50%.

Sympathetic cooling of trapped Cd ¿ isotopes

A collection of cold trapped ions offers one of the most promising avenues towards realizing a quantum computer @1‐4 #. Quantum information is stored in the internal states of individual trapped ions, while entangling quantum logic gates are implemented via a collective quantized mode of motion of the ion crystal. The internal qubit states can have extremely long coherence times @5#, but decoherence of the motion of the ion crystal may limit quantum logic gate fidelity @6#. Furthermore, when ions are nonadiabatically shuttled between different trapping regions for large-scale quantum computer schemes @4,7#, their motion must be recooled for subsequent logic operations. Direct laser cooling of the qubit ions is not generally possible without disturbing coherence of the internal qubit states. Instead, additional ‘‘refrigerator’’ ions in the crystal can be directly laser cooled, with the qubit ions cooled in sympathy by virtue of their Coulomb-coupled motion @8#. The laser cooling of the refrigerator ions can quench unwanted motion of the ion crystal, while not affecting the internal states of the qubit ions @4,9,10#. Sympathetic cooling has been observed in large ensembles of ions in Penning traps @8,11#, impurities in small collections of ion crystals, and in small ion crystals consisting of a single species, where strong laser focusing was required to access a particular ion without affecting the others @12#. Here, we report the first demonstration of sympathetic cooling in a small ion crystal with two different species where both species are independently optically addressed. We study sympathetic cooling of individual Cd 1 isotopes confined in a rf trap. One ion isotope~the refrigerator ion! is continuously Doppler-cooled by a laser beam red detuned from its D2 line ( S1/2-P3/2), while the other isotope ~the probe ion! is either Doppler cooled or Doppler heated by another beam, whose frequency is scanned around its D2 resonance line. The effect of the sympathetic cooling is to enable measuring fluorescence on the blue side of the probe ion’s resonance. Ordinarily, when the probe laser beam is tuned to the blue of the probe ion’s resonance, the ion ceases fluorescing due to Doppler heating, but the sympathetic cooling from the refrigerator ion keeps the probe ion cold and fluorescing regardless of the probe tuning. In the experiment, the probe ion is 112 Cd 1 , while the refrigerator ion is 114 Cd 1 ~both isotopes have zero nuclear spin!. The respective D2 lines of these two neighboring isotopes are separated by about 680 MHz, with the heavier ion at lower frequency, and the natural linewidth of each ion’s excited P3/2 state is g/2p.47 MHz. The experimental apparatus is schematically shown in Fig. 1. The Cd 1 D2 line resonant light near 214.5 nm is generated by quadrupling a Ti:sapphire laser. The laser is stabilized to a molecular tellurium feature near 429 nm to better than 1 MHz. The quadrupled UV output is split into two parts; one part is upshifted by ;420 MHz, while the

Phase control of trapped ion quantum gates

There are several known schemes for entangling trapped ion quantum bits for large-scale quantum computation. Most are based on an interaction between the ions and external optical fields, coupling internal qubit states of trapped ions to their Coulomb-coupled motion. In this paper, we examine the sensitivity of these motional gate schemes to phase fluctuations introduced through noisy external control fields, and suggest techniques for suppressing the resulting phase decoherence.

Zero-point cooling and low heating of trapped {sup 111}Cd{sup +} ions

We report on ground-state laser cooling of single {sup 111}Cd{sup +} ions confined in radio-frequency (Paul) traps. Heating rates of trapped ion motion are measured for two different trapping geometries and electrode materials, where no effort was made to shield the electrodes from the atomic Cd source. The low measured heating rates suggest that trapped {sup 111}Cd{sup +} ions may be well suited for experiments involving quantum control of atomic motion, including applications in quantum information science.

Atomic qubit manipulations with an electro-optic modulator

We report new techniques for driving high-fidelity stimulated Raman transitions in trapped-ion qubits. An electro-optic modulator induces sidebands on an optical source, and interference between the sidebands allows coherent Rabi transitions to be efficiently driven between hyperfine ground states separated by 14.53 GHz in a single trapped 111Cd+ ion.

Efficient photoionization loading of trapped ions with ultrafast pulses

Atomic cadmium ions are loaded into radiofrequency ion traps by photoionization of atoms in a cadmium vapor with ultrafast laser pulses. The photoionization is driven through an intermediate atomic resonance with a frequency-quadrupled mode-locked Ti:sapphire laser that produces pulses of either 100-fs or 1-ps duration at a central wavelength of 229 nm. The large bandwidth of the pulses photoionizes all velocity classes of the Cd vapor, resulting in a high loading efficiency compared to previous ion trap loading techniques. Measured loading rates are compared with a simple theoretical model, and we conclude that this technique can potentially ionize every atom traversing the laser beam within the trapping volume. This may allow the operation of ion traps with lower levels of background pressures and less trap electrode surface contamination. The technique and laser system reported here should be applicable to loading most laser-cooled ion species.

Ion Trap Networking: Cold, Fast, and Small

A large-scale ion trap quantum computer will require low-noise entanglement schemes and methods for networking ions between different regions. We report work on both fronts, with the entanglement of two trapped cadmium ions following a phase-insensitive Molmer-Sorensen quantum gate, the entanglement between a single ion and a single photon, and the development of advanced ion traps at the micrometer scale, including the first ion trap integrated on a semiconductor chip. We additionally report progress on the interaction of ultrafast resonant laser pulses with cold trapped ions. This includes fast Rabi oscillations on optical S-P transitions and broadband laser cooling, where the pulse laser bandwidth is much larger than the atomic linewidth. With these fast laser pulses, we also have developed a new method for precision measurement of excited state lifetimes.

Clean and scalable quantum logic with trapped ions

Summary form only given. Experimental progress in constructing a quantum computer with a collection of trapped ions is discussed, with emphasis on how various technical hurdles may be circumvented.