Session G6: Ion Trap Techniques and Measurements

Ultrafast Generation of Large Schrodinger Cat States

Using a series of spin-dependent kicks on a trapped Yb$+$ ion, we create large, entangled, Schrodinger cat states. We prepare the ion in a superposition of its two m$_{\mathrm{f}}=$0 hyperfine ground states, representing an effective spin-1/2 system. Trapped in a harmonic potential, the ion is illuminated with a specially shaped, 1.5 ns pulse that imparts a momentum kick on the ion with a spin-dependent direction. A fast Pockels cell allows us to change the direction of the spin-dependent kick from each subsequent pulse out of an 80MHz mode-locked laser. By concatenating a series of these very high fidelity spin-dependent kicks, we separate the ion's wave packet into two, spatially distinct states separated by about 200 recoil momenta and involving about 70 phonons. This method for creating a Schrodinger cat state is not time-limited by the trap frequency, and does not rely on confinement in the Lamb-Dicke regime.   

A scalable monolithic ion trap in three-dimensional geometry

We developed a three-dimensional monolithic radio frequency (RF) trap that has a deep confining potential and can be extended to contain multiple zones similar to two-dimensional surface traps. The trap is fabricated by gold coating on a laser-machined alumina plate which has been successfully used for trapping ions. The basic structure of the trap is analogous to the combination of the structure of three-layer trap [1] and the symmetric trap [2], but the post-processing assembly of multi-layers is not required in the fabrication. On a single layer of alumina plate, we implement RF electrode and twenty DC electrodes. We successfully load ions in the trap. We report the basic properties of the trap such as axial and radial trap frequencies, heating rates and compare them to the theoretical expectations. The monolithic trap is able to produce a uniformly spaced ion chain and the technology can be extended to implement a junction structure on the trap to transport ions for connecting different trap zones.\\[4pt] [1] W. K. Hensinger, et al., Appl. Phys. Lett. 88, 034101 (2006).\\[0pt] [2] F. Shaikh, et al., arXiv:1105.4909.   

Scalable imaging of trapped ions with integrated diffractive mirrors

The standard roadmap to large-scale trapped-ion quantum information processing requires simultaneous fluorescence collection from ions at a large array of trap sites. We experimentally demonstrate scalable, monolithically integrated optics for fluorescence collection. We lithographically fabricate high-numerical-aperture diffractive mirrors directly on a microfabricated surface ion trap array, using the trap electrodes as the reflective element. These mirrors collimate the fluorescence from ions trapped at particular array sites. The collection efficiency of the diffractive mirrors exceeds 4\%, on the order of standard bulk-optics collection systems. Since the diffractive mirrors are designed to be aberration-free, we anticipate that we will also achieve high-efficiency collection into single-mode fiber for quantum communications applications.   

Robust Quantum Information Processing with Trapped Ions in a Surface Trap

Microfabricated surface ion traps provide a scalable platform for building a trapped ion quantum information processor. These multi-segmented traps are fabricated using existing silicon processing technology and can provide the capability to store a chain of ions and shuttle parts of the chain to various locations within the trap structure. Utilizing micro-mirrors fabricated using microelectromechanical systems (MEMS) technology, we focus and shift Raman laser beams to individual ions in the chain to perform quantum logic gates on them. Using a microfabricated surface trap made by Sandia National Laboratories we demonstrate individually addressed single qubit gates on a chain of ions driven by a repetition-rate-stabilized frequency comb. Compensating pulse sequences were utilized to mitigate the effect of the intensity fluctuations of the Raman beams. Our MEMS-based individual addressing system requires around 5 $\mu$s to switch between different ions in the chain with crosstalk to neighboring qubits on the order of 10$^{-5}$ characterized by the intensity spillover of the addressing beams. Here we present full state tomography results on un-compensated and compensated single qubit gates, single qubit gate fidelities measured by randomized benchmarking techniques, and progress towards entangling gates and their characterization.   

Cryogenic surface-electrode ion trap apparatus

In this talk we describe the infrastructure necessary to operate a surface-electrode ion trap with integrated microwave conductors for near-field quantum control of $^{9}$Be$^{+}$ in a cryogenic environment. These traps are promising systems for analog quantum simulators and for quantum logic applications. Our group recently developed a trap with an integrated meander-like microwave guide for driving motional sidebands on an $^{9}$Be$^{+}$ ion [1]. The trap will be operated in a cryogenic vacuum chamber. We will discuss the vibrational isolated closed cycle cryostat and the design of the vacuum chamber with all electrical supplies necessary to apply two different microwave currents, dc voltages and three independent rf supplies to generate a reconfigurable rf trapping potential. We will also discuss the used hyperfine qubit and the laser systems required to cool and repump. Furthermore we will present the cryogenic, high aperture and fully acromatic imaging system.\\[4pt] [1] Carsjens \textit{et al}., Applied Physics B - 10.1007/s00340-013-5689-6 (2013)   

Trapping Ions in a 2-pi Parabolic Mirror

Trapped ion qubit is an excellent candidate for quantum computation and information due to its low decoherence, ease of control and detection, and ability to couple to a photon. Efficient coupling between ions and resonant photons is crucial for ion-photon and remote-ion entanglement protocols. We describe an operation of a RF ion trap in which a reflective parabolic surface serves as the trap's electrodes. This parabolic mirror covers a solid angle of approximately 2 Pi around the trapped ion, while a movable needle electrode allows precise ion placement at the focal point of the parabola. We measured approximately 40{\%} solid angle fluorescence collection from a single Ba$+$ ion with this setup, with an image spot size of about twice the diffraction limit. Progress on image correction and fiber coupling will be reported.   

Limits to optical imaging of trapped ions

Trapped ions are an important system for quantum information processing, molecular analysis, and precision metrology. Recent efforts have focused on development of large aperture optics for efficient fluorescence detection, coherent coupling to single mode fibres, and imaging with wavelength-scale resolution. The latter opens up the prospect of applying super-resolution imaging techniques to increase the sensitivity of precision metrology experiments with atomic ions or to probe the structure and dynamics of single well-isolated biomolecular ions. Laser-cooled atomic ions have frequently been approximated as point sources due to their strong confinement and low temperatures relative to the resolution and depth-of-focus of their associated imaging systems. We expand upon this to account for the effect of quantum or thermal motion in imaging trapped ions. To minimise the imaged spot size we calculate the optimal confinement geometry for a trapped ion based on the imaging system performance. Excursions along the optical axis introduce Laguerre-Gaussian ``donut-mode'' components to the imaged spot. Other factors that would increase the imaged size, such as Brownian motion in a buffer gas environment, are also considered.   

Trapped Ion Thermometry and Mass Determination through Imaging

Laser cooled atomic ions and sympathetically cooled molecular ions form ordered structures in an ion trap. In the weak binding limit, where the linewidth of the cooling transition is much larger than the trap frequency, we measure the temperature of a string of laser cooled ions using the dependence of the spatial width of the ions on the normal modes of vibration of the string. This thermometry method is passive and avoids any resonant excitation and consequent heating of the ions. In the case of a mixed string, we use an active imaging method to determine the molecular composition of the co-trapped sympathetically cooled species. Here the collective motion of the ions is resonantly excited via an rf voltage applied to the endcaps. Our apparatus for making molecular ions without significant increase in background pressure could be of interest for future experiments, since it can potentially be applied to investigate chemical reactions with small cross-sections. Our molecule production and mass determination techniques are also promising for recyclable spectroscopy, where the pure string of atomic ions is restored after spectroscopy on a particular quantum state of the molecular ions.   

Flexible control of two-species ion chains for quantum information processing

We will describe control of beryllium-calcium ion chains in a three-dimensional segmented linear Paul trap, which is relevant for scaling up quantum information processing based on trapped ions [1,2]. We have loaded multi-species ion strings, and imaged both species simultaneously using a bichromatic imaging system. We will describe experiments in quantum control using this system, including demonstrations of dissipative methods for preparing motional superpositions including squeezed states of motion, and new diagnostic methods for these states based on bichromatic Hamiltonians. These methods provide routes to open-systems quantum simulations with trapped ions. \\[4pt] [1] D. Kielpinski, C. Monroe, and D. J. Wineland, \textit{Nature} \textbf{417}, 709 (2002)\\[0pt] [2] J. P. Home, D. Hanneke, J. D. Jost, J. Amini, D. Leibfried and D. J. Wineland, \textit{Science} \textbf{325}, 1227 (2009).   

Tritium Helium-3 Mass Difference

By trapping and manipulating pairs of ions in a cryogenic Penning trap we are measuring the cyclotron frequency ratios HD$^{\mathrm{+}}$/$^{\mathrm{3}}$He$^{\mathrm{+}}$ and HD$^{\mathrm{+}}$/T$^{\mathrm{+}}$. From these ratios a more precise value for the atomic mass difference between T and $^{\mathrm{3}}$He, and hence the Q-value of tritium beta-decay can be derived. This will enable a strong test of the systematics in the large-scale tritium beta-decay spectrometer KATRIN, which aims for a ten-fold improvement in the laboratory measurement of the electron neutrino mass.