Dense arrays of trapped ions provide one way of scaling up ion trap quantum information processing. However, miniaturization of ion traps is currently limited by sharply increasing motional state decoherence at sub-100 µm ion-electrode distances. We characterize heating rates in cryogenically cooled surface-electrode traps, with characteristic sizes in 75 µm to 150 µm range. Upon cooling to 6 K, the measured rates are suppressed by 7 orders of magnitude, two orders of magnitude below previously published data of similarly sized traps operated at room temperature. The observed noise depends strongly on fabrication process, which suggests further improvements are possible.PACS numbers: 32.80. Pj, 39.10.+j, 42.50.Vk Quantum information processing offers a tantalizing possibility of a significant speedup in execution of certain algorithms [1,2], as well as enabling previously unmanageable simulations of large quantum systems [3,4]. One of the most promising avenues towards practical quantum computation uses trapped ions as qubits. Interaction between qubits can be mediated by superconductive wires [5], photons [6,7] or by shared phonon modes [8]. The last scheme has been most successful so far, having demonstrated one and two qubit gates [9], teleportation [10,11], error correction[12] and shuttling [13]. Scaling of these experiments to a large number of ions will require arrays of small traps, on the order of 10 µm, to achieve dense qubit packing, improve the gate speed and reduce the time necessary to shuttle ions between different traps in the array [14,15,16,17]. Micro-fabrication techniques have been successfully used to fabricate a new generation of ion traps, demonstrating trap sizes down to 30 µm [18,19,20]. However, as the trap size is decreased, ion heating and decoherence of the motional quantum state increases rapidly, approximately as the fourth power of the trap size [21,22,23]. At currently observed values, the heating rate in a 10 µm trap would exceed 10 6 quanta/s, precluding ground-state cooling or qubit operations mediated by the motional state.The strong distance dependence of the heating rate suggests that the electric field noise is generated by surface charge fluctuations, which are small compared to the distance to the ion. Charge noise is also observed in condensed matter systems, where device fabrication has proven critical in reducing the problem [24,25]. Similar advances in ion traps are impeded by lack of data and models accurately predicting measured noise [26,27,28]. The charge fluctuations have been demonstrated to be thermally driven, providing another plausible route to reduce the heating. Cooling of the electrodes to 150 K has been shown to significantly decrease the heating rate [23].In this Letter, we present the first measurements of heating rates in ion traps cooled to 6 K. We designed and built a range of surface-electrode traps, in which we are able to cool a single ion to motional ground state with high fidelity and observe heating on a quantum level. Although the traps h...

Electric field noise from fluctuating patch potentials is a significant problem for a broad range of precision experiments, including trapped ion quantum computation and single spin detection. Recent results demonstrated strong suppression of this noise by cryogenic cooling, suggesting an underlying thermal process. We present measurements characterizing the temperature and frequency dependence of the noise from 7 to 100 K, using a single Sr+ ion trapped 75 mum above the surface of a gold plated surface electrode ion trap. The noise amplitude is observed to have an approximate 1/f spectrum around 1 MHz, and grows rapidly with temperature as T;{beta} for beta from 2 to 4. The data are consistent with microfabricated cantilever measurements of noncontact friction but do not extrapolate to the dc measurements with neutral atoms or contact potential probes.

We present a novel system where an optical cavity is integrated with a microfabricated planar-electrode ion trap. The trap electrodes produce a tunable periodic potential allowing the trapping of up to 50 separate ion chains aligned with the cavity and spaced by 160 µm in a one-dimensional array along the cavity axis. Each chain can contain up to 20 individually addressable Yb + ions coupled to the cavity mode. We demonstrate deterministic distribution of ions between the sites of the electrostatic periodic potential and control of the ion-cavity coupling. The measured strength of this coupling should allow access to the strong collective coupling regime with 10 ions. The optical cavity could serve as a quantum information bus between ions or be used to generate a strong wavelength-scale periodic optical potential.

We fabricate superconducting ion traps with niobium and niobium nitride and trap single 88 Sr ions at cryogenic temperatures. The superconducting transition is verified and characterized by measuring the resistance and critical current using a 4-wire measurement on the trap structure, and observing change in the rf reflection. The lowest observed heating rate is 2.1(3) quanta/sec at 800 kHz at 6 K and shows no significant change across the superconducting transition, suggesting that anomalous heating is primarily caused by noise sources on the surface. This demonstration of superconducting ion traps opens up possibilities for integrating trapped ions and molecular ions with superconducting devices.Microfabricated surface electrode ion traps have significantly advanced the capabilities of trapped ion systems for quantum information processing 1-3 , by enabling increased level of precision, density, and system integration. The ions trapped in these devices represent quantum bits and are confined by oscillating electric fields. While typical ion traps currently employ aluminum, gold, or doped semiconductor as the electrode material 2,4,5 , the anomalous electric field noise 6 affecting such traps provides significant motivation to explore qualitatively different materials for microfabricated ion traps, such as superconductors. In particular, the fact that a superconductor expels electric fields provides an opportunity to test the theoretical understanding that anomalous noise results from surface patch potentials 6,7 , rather than sources in the bulk, since the bulk noise sources would be screened by the superconductor. A similar approach was taken for neutral atoms, where in superconducting traps it was found that magnetic near-field noise is suppressed resulting in lower heating rate and longer spin-flip lifetimes 8,9 . For a thin film superconducting ion trap, blue lasers are typically employed for doppler cooling and state detection of trapped ions, and the short (279−422 nm 10 ) wavelengths may create quasiparticles in the superconductor, driving it into a normal state. Therefore, verifying that the superconductor employed is actually superconducting during an experiment is required.Here, we demonstrate the operation of several superconducting microfabricated ion traps made with niobium and niobium nitride, describe how superconductivity is verified during trap operation, and apply these traps to test the physical mechanisms of anomalous noise. The demonstration of superconducting ion traps opens up possibilities for integrating trapped ions and molecular ions with superconducting devices, such as photon counting detectors, microwave resonators 11 , and circuit-QED systems 12 .The ion traps used in this experiment consist of Nb or NbN on a sapphire substrate. One Nb and one NbN trap a) sxwang@mit.edu are identical to a prior five-electrode design 13 . An additional Nb trap (Nb-g) includes a thin wire structure 14 on the center ground electrode that is electrically connected in a 4-wire configuration to measure...

Dense array of ions in microfabricated traps represent one possible way to scale up ion trap quantum computing. The ability to address individual ions is an important component of such a scheme. We demonstrate individual addressing of trapped ions in a microfabricated surface-electrode trap using a magnetic field gradient generated on-chip. A frequency splitting of 310(2) kHz for two ions separated by 5 µm is achieved. Selective single qubit operations are performed on one of two trapped ions with an average of 2.2±1.0% crosstalk. Coherence time as measured by the spin-echo technique is unaffected by the field gradient.Trapped ion systems are promising candidates for implementing quantum computation [1]. Significant progress has been made in realizing quantum operations using a small number of ions as qubits [2,3,4,5,6]. Meaningful quantum computation involves a large number of qubits, and one possible way to scale up ion traps is to create a dense array of ions in microfabricated traps. The ability to individually address ions is an essential component of such a system. In this letter, we report the successful demonstration of individual addressing of single trapped ions in a linear crystal, using a magnetic field gradient generated in a microfabricated surface-electrode trap.Previous methods of individual ion addressing have included spatial and frequency separation. An example of spatial separation is the use of precisely focused laser beams aimed at only one ion at a time [7], which poses a significant technical challenge in laser beam control. Another approach is to transport ions between trap zones using varying DC potentials [4,8], which requires precision voltage and timing control. Separation in frequency space has been proposed and implemented by creating AC stark shifts using a far off-resonant laser [9,10]. Frequency selectivity can also be achieved by applying an inhomogeneous magnetic field gradient for spatiallyseparated ions [11]. This has been demonstrated in a linear Paul trap using hyperfine states of trapped 172 Yb + ions probed with rf radiation [12], and with neutral Cs atoms in an optical dipole trap [13]. A similar technique can be applied to magnetic-field-sensitive qubit transitions in the optical range. Such an approach requires separating the ion frequencies by much more than the desired gate speed [1].Here, we present the first design and implementation of a microfabricated trap to perform individual addressing for optical transitions using a magnetic field gradient. Addressability is demonstrated by observing distinct peaks in the frequency spectrum, and by performing Rabi oscillations on one of two trapped ions with a low probability of unwanted excitation on the neighbouring ion. Integrating the gradient-generating wires with microfabricated traps assures position stability. We evaluate this individual addressing scheme to show that we can achieve reasonable gate speeds, and minimal crosstalk between neighbouring ions, with no decrease in coherence time measured with spin-echo. Fi...

We present two simple cryogenic RF ion trap systems in which cryogenic temperatures and ultra high vacuum pressures can be reached in as little as 12 hours. The ion traps are operated either in a liquid helium bath cryostat or in a low vibration closed cycle cryostat. The fast turn around time and availability of buffer gas cooling made the systems ideal for testing surface-electrode ion traps. The vibration amplitude of the closed cycled cryostat was found to be below 106 nm. We evaluated the systems by loading surface-electrode ion traps with 88 Sr + ions using laser ablation, which is compatible with the cryogenic environment. Using Doppler cooling we observed small ion crystals in which optically resolved ions have a trapped lifetime over 2500 minutes.

Electrical charging of metal surfaces due to photoelectric generation of carriers is of concern in trapped ion quantum computation systems, due to the high sensitivity of the ions' motional quantum states to deformation of the trapping potential. The charging induced by typical laser frequencies involved in doppler cooling and quantum control is studied here, with microfabricated surface electrode traps made of aluminum, copper, and gold, operated at 6 K with a single Sr$^+$ ion trapped 100 $\mu$m above the trap surface. The lasers used are at 370, 405, 460, and 674 nm, and the typical photon flux at the trap is 10$^{14}$ photons/cm$^2$/sec. Charging is detected by monitoring the ion's micromotion signal, which is related to the number of charges created on the trap. A wavelength and material dependence of the charging behavior is observed: lasers at lower wavelengths cause more charging, and aluminum exhibits more charging than copper or gold. We describe the charging dynamic based on a rate equation approach.Comment: 8 pages, 8 figure

CitationWang, Shannon X. et al. "Demonstration of a quantum logic gate in a cryogenic surface-electrode ion trap." Physical Review A 81.6 (2010): 062332.We demonstrate quantum control techniques for a single trapped ion in a cryogenic, surface-electrode trap. A narrow optical transition of Sr + along with the ground and first excited motional states of the harmonic trapping potential form a two-qubit system. The optical qubit transition is susceptible to magnetic field fluctuations, which we stabilize with a simple and compact method using superconducting rings. Decoherence of the motional qubit is suppressed by the cryogenic environment. ac Stark shift correction is accomplished by controlling the laser phase in the pulse sequencer, eliminating the need for an additional laser. Quantum process tomography is implemented on atomic and motional states by use of conditional pulse sequences. With these techniques, we demonstrate a Cirac-Zoller controlled-NOT gate in a single ion with a mean fidelity of 91(1)%.