Session C4: Quantum Computing

One-way quantum computing in optical lattices with many atom measurements.

With one-way quantum computation universality can be achieved using only single qubit measurements on a highly entangled state, known as a cluster state. By manipulating ultracold atoms in optical lattices it is possible to efficiently generate large, many qubit cluster states. Although this approach is very promising, the small spacing between lattice sites severely restricts our ability to sequentially measure the states of individual atoms by external lasers, a necessary condition for universal computing. Working within the limitations of current technology we must generally consider many atom measurements. We have developed a deterministic protocol for one-way quantum computing based on many atom measurements on an optical lattice cluster state, with only polynomial classical overhead. Our scheme opens the way toward concrete experimental quantum computing in neutral atom systems.   

Entangling operations and rapid measurement of atomic clock-state qubits for violating Bell inequalities

Optical clock-transitions such as the ones in Ytterbium/Strontium and Cesium are prime candidates for encoding qubits for quantum information processing applications due to very low decoherence rates. In this work, we investigate the challenges involved in using these prime candidates. We devise entangling operations for atoms trapped in optical tweezers, as well as determine the feasibility of rapid qubit rotation and measurement of qubits encoded in these desirable low-decoherence clock transitions. We propose ultracold collisions for entangling operations and multi-photon transitions for fast rotation of qubits, followed by ultrafast readout via resonant multiphoton ionization. The rapid control of atomic qubits is crucial for high-speed synchronization of quantum information processors, but is also of interest for tests of Bell inequalities. We investigate rapid measurement of clock-state qubits in the context of a Bell inequality test that avoids the detection loophole in spacelike separated entangled qubits.   

Quantum Simulators, Spin Systems, and Trapped Ions

Many-quantum-spin systems cannot be efficiently simulated on classical computers as they require exponentially large resources. Yet many such systems can be simulated efficiently with quantum simulators (QS) that do not require universal control like quantum computers. Following the work of Porras and Cirac [Phys. Rev. Lett. 92, 207901-1 (2004)] we discuss current experimental efforts at Los Alamos to implement a QS for Ising-like and Heisenberg-like models with trapped ion qubit ``spins''. The states of the QS follow nearly the same equations of motion as the systems of interest, and unlike with real materials, the experimenter has the advantage of direct access to and control over the spins. We will discuss progress towards proof-of-principle investigations of two-ion simulations in a single-well trap, in which we use state-selective optical forces to induce ion-ion interactions.   

Sympathetic cooling of a atom in a transported trap via superfluid immersion, preserving quantum information

We investigate the possibility of using sympatheitic cooling via super-fluid immersion in order to suppress diabatic transitions in a system governed by a time-dependent Hamiltonian. A simple model is constructed in order to study how to store quantum information can be stored in the nuclear spin of a group-II atom that is trapped in a harmonic oscillator, while it is traveling at a constant velocity inside of a stationary BEC. While the motion of the trap acts to heat the atom in the trap to higher vibrational levels, the motion of the trapped atom creates excitations in the BEC and carries the energy away in the form of phonons and decreases the effective heating. Quantum information is preserved as the nuclear spin is decoupled from all other degrees of freedom.   

Bright Source of Cold Ions for Surface-Electrode Traps

We produce large numbers of low-energy ions by photoionization of laser-cooled atoms inside a surface-electrode-based Paul trap. The isotope-selective trap loading rate of $4\times10^{5}$ $^{174}$Yb$^{+}$ ions/s exceeds that attained by photoionization (electron impact ionization) of an atomic beam by four (six) orders of magnitude. Our high loading rate could enable rapid, isotope-selective loading of large ion trap arrays for use in quantum computing or atomic clocks. The ions are confined in the same spatial region as the laser-cooled atoms, which will also allow experimental investigation of the interactions between cold ions and cold atoms.   

Spatially imaging single atoms in a 3D optical lattice

We have built a 3D far-off-resonance optical lattice with 4.5 $\mu $m spacing. We load $\sim $6 atoms per site in the lattice. During cooling, photon-assisted interactions cause atom loss in pairs, until a random half of the central 500 sites are occupied by single atoms. We image the 3D pattern of atoms, one plane at a time, using high-numerical-aperture optics. Since the image is formed with the laser cooling light, and the steady state average energy of the atoms is well below the lattice depth, the process of identifying which sites are occupied and which are not does not change the atom locations. Atoms in this site-addressable optical lattice are promising qubits.   

Efficient qubit state detection using integrated optics in ion trap quantum computation.

Efficient and scalable detection of qubits in trapped ion systems is a major bottleneck in achieving scalable quantum information processor. High fidelity qubit detection is possible utilizing cycling transition, and monitoring the presence of scattered photons. The efficiency and speed of this detection process critically depends on the effectiveness of photon collection optics and the performance of the photon detectors used. In this study, we explore utilization of micro-optical components for effective collection of the scattered photons in a scalable manner. We also analyze the signal-to-noise ratio and corresponding bit-error-rate (BER) of the state detection based on different types of detectors available to date. The BER in the detection process critically depends on the quantum efficiency, internal gain, and the excess noise factor associated with the gain process. Based on this analysis, we propose an ideal photon detector based on visible light photon counter (VLPC) technology that provides the best BER performance. Combining the micro-optical collection and ideal photon detector is shown to improve the integration time required for the state detection by almost an order of magnitude at the same BER level as compared to current approaches using photomultiplier tubes.   

Spin exchange in a double-well optical lattice for a $\sqrt{SWAP}$ gate

We report the observation of coherent spin-exchange between pairs of atoms in a double-well optical lattice, from which a $\sqrt{SWAP}$ gate can be constructed. First, we perform qubit rotations selectively to atoms in either side of the double well to prepare pairs of qubits in the initial state. Each pair of neighboring atoms are then brought together in a single well and entangled through controlled coherent collisions. The ``exchange blockade'' arising from the symmetry of identical particles results in $\sqrt{SWAP}$, a universal two-qubit entangling gate. These demonstrations provide important tools for quantum information processing and simulation of condensed matter systems. We will also discuss how they can be extended to a scalable system for quantum computation.   

Excitation and interaction of Rydberg states in optical dipole traps

We present recent progress in two-photon excitation of Rydberg levels with $28 [Preview Abstract]

Femtosecond optical excitation of trapped barium ions

Ion-photon entanglement and the remote entanglement of trapped ions are crucial building blocks of several proposed quantum computing architectures. The creation and detection of single photons from the trapped ions is a fundamental step in this process. Ultrafast laser pulses with an optical bandwidth broad enough to coherently excite both hyperfine levels of the ground state can be used to create optical qubit photons differentiable by their frequency or polarization upon decay to their respective ground states. Preliminary results have been achieved with this technique using trapped $^{138}$Ba$^{+}$ ions, which displays Rabi oscillations between S$_{1/2}$ and P$_{3/2}$ states driven by near-resonant 400 fs pulses at 455 nm. We plan to use the odd isotope ($^{137}$Ba$^{+})$ whose nuclear spin leads to ground state hyperfine splitting, where the same excitation method would create optical qubits and ion-photon entangled states.   

Microfabricated surface-electrode ion traps for scalable quantum information processing

We confine individual atomic ions in rf Paul traps with a novel geometry where the electrodes are located in a single plane and the ions are confined above this plane \footnote{J. Chiaverini \emph{et al.}, Quantum Inf. Comput. \textbf{5}, 419 (2005).}$^,$ \footnote{S. Seidelin \emph{et al.}, Phys. Rev. Lett. \textbf{96}, 253003 (2006).}$^,$ \footnote{J. Britton \emph{et al.}, quant-ph/0605170.}. These devices are realized with simple fabrication procedures, making them potentially scalable for quantum information processing using large numbers of ions. For traps fabricated from gold on fused quartz, the ions are 40 micrometers above the planar electrodes and their heating rate is low enough to make the traps useful for quantum information processing.   

High fidelity quantum gates for ion qubits in optical transitions

One of the most important challenges in ion trap quantum computing is the implementation of a high fidelity two--qubit quantum gate. For hyperfine qubits state dependend AC--Stark shifts were used to demonstrate a two qubit gate with a fidelity of 0.97 [1]. Here, we show that similar forces can also be employed for qubits based on optical transitions as for instance the $S_{1/2} \rightarrow D_{5/2}$ transition in Calcium. In contrast to previous work, our proposal can be applied to magnetic--field insensitive transitions. It also allows for negligible spontaneous emission rates and can be implemented with a pair of co--propagating beams. According to simulations with current experimental imperfections, the proposed gate will allow for fidelities exceeding 0.99. \newline \newline [1] D. Leibfried, {\it et al.}, Nature {\bf 422}, 412 (2003).