QIC Abstracts


Vol.1 Special Issue Dec. 8, 2001 (in print: Dec 28, 2001)
Implementation of Quantum Computation

Editorial  (ppi-ii)
        R. Clark
Dogma and heresy in quantum computing  (pp1-6)
        D.P. DiVincenzo

Important new ideas for the physical implementation of quantum computers are reviewed.

Quantum networks based on cavity QED (pp7-12)
        H. Mabuchi, M. Armen, B. Lev, M. Loncar, J. Vuckovic, H.J. Kimble,  J.Preskill, M. Roukes, A. Scherer, and S.J. van Enk
We review an ongoing program of interdisciplinary research aimed at developing hardware and protocols for quantum communication networks. Our primary experimental goals are to demonstrate quantum state mapping from storage/processing media (internal states of trapped atoms) to transmission media (optical photons), and to investigate a nanotechnology paradigm for cavity QED that would involve the integration of magnetic microtraps with photonic bandgap structures.

Efficient linear optics quantum computation (pp13-19)
        G.J. Milburn, T. Ralph, A. White, E. Knill, and R. Laflamme
Two qubit gates for photons are generally thought to require exotic materials with huge optical nonlinearities. We show here that, if we accept two qubit gates that only work conditionally, single photon sources, passive linear optics and particle detectors are sufficient for implementing reliable quantum algorithms. The conditional nature of the gates requires feed-forward from the detectors to the optical elements. Without feed forward, non-deterministic quantum computation is possible. We discuss one proposed single photon source based on the surface acoustic wave guiding of single electrons.

Quantum control and information processing in optical lattices (pp20-32)
        P.S. Jessen, D.L. Haycock, G. Klose and G.A. Smith, I.H. Deutsch, and G.K. Brennen
Neutral atoms offer a promising platform for single- and many-body quantum control, as required for quantum information processing.  This includes excellent isolation from the decohering influence of the environment, and the existence of well developed techniques for atom trapping and coherent manipulation.  We present a review of our work to implement quantum control and measurement for ultra-cold atoms in far-off-resonance optical lattice traps.  In recent experiments we have demonstrated coherent behavior of mesoscopic atomic spinor wavepackets in optical double-well potentials, and carried out quantum state tomography to reconstruct the full density matrix for the atomic spin degrees of freedom.  This model system shares a number of important features with proposals to implement quantum logic and quantum computing in optical lattices.  We present a theoretical analysis of a protocol for universal quantum logic via single qubit operations and an entangling gate based on electric dipole-dipole interactions. Detailed calculations including the full atomic hyperfine structure suggests that high-fidelity quantum gates are possible under realistic experimental conditions.

Encoded universality from a single physical interaction (pp33-55)
        J. Kempe, D.  Bacon, D.P. DiVincenzo, and K.B. Whaley
We present a theoretical analysis of the paradigm of encoded universality, using a Lie algebraic analysis to derive specific conditions under which physical interactions can provide universality. We discuss the significance of the tensor product structure in the quantum circuit model and use this to define the conjoining of encoded qudits. The construction of encoded gates between conjoined qudits is discussed in detail. We illustrate the general procedures with several examples from exchange-only quantum computation. In particular, we extend our earlier results showing universality with the isotropic exchange interaction to the derivation of encoded universality with the anisotropic exchange interaction, i.e., to the XY model. In this case the minimal encoding for universality is into qutrits rather than into qubits as was the case for isotropic (Heisenberg) exchange. We also address issues of fault-tolerance, leakage and correction of encoded qudits.

Solid-state crystal lattice NMR quantum computation (pp56-81)
        T.D. Ladd, Y. Yamamoto, J.R. Goldman, and F. Yamaguchi

Construction of a silicon-based solid state quantum computer  (pp82-95)
        A.S. Dzurak, M.Y. Simmons, A.R. Hamilton, R.G. Clark, R. Brenner, T.M. Buehler, N.J. Curson, E. Gauja, R.P. McKinnon, L.D. Macks, M. Mitic, J.L. O’brien, L. Oberbeck, D.J. Reilly, S.R. Schofield, and F.E. Stanley
We discuss progress towards the fabrication and demonstration of a prototype silicon-based quantum computer. The devices are based on a precise array of 31P dopants embedded in 28Si.  Fabrication is being pursued via two complementary pathways – a ‘top-down’ approach for near-term production of few-qubit demonstration devices and a ‘bottom-up’ approach for large-scale qubit arrays.  The ‘top-down’ approach employs ion implantation through a multi-layer resist structure which serves to accurately register the donors to metal control gates and single-electron transistor (SET) read-out devices.  In contrast the ‘bottom-up’ approach uses STM lithography and epitaxial silicon overgrowth to construct devices at an atomic scale.  Techniques for qubit read-out, which utilise coincidence measurements on novel twin-SET devices, are also presented.

Quantum computation using electrons trapped by surface acoustic waves (pp96-101)
        C.H. W. Barnes, J.M. Shilton and A.M. Robinson
We outline a set of ideas for implementing a quantum processor based on technology used in surface acoustic wave (SAW) single-electron transport devices. These devices allow single electrons to be captured from a two-dimensional electron gas by a SAW. We discuss how these devices can be adapted to capture electrons in pure spin states and how both single and two-qubit gates can be constructed. We give designs for readout gates and discuss possible sources of error and decoherence.

Quantum computing with electrons floating on liquid helium  (pp102-107)
        M.I. Dykman and P.M. Platzman
Electrons on a helium surface form a quasi two-dimensional system which displays the highest mobility reached in condensed matter physics. We propose to use this system as a set of interacting quantum bits. We will briefly describe the system and discuss how the qubits can be addressed and manipulated. The working frequency of the proposed quantum computer is ~ 1GHz. Careful analysis shows that the relaxation rate can be at least 5 orders of magnitude smaller, for low temperatures.

Fabrication of the structure for qubits using electrons on liquid helium (pp108-112)
        J. Goodkind and S. Pilla
In the previous papers, the system of qubits using electrons on a liquid helium film was described. In this paper we describe the physical realization of the system that we have begun to fabricate. We will not in this brief discussion describe how we intend to operate the system. We will show that we are nano- and micro-fabricating a new type of electronic device that differs from other microelectronic devices in that the final step of the fabrication deposits a layer of helium rather than some other dielectric. The operation of the device will differ in that it manipulates single electrons and it must operate at low temperatures.

Recent results in trapped-ion quantum computing at NIST   (pp113-123)
        D. Kielpinski, A. Ben-Kish, J. Britton, V. Meyer, M.A. Rowe, W.M. Itano,   D.J. Wineland, C. Sackett, and C. Monroe
We review recent experiments on entanglement, Bell's inequality, and decoherence-free subspaces in a quantum register of trapped {9Be+} ions. We have demonstrated entanglement of up to four ions using the technique of Molmer and Sorensen. This method produces the state ({|\uparrow\uparrow\rangle}+{|\downarrow\downarrow\rangle})/\sqrt{2} for two ions and the state ({\downarrow}{\downarrow}{\downarrow}{\downarrow} \rangle + | {\uparrow}{\uparrow}{\uparrow}{\uparrow} \rangle)/\sqrt{2} for four ions. We generate the entanglement deterministically in each shot of the experiment. Measurements on the two-ion entangled state violates Bell's inequality at the 8\sigma level. Because of the high detector efficiency of our apparatus, this experiment closes the detector loophole for Bell's inequality measurements for the first time. This measurement is also the first violation of Bell's inequality by massive particles that does not implicitly assume results from quantum mechanics. Finally, we have demonstrated reversible encoding of an arbitrary qubit, originally contained in one ion, into a decoherence-free subspace (DFS) of two ions. The DFS-encoded qubit resists applied collective dephasing noise and retains coherence under ambient conditions 3.6 times longer than does an unencoded qubit. The encoding method, which uses single-ion gates and the two-ion entangling gate, demonstrates all the elements required for two-qubit universal quantum logic.

Qubit utilizing charge-number state in super conducting nanostructure  (pp124-128)
        J.S. Tsai, Y. Nakamura, and YU. Pashkin
In single-Cooper-pair box, the number of electrons in the box is quantized and they form a single macroscopic quantum charge-number state, corresponding to the number of excess electrons in the box. By making all the electrodes superconducting, we can couple two neighboring charge-number states coherently.  In this way one can create an artificial two-level system. Qubit operations were demonstrated in two different control techniques, dc electric-field gate bias and ac field bias. The dc method was unique compared with the commonly used Rabi-oscillation-type operation. Here the system was biased at the degenerate point of the two states so that the dynamical phase does not develop during the operation. This was the first time that the quantum coherent oscillation was observed in a solid-state device whose quantum states involved a macroscopic number of quantum particles. Multiple-pulse experiments were also carried out and phase control was also demonstrated.

Fabricating an all-epitaxial silicon quantum computer (pp129-133)
        J.R. Tucker and T.-C. Shen 
Scalable silicon quantum computers will require a material perfection that has never been
attempted. Ground state wavefunctions for conduction electrons orbiting individual phosphorous donors must be polarized electronically and coupled to nearest neighbors with great precision. Elimination of all randomizing influences can be achieved only with a fully epitaxial structure; and we believe that output circuitry must also be integrated into the qubit arrays in order to achieve the uniformity needed for large-scale integration. A process that could potentially accomplish this will be outlined, based on scanning tunneling microscope (STM) removal of individual hydrogen atoms from the H-terminated silicon surface followed by phosphine dosing and ultra-low-temperature overgrowth. Self-ordering of PH3 molecules onto extended areas of bare silicon should permit patterning of planar single-electron transistors along with P-donor qubits in the same lithographic step. Initial plans for an experiment to characterize exchange coupling under gate control will be described.

NMR quantum computing - lessons for the future (pp134-142)
        L. Vandersypen and I. Chuang
Future physical implementations of large-scale quantum computers will face significant practical challenges. Many useful lessons can be drawn from present results with Nuclear Magnetic Resonance realizations of controllable two, three, five, and seven qubit quantum systems. We summarize various experimental methods and theoretical procedures learned in this work which will be of considerable value in building and testing quantum processors with a wide variety of physical systems.

Universal quantum gates for single cooper pair box based quantum  computing  (pp143-150)
        P. Echternach, C.P. Williams, S.C. Dultz, S. Braunstein, and J.P. Dowling

back to QIC online Front page