Michael J. Biercuk

Carbon nanotube composites for thermal management

Single-wall carbon nanotubes (SWNTs) were used to augment the thermal transport properties of industrial epoxy. Samples loaded with 1 wt % unpurified SWNT material showed a 70% increase in thermal conductivity at 40 K, rising to 125% at room temperature; the enhancement due to 1 wt % loading of vapor grown carbon fibers was three times smaller. Electrical conductivity data showed a percolation threshold between 0.1 and 0.2 wt % SWNT loading. The Vickers hardness rose monotonically with SWNT loading up to a factor of 3.5 at 2 wt %. These results suggest that the thermal and mechanical properties of SWNT-epoxy composites are improved, without the need to chemically functionalize the nanotubes.

Engineered two-dimensional Ising interactions in a trapped-ion quantum simulator with hundreds of spins.

The presence of long-range quantum spin correlations underlies a variety of physical phenomena in condensed-matter systems, potentially including high-temperature superconductivity. However, many properties of exotic, strongly correlated spin systems, such as spin liquids, have proved difficult to study, in part because calculations involving N-body entanglement become intractable for as few as N ≈ 30 particles. Feynman predicted that a quantum simulator—a special-purpose ‘analogue’ processor built using quantum bits (qubits)—would be inherently suited to solving such problems. In the context of quantum magnetism, a number of experiments have demonstrated the feasibility of this approach, but simulations allowing controlled, tunable interactions between spins localized on two- or three-dimensional lattices of more than a few tens of qubits have yet to be demonstrated, in part because of the technical challenge of realizing large-scale qubit arrays. Here we demonstrate a variable-range Ising-type spin–spin interaction, Ji,j, on a naturally occurring, two-dimensional triangular crystal lattice of hundreds of spin-half particles (beryllium ions stored in a Penning trap). This is a computationally relevant scale more than an order of magnitude larger than previous experiments. We show that a spin-dependent optical dipole force can produce an antiferromagnetic interaction , where 0 ≤ a ≤ 3 and di,j is the distance between spin pairs. These power laws correspond physically to infinite-range (a = 0), Coulomb–like (a = 1), monopole–dipole (a = 2) and dipole–dipole (a = 3) couplings. Experimentally, we demonstrate excellent agreement with a theory for 0.05 ≲ a ≲ 1.4. This demonstration, coupled with the high spin count, excellent quantum control and low technical complexity of the Penning trap, brings within reach the simulation of otherwise computationally intractable problems in quantum magnetism.

Optimized dynamical decoupling in a model quantum memory

Any quantum system, such as those used in quantum information or magnetic resonance, is subject to random phase errors that can dramatically affect the fidelity of a desired quantum operation or measurement. In the context of quantum information, quantum error correction techniques have been developed to correct these errors, but resource requirements are extraordinary. The realization of a physically tractable quantum information system will therefore be facilitated if qubit (quantum bit) error rates are far below the so-called fault-tolerance error threshold, predicted to be of the order of 10-3–10-6. The need to realize such low error rates motivates a search for alternative strategies to suppress dephasing in quantum systems. Here we experimentally demonstrate massive suppression of qubit error rates by the application of optimized dynamical decoupling pulse sequences, using a model quantum system capable of simulating a variety of qubit technologies. We demonstrate an analytically derived pulse sequence, UDD, and find novel sequences through active, real-time experimental feedback. The latter sequences are tailored to maximize error suppression without the need for a priori knowledge of the ambient noise environment, and are capable of suppressing errors by orders of magnitude compared to other existing sequences (including the benchmark multi-pulse spin echo). Our work includes the extension of a treatment to predict qubit decoherence under realistic conditions, yielding strong agreement between experimental data and theory for arbitrary pulse sequences incorporating nonidealized control pulses. These results demonstrate the robustness of qubit memory error suppression through dynamical decoupling techniques across a variety of qubit technologies.

Low-temperature atomic-layer-deposition lift-off method for microelectronic and nanoelectronic applications

We report a method for depositing patterned dielectric layers with submicron features using atomic layer deposition. The patterned films are superior to sputtered or evaporated films in continuity, smoothness, conformality, and minimum feature size. Films were deposited at 100–150 °C using several different precursors and patterned using either electron-beam or photoresist. The low deposition temperature permits uniform film growth without significant outgassing or hardbaking of resist layers. A lift-off technique presented here gives sharp step edges with edge roughness as low as ∼10 nm. We also measure dielectric constants (κ) and breakdown fields for the high-κ materials aluminum oxide (κ∼8–9), hafnium oxide (κ∼16–19), and zirconium oxide (κ∼20–29), grown under similar low temperature conditions.

Electrical Transport in Single-Wall Carbon Nanotubes

We review recent progress in the measurement and understanding of the electricalproperties of individual metal and semiconducting single-wall carbon nanotubes. Thefundamental scattering mechanisms governing the electrical transport in nanotubesare discussed, along with the properties of p–n and Schottky-barrier junctions insemiconductor tubes. The use of advanced nanotube devices for electronic,high-frequency, and electromechanical applications is discussed. We thenexamine quantum transport in carbon nanotubes, including the observation ofquantized conductance, proximity-induced supercurrents, and spin-dependentballistic transport. We move on to explore the properties of single and coupledcarbon-nanotube quantum dots. Spin and orbital (isospin) magnetic moments lead tofourfold shell structure and unusual Kondo phenomena. We conclude with adiscussion of unanswered questions and a look to future research directions.

Ultrasensitive detection of force and displacement using trapped ions

The ability to detect extremely small forces and nanoscale displacements is vital for disciplines such as precision spin-resonance imaging, microscopy, and tests of fundamental physical phenomena. Current force-detection sensitivity limits have surpassed 1 aN Hz(-1/2) (refs 6,7) through coupling of nanomechanical resonators to a variety of physical readout systems. Here, we demonstrate that crystals of trapped atomic ions behave as nanoscale mechanical oscillators and may form the core of exquisitely sensitive force and displacement detectors. We report the detection of forces with a sensitivity of 390 +/- 150 yN Hz(-1/2), which is more than three orders of magnitude better than existing reports using nanofabricated devices(7), and discriminate ion displacements of approximately 18 nm. Our technique is based on the excitation of tunable normal motional modes in an ion trap and detection through phase-coherent Doppler velocimetry, and should ultimately allow force detection with a sensitivity better than 1 yN Hz(-1/2) (ref. 16). Trapped-ion-based sensors could enable scientists to explore new regimes in materials science where augmented force, field and displacement sensitivity may be traded against reduced spatial resolution.

Gate-Defined Quantum Dots on Carbon Nanotubes

We report the realization of nanotube-based multiple quantum dots that are fully defined and controlled with electrostatic gates. Metallic top-gates are used to produce localized depletion regions in the underlying tubes; a pair of such depletion regions in a nanotube with ohmic contact electrodes defines the quantum dot. Top-gate voltages tune the transparencies of tunnel barriers as well as the electrostatic energies within single and multiple dots. This approach allows precise control over multiple devices on a single tube, and serves as a design paradigm for nanotube-based electronics and quantum systems.

Experimental Uhrig dynamical decoupling using trapped ions

We present a detailed experimental study of the Uhrig dynamical decoupling UDD sequence in a variety of noise environments. Our qubit system consists of a crystalline array of 9 Be + ions confined in a Penning trap. We use an electron-spin-flip transition as our qubit manifold and drive qubit rotations using a 124 GHz microwave system. We study the effect of the UDD sequence in mitigating phase errors and compare against the well known Carr-Purcell-Meiboom-Gill-style multipulse spin echo as a function of pulse number, rotation axis, noise spectrum, and noise strength. Our results agree well with theoretical predictions for qubit decoherence in the presence of classical phase noise, accounting for the effect of finite-duration pulses. Finally, we demonstrate that the Uhrig sequence is more robust against systematic over- or under-rotation and detuning errors than is multipulse spin echo, despite the precise prescription for pulse timing in UDD.

Local gating of carbon nanotubes

Local effects of multiple electrostatic gates placed beneath carbon nanotubes grown by chemical vapor deposition (CVD) are reported. Single-walled carbon nanotubes were grown by CVD from Fe catalyst islands across thin Mo “finger gates” (∼150 nm × 10 nm). Prior to tube growth, several finger gates were patterned lithographically and subsequently coated with a patterned high-κ dielectric using low-temperature atomic layer deposition. Transport measurements demonstrate that local finger gates have an effect that is distinct from that of a global backgate.

Near-ground-state transport of trapped-ion qubits through a multidimensional array

We have demonstrated transport of {sup 9}Be{sup +}ions through a two-dimensional Paul-trap array that incorporates an X junction, while maintaining the ions near the motional ground state of the confining potential well. We expand on the first report of the experiment in Blakestad et al.[Phys. Rev. Lett. 102, 153002 (2009)], including a detailed discussion of how the transport potentials were calculated. Two main mechanisms that caused motional excitation during transport are explained, along with the methods used to mitigate such excitation. We reduced the motional excitation below the results in the above reference by a factor of approximately 50. The effect of a mu-metal shield on qubit coherence is also reported. Finally, we examined a method for exchanging energy between multiple motional modes on the few-quanta level, which could be useful for cooling motional modes without directly accessing the modes with lasers. These results establish how trapped ions can be transported in a large-scale quantum processor with high fidelity.