Patrick See

Time-of-Flight Measurements of Single-Electron Wave Packets in Quantum Hall Edge States.

We report time-of-flight measurements on electrons traveling in quantum Hall edge states. Hot-electron wave packets are emitted one per cycle into edge states formed along a depleted sample boundary. The electron arrival time is detected by driving a detector barrier with a square wave that acts as a shutter. By adding an extra path using a deflection barrier, we measure a delay in the arrival time, from which the edge-state velocity v is deduced. We find that v follows 1/B dependence, in good agreement with the E[over →]×B[over →] drift. The edge potential is estimated from the energy dependence of v using a harmonic approximation.

A monolithic array of three-dimensional ion traps fabricated with conventional semiconductor technology

The coherent control of quantum-entangled states of trapped ions has led to significant advances in quantum information, quantum simulation, quantum metrology and laboratory tests of quantum mechanics and relativity. All of the basic requirements for processing quantum information with arrays of ion-based quantum bits (qubits) have been proven in principle. However, so far, no more than 14 ion-based qubits have been entangled with the ion-trap approach, so there is a clear need for arrays of ion traps that can handle a much larger number of qubits. Traps consisting of a two-dimensional electrode array have undergone significant development, but three-dimensional trap geometries can create a superior confining potential. However, existing three-dimensional approaches, as used in the most advanced experiments with trap arrays, cannot be scaled up to handle greatly increased numbers of ions. Here, we report a monolithic three-dimensional ion microtrap array etched from a silica-on-silicon wafer using conventional semiconductor fabrication technology. We have confined individual (88)Sr(+) ions and strings of up to 14 ions in a single segment of the array. We have measured motional frequencies, ion heating rates and storage times. Our results demonstrate that it should be possible to handle several tens of ion-based qubits with this approach.

Detection and susceptibility measurements of a single Dynal bead

In this work we present detection and susceptibility measurement experiments on a single superparamagnetic Dynal bead with a diameter of 1 μm and a magnetic moment of ≈4×108μB. Accurate bead positioning was achieved via non-invasive AFM nanomanipulation. The detection and magnetic characterization of the bead were performed using ultra-sensitive InSb Hall devices. Single bead detection was demonstrated using a step-wise change of the dc magnetic field; measurements were performed using only the in-phase component of the total ac Hall voltage. Very clear evidence of the bead presence is demonstrated simultaneously with explicit separation of parasitic inductive signals. Additional experiments performed using a sweeping change of the dc field allowed susceptibility measurements of a single Dynal bead. The numerical outcomes of both sweeping and stepping experiments are in a very good agreement. The method presented here opens up new possibilities for the reliable and accurate detection of small magnetic mom...

Magnetic nanoparticle detection using nano-SQUID sensors

We demonstrate detection of a single core-shell magnetite?silica nanoparticle (outer diameter ~120?nm, moment ~104?B) using an Nb dc superconducting quantum interference device (SQUID) with the loop size of 350?nm operational at T < 10?K. The system noise was minimized down to 0.2???0?Hz?1/2 using a cryogenic SQUID series array pre-amplifier. Initial measurements of an individual magnetic nanoparticle were performed and a clear change of the noise spectra of the nano-SQUID was detected at low frequencies in the presence of the nanoparticle. Similar behaviour was confirmed with an FePt nanoparticle with a larger magnetic moment (diameter ~150?nm, moment ~106?B). Thus, we demonstrate a magnetic sensor based on a dc nano-SQUID and enabling detection of small moments (potentially down to a few electron spins). Such a sensor is of considerable significance for nanomagnetic metrology and quantum information processing based on spin systems.

Ultrasmall particle detection using a submicron Hall sensor

We demonstrate detection of a single FePt nanoparticle (diameter 150 nm, moment ∼107u2002μB) using an ultrasensitive InSb Hall sensor with the bar lateral width of 600 nm. The white noise of a typical nanodevice, SV1/2≈28u2002nV/√Hz, is limited only by two-terminal resistance of the voltage leads which results in a minimum field sensitivity of the device Bmin=0.87u2002μT/√Hz. To detect a single FePt bead, we employed a phase-sensitive method based on measuring the ac susceptibility change in a bead when exposed to a switched dc magnetic field. Such nano-Hall devices, enabling detection of potentially even smaller moments, are of considerable significance both for nanomagnetic metrology and high sensitivity biological and environmental detectors.

Anomalous critical fields in quantum critical superconductors

Fluctuations around an antiferromagnetic quantum critical point (QCP) are believed to lead to unconventional superconductivity and in some cases to high-temperature superconductivity. However, the exact mechanism by which this occurs remains poorly understood. The iron-pnictide superconductor BaFe2(As1−xPx)2 is perhaps the clearest example to date of a high-temperature quantum critical superconductor, and so it is a particularly suitable system to study how the quantum critical fluctuations affect the superconducting state. Here we show that the proximity of the QCP yields unexpected anomalies in the superconducting critical fields. We find that both the lower and upper critical fields do not follow the behaviour, predicted by conventional theory, resulting from the observed mass enhancement near the QCP. Our results imply that the energy of superconducting vortices is enhanced, possibly due to a microscopic mixing of antiferromagnetism and superconductivity, suggesting that a highly unusual vortex state is realized in quantum critical superconductors.

Fabrication of a Monolithic Array of Three Dimensional Si-based Ion Traps

Segmented linear ion trap arrays are versatile devices that are increasingly used to study quantum physics and demonstrate the fundamental principles of quantum information processing. Traps with three-dimensional (3-D) electrode geometries can create a superior confining potential for ions. However, the realization of a monolithic 3-D microchip trap with scalable fabrication technology remains challenging. In this paper the microfabrication of a monolithic array of 3-D ion microtraps in a semiconductor chip is presented. The electrode structure is formed by micromachining a silica-on-silicon wafer and metallizing the dielectric with gold. The fabrication method uses conventional semiconductor wafer processing tools and techniques. The specific operating characteristics which demonstrate the suitability of the chosen material system and fabrication process are presented.

Detection of a Micron-Sized Magnetic Particle Using InSb Hall Sensor

We present our results on the realization of ultra-sensitive and non-invasive magnetic sensors based on double-cross InSb Hall bars with dimensions down to 1.5 mum. Hall sensors were patterned from an epitaxial InSb film characterized by the carrier concentration of 3.9 times 1016 cm-3 and high carrier mobility of 1.3 m 2/Vs. We study noise characteristics and magnetotransport properties of Hall sensors as well as their performance with respect to the lateral dimensions of devices. The adjacent crosses are characterized by very similar transport properties, which make them suitable for single magnetic particle detection. The high Hall coefficient, RH les 1000 V/AT, was measured for all devices. The white noise spectral density is of the order of 80 nV/radic Hz providing room-temperature magnetic field sensitivity better than 0.5 muT/ radic Hz. We demonstrate that micron-sized InSb Hall sensors can be successfully used for detection of a single paramagnetic bead with the diameter of 1 mu m. We also model the response of the Hall sensor to an inhomogeneous magnetic field caused by the presence of a magnetic bead.

Interference effects in a tunable quantum point contact integrated with an electronic cavity

We show experimentally how quantum interference can be produced using an integrated quantum system comprising an arch-shaped short quantum wire (or quantum point contact, QPC) of 1D electrons and a reflector forming an electronic cavity. On tuning the coupling between the QPC and the electronic cavity, fine oscillations are observed when the arch QPC is operated in the quasi-1D regime. These oscillations correspond to interference between the 1D states and a state which is similar to the Fabry-Perot state and suppressed by a small transverse magnetic field of n± n60 nu2009 nu2009 nmT n. Tuning the reflector, we find a peak in resistance which follows the behavior expected for a Fano resonance. We suggest that this is an interesting example of a Fano resonance in an open system which corresponds to interference at or near the Ohmic contacts due to a directly propagating, reflected discrete path and the continuum states of the cavity corresponding to multiple scattering. Remarkably, the Fano factor shows an oscillatory behavior taking peaks for each fine oscillation, thus, confirming coupling between the discrete and continuum states. The results indicate that such a simple quantum device can be used as building blocks to create more complex integrated quantum circuits for possible applications ranging from quantum-information processing to realizing the fundamentals of complex quantum systems.

Fano resonance in a cavity-reflector hybrid system

© 2017 authors. Published by the American Physical Society.We present the results of transport measurements in a hybrid system consisting of an arch-shaped quantum point contact (QPC) and a reflector; together, they form an electronic cavity in between them. On tuning the arch-QPC and the reflector, an asymmetric resonance peak in resistance is observed at the one-dimension to two-dimension transition. Moreover, a dip in resistance near the pinch-off of the QPC is found to be strongly dependent on the reflector voltage. These two structures fit very well with the Fano line shape. The Fano resonance was found to get weakened on applying a transverse magnetic field, and smeared out at 100 mT. In addition, the Fano-like shape exhibited a strong temperature dependence and gradually smeared out when the temperature was increased from 1.5 to 20 K. The results might be useful in realizing devices for quantum information processing.