# Publications

## Speed limit of the insulator-metal transition in magnetite.

Nature materials **12** (2013) 882-886

As the oldest known magnetic material, magnetite (Fe3O4) has fascinated mankind for millennia. As the first oxide in which a relationship between electrical conductivity and fluctuating/localized electronic order was shown, magnetite represents a model system for understanding correlated oxides in general. Nevertheless, the exact mechanism of the insulator-metal, or Verwey, transition has long remained inaccessible. Recently, three-Fe-site lattice distortions called trimerons were identified as the characteristic building blocks of the low-temperature insulating electronically ordered phase. Here we investigate the Verwey transition with pump-probe X-ray diffraction and optical reflectivity techniques, and show how trimerons become mobile across the insulator-metal transition. We find this to be a two-step process. After an initial 300 fs destruction of individual trimerons, phase separation occurs on a 1.5±0.2 ps timescale to yield residual insulating and metallic regions. This work establishes the speed limit for switching in future oxide electronics.

## Applied physics. Storing quantum information in Schrödinger's cats.

Science **342** (2013) 568-569

## Quantum dot admittance probed at microwave frequencies with an on-chip resonator

Physical Review B - Condensed Matter and Materials Physics **86** (2012)

We present microwave frequency measurements of the dynamic admittance of a quantum dot tunnel-coupled to a two-dimensional electron gas. The measurements are made via a high-quality 6.75 GHz on-chip resonator capacitively coupled to the dot. The resonator frequency is found to shift both down and up close to conductance resonance of the dot corresponding to a change of sign of the reactance of the system from capacitive to inductive. The observations are consistent with a scattering matrix model. The sign of the reactance depends on the detuning of the dot from conductance resonance and on the magnitude of the tunnel rate to the lead with respect to the resonator frequency. Inductive response is observed on a conductance resonance when tunnel coupling and temperature are sufficiently small compared to the resonator frequency. © 2012 American Physical Society.

## Dipole Coupling of a Double Quantum Dot to a Microwave Resonator

PHYSICAL REVIEW LETTERS **108** (2012) ARTN 046807

## Characterization of a microwave frequency resonator via a nearby quantum dot

APPLIED PHYSICS LETTERS **98** (2011) ARTN 262105

## Correlation measurements of individual microwave photons emitted from a symmetric cavity

Journal of Physics: Conference Series **264** (2011)

Superconducting circuits have been successfully established as systems to prepare and investigate microwave light fields at the quantum level. In contrast to optical experiments where light is detected using photon counters, microwaves are usually measured with well developed linear amplifiers. This makes measurements of correlation functions - one of the important tools in optics - harder to achieve because they traditionally rely on photon counters and beam splitters. Here, we demonstrate a system where we can prepare on demand single microwave photons in a cavity and detect them at the two outputs of the cavity using linear amplifiers. Together with efficient data processing, this allows us to measure different observables of the cavity photons, including the first-order correlation function. Using these techniques we demonstrate cooling of a thermal background field in the cavity. © Published under licence by IOP Publishing Ltd.

## Fabrication and heating rate study of microscopic surface electrode ion traps

NEW JOURNAL OF PHYSICS **13** (2011) ARTN 013032

## Measurements of the Correlation Function of a Microwave Frequency Single Photon Source

ArXiv (0)

At optical frequencies the radiation produced by a source, such as a laser, a black body or a single photon source, is frequently characterized by analyzing the temporal correlations of emitted photons using single photon counters. At microwave frequencies, however, there are no efficient single photon counters yet. Instead, well developed linear amplifiers allow for efficient measurement of the amplitude of an electromagnetic field. Here, we demonstrate how the properties of a microwave single photon source can be characterized using correlation measurements of the emitted radiation with such detectors. We also demonstrate the cooling of a thermal field stored in a cavity, an effect which we detect using a cross-correlation measurement of the radiation emitted at the two ends of the cavity.

## Using Sideband Transitions for Two-Qubit Operations in Superconducting Circuits

ArXiv (0)

We demonstrate time resolved driving of two-photon blue sideband transitions between superconducting qubits and a transmission line resonator. Using the sidebands, we implement a pulse sequence that first entangles one qubit with the resonator, and subsequently distributes the entanglement between two qubits. We show generation of 75% fidelity Bell states by this method. The full density matrix of the two qubit system is extracted using joint measurement and quantum state tomography, and shows close agreement with numerical simulation. The scheme is potentially extendable to a scalable universal gate for quantum computation.

## Coplanar Waveguide Resonators for Circuit Quantum Electrodynamics

ArXiv (0)

We have designed and fabricated superconducting coplanar waveguide resonators with fundamental frequencies from 2 to $9 \rm{GHz}$ and loaded quality factors ranging from a few hundreds to a several hundred thousands reached at temperatures of $20 \rm{mK}$. The loaded quality factors are controlled by appropriately designed input and output coupling capacitors. The measured transmission spectra are analyzed using both a lumped element model and a distributed element transmission matrix method. The experimentally determined resonance frequencies, quality factors and insertion losses are fully and consistently characterized by the two models for all measured devices. Such resonators find prominent applications in quantum optics and quantum information processing with superconducting electronic circuits and in single photon detectors and parametric amplifiers.

## Resolving Vacuum Fluctuations in an Electrical Circuit by Measuring the Lamb Shift

SCIENCE **322** (2008) 1357-1360

## Adiabatic charge pumping in carbon nanotube quantum dots

PHYSICAL REVIEW LETTERS **101** (2008) ARTN 126803

## Climbing the Jaynes-Cummings ladder and observing its root n nonlinearity in a cavity QED system

NATURE **454** (2008) 315-318

## Observation of Berry's phase in a solid-state qubit

SCIENCE **318** (2007) 1889-1892

## Charge pumping in carbon nanotubes

PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES **34** (2006) 662-665

## Charge pumping and current quantization in surface acoustic-wave-driven carbon nanotube devices

SEMICONDUCTOR SCIENCE AND TECHNOLOGY **21** (2006) S69-S77

## Charge pumping in carbon nanotubes

PHYSICAL REVIEW LETTERS **95** (2005) ARTN 256802

## Excitation and detection of propagating spin waves at the single magnon level

ArXiv (0)

Ferro- and ferrimagnets play host to small-signal, microwave-frequency magnetic excitations called spin waves, the quanta of which are known as magnons. Over the last decade, the field of spin-wave dynamics has contributed much to our understanding of fundamental magnetism. To date, experiments have focussed overwhelmingly on the study of room-temperature systems within classical limits. Here we demonstrate, for the first time, the excitation and detection of propagating spin waves at the single magnon level. Our results allow us to project that coupling of propagating spin-wave excitations to quantum circuits is achievable, enabling fundamental quantum-level studies of magnon systems and potentially opening doors to novel hybrid quantum measurement and information processing devices.

## Realization of a Carbon-Nanotube-Based Superconducting Qubit

ArXiv (0)

Hybrid circuit quantum electrodynamics (QED) involves the study of coherent quantum physics in solid state systems via their interactions with superconducting microwave circuits. Here we present an implementation of a hybrid superconducting qubit that employs a carbon nanotube as a Josephson junction. We realize the junction by contacting a carbon nanotube with a superconducting Pd/Al bi-layer, and implement voltage tunability of the qubit frequency using a local electrostatic gate. We demonstrate strong dispersive coupling to a coplanar waveguide resonator via observation of a resonator frequency shift dependent on applied gate voltage. We extract qubit parameters from spectroscopy using dispersive readout and find qubit relaxation and coherence times in the range of $10-200~\rm{ns}$.

## Calibration of the cross-resonance two-qubit gate between directly-coupled transmons

ArXiv (0)

Quantum computation requires the precise control of the evolution of a quantum system, typically through application of discrete quantum logic gates on a set of qubits. Here, we use the cross-resonance interaction to implement a gate between two superconducting transmon qubits with a direct static dispersive coupling. We demonstrate a practical calibration procedure for the optimization of the gate, combining continuous and repeated-gate Hamiltonian tomography with step-wise reduction of dominant two-qubit coherent errors through mapping to microwave control parameters. We show experimentally that this procedure can enable a $\hat{ZX}_{-\pi/2}$ gate with a fidelity $F=97.0(7)\%$, measured with interleaved randomized benchmarking. We show this in a architecture with out-of-plane control and readout that is readily extensible to larger scale quantum circuits.