# Publications

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

Science 342 (2013) 568-569

PJ Leek

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

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

T Frey, PJ Leek, M Beck, J Faist, A Wallraff, K Ensslin, T Ihn, M Büttiker

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

T Frey, PJ Leek, M Beck, A Blais, T Ihn, K Ensslin, A Wallraff

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

APPLIED PHYSICS LETTERS 98 (2011) ARTN 262105

T Frey, PJ Leek, M Beck, K Ensslin, A Wallraff, T Ihn

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

Journal of Physics: Conference Series 264 (2011)

D Bozyigit, C Lang, L Steffen, JM Fink, C Eichler, M Baur, R Bianchetti, PJ Leek, S Filipp, A Wallraff, MP Da Silva, A Blais

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

N Daniilidis, S Narayanan, SA Moeller, R Clark, TE Lee, PJ Leek, A Wallraff, S Schulz, F Schmidt-Kaler, H Haeffner

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

ArXiv (0)

D Bozyigit, C Lang, L Steffen, JM Fink, M Baur, R Bianchetti, PJ Leek, S Filipp, MPD Silva, A Blais, A Wallraff

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.

## Quantum-to-Classical Transition in Cavity Quantum Electrodynamics

ArXiv (0)

JM Fink, L Steffen, P Studer, LS Bishop, M Baur, R Bianchetti, D Bozyigit, C Lang, S Filipp, PJ Leek, A Wallraff

The quantum properties of electromagnetic, mechanical or other harmonic oscillators can be revealed by investigating their strong coherent coupling to a single quantum two level system in an approach known as cavity quantum electrodynamics (QED). At temperatures much lower than the characteristic energy level spacing the observation of vacuum Rabi oscillations or mode splittings with one or a few quanta asserts the quantum nature of the oscillator. Here, we study how the classical response of a cavity QED system emerges from the quantum one when its thermal occupation -- or effective temperature -- is raised gradually over 5 orders of magnitude. In this way we explore in detail the continuous quantum-to-classical crossover and demonstrate how to extract effective cavity field temperatures from both spectroscopic and time-resolved vacuum Rabi measurements.

## Cavity QED with separate photon storage and qubit readout modes

ArXiv (0)

PJ Leek, M Baur, JM Fink, R Bianchetti, L Steffen, S Filipp, A Wallraff

We present the realization of a cavity quantum electrodynamics setup in which photons of strongly different lifetimes are engineered in different harmonic modes of the same cavity. We achieve this in a superconducting transmission line resonator with superconducting qubits coupled to the different modes. One cavity mode is strongly coupled to a detection line for qubit state readout, while a second long lifetime mode is used for photon storage and coherent quantum operations. We demonstrate sideband based measurement of photon coherence, generation of n photon Fock states and the scaling of the sideband Rabi frequency with the square root of n using a scheme that may be extended to realize sideband based two-qubit logic gates.

## Thermal Excitation of Multi-Photon Dressed States in Circuit Quantum Electrodynamics

ArXiv (0)

JM Fink, M Baur, R Bianchetti, S Filipp, M Göppl, PJ Leek, L Steffen, A Blais, A Wallraff

The exceptionally strong coupling realizable between superconducting qubits and photons stored in an on-chip microwave resonator allows for the detailed study of matter-light interactions in the realm of circuit quantum electrodynamics (QED). Here we investigate the resonant interaction between a single transmon-type multilevel artificial atom and weak thermal and coherent fields. We explore up to three photon dressed states of the coupled system in a linear response heterodyne transmission measurement. The results are in good quantitative agreement with a generalized Jaynes-Cummings model. Our data indicates that the role of thermal fields in resonant cavity QED can be studied in detail using superconducting circuits.

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

ArXiv (0)

PJ Leek, S Filipp, P Maurer, M Baur, R Bianchetti, JM Fink, M Göppl, L Steffen, A Wallraff

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)

M Göppl, A Fragner, M Baur, R Bianchetti, S Filipp, JM Fink, PJ Leek, G Puebla, L Steffen, A Wallraff

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

A Fragner, M Goeppl, JM Fink, M Baur, R Bianchetti, PJ Leek, A Blais, A Wallraff

## Adiabatic charge pumping in carbon nanotube quantum dots

PHYSICAL REVIEW LETTERS 101 (2008) ARTN 126803

MR Buitelaar, V Kashcheyevs, PJ Leek, VI Talyanskii, CG Smith, D Anderson, GAC Jones, J Wei, DH Cobden

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

NATURE 454 (2008) 315-318

JM Fink, M Goeppl, M Baur, R Bianchetti, PJ Leek, A Blais, A Wallraff

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

SCIENCE 318 (2007) 1889-1892

PJ Leek, JM Fink, A Blais, R Bianchetti, M Goeppl, JM Gambetta, DI Schuster, L Frunzio, RJ Schoelkopf, A Wallraff

## Charge pumping in carbon nanotubes

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

VI Talyanskii, P Leek, M Buitelaar, CG Smith, D Anderson, G Jones, J Wei, D Cobden

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

SEMICONDUCTOR SCIENCE AND TECHNOLOGY 21 (2006) S69-S77

MR Buitelaar, PJ Leek, VI Talyanskii, CG Smith, D Anderson, GAC Jones, J Wei, DH Cobden

## Charge pumping in carbon nanotubes

PHYSICAL REVIEW LETTERS 95 (2005) ARTN 256802

PJ Leek, MR Buitelaar, VI Talyanskii, CG Smith, D Anderson, GAC Jones, J Wei, DH Cobden

ArXiv (0)