Direct observation of time correlated single-electron tunneling

We report a direct detection of time correlated single-electron tunneling oscillations in a series array of small tunnel junctions. Here the current, I, is made up of a lattice of charge solitons moving throughout the array by time correlated tunneling with the frequency f=I/e, where e is the electron charge. To detect the single charges, we have integrated the array with a radio-frequency single-electron transistor (RF-SET) and employed two different methods to couple the array to the SET input: by direct injection through a tunnel junction, and by capacitive coupling. In this paper we report the results from the latter type of charge input, where we have observed the oscillations in the frequency domain and measured currents from 50 to 250 fA by means of electron counting.Comment: 2 pages, 1 figure; submitted to the 10th International Superconductive Electronics Conference (ISEC'05), the Netherlands, Sept. 200

Line Widths of Single-Electron Tunneling Oscillations: Experiment and Numerical Simulations

We present experimental and numerical results from a real-time detection of time-correlated single-electron tunneling oscillations in a one-dimensional series array of small tunnel junctions. The electrons tunnel with a frequency f=I/e, where I is the current and e is the electron charge. Experimentally, we have connected a single-electron transistor to the last array island, and in this way measured currents from 5 fA to 1 pA by counting the single electrons. We find that the line width of the oscillation is proportional to the frequency f. The experimental data agrees well with numerical simulations.Comment: 2 pages, 1 figure. Submitted to the 24th International Conference on Low Temperature Physics (LT24), Orlando, FL, USA, Aug. 2005; to be published in the AIP Conference Proceedings serie

Designing frequency-dependent relaxation rates and Lamb shift for a giant artificial atom

In traditional quantum optics, where the interaction between atoms and light at optical frequencies is studied, the atoms can be approximated as point-like when compared to the wavelength of light. So far, this relation has also been true for artificial atoms made out of superconducting circuits or quantum dots, interacting with microwave radiation. However, recent and ongoing experiments using surface acoustic waves show that a single artificial atom can be coupled to a bosonic field at several points wavelengths apart. Here, we theoretically study this type of system. We find that the multiple coupling points give rise to a frequency dependence in the coupling strength between the atom and its environment, and also in the Lamb shift of the atom. The frequency dependence is given by the discrete Fourier transform of the coupling point coordinates and can therefore be designed. We discuss a number of possible applications for this phenomenon, including tunable coupling, single-atom lasing, and other effects that can be achieved by designing the relative coupling strengths of different transitions in a multi-level atom.Comment: 14 pages, 8 figure

Coulomb blockade thermometry using a two-dimensional array of tunnel junctions

We have measured current-voltage characteristics of two-dimensional arrays of small tunnel junctions at temperatures from 1.5 K to 4.2 K. This corresponds to thermal energies larger than the charging energy. We show that 2D-arrays can be used as primary thermometers in the same way as 1D-arrays, and even have some advantages over 1D-arrays. We have carried out Monte Carlo simulations, which agree with our experimental results.Comment: 4 pages, 4 eps figures. Also available from Journal of Applied Physics (http://link.aip.org/link/?jap/86/3844

A fast, primary Coulomb blockade thermometer

We have measured the third derivative of the current-voltage characteristics, d^3I/dV^3, in a two-dimensional array of small tunnel junctions using a lock-in amplifier. We show that this derivative is zero at a voltage which scales linearly with the temperature and depends only on the temperature and natural constants, thus providing a primary thermometer. We demonstrate a measurement method which extracts the zero crossing voltage directly using a feedback circuit. This method requires only one voltage measurement, which makes it substantially faster than the original Coulomb blockade thermometry method.Comment: 3 pages, 4 figures. This article has been submitted to Applied Physics Letters (http://ojps.aip.org/aplo

Thermal properties of charge noise sources

Measurements of the temperature and bias dependence of Single Electron Transistors (SETs) in a dilution refrigerator show that charge noise increases linearly with refrigerator temperature above a voltage-dependent threshold temperature, and that its low temperature saturation is due to SET self-heating. We show further that the two-level fluctuators responsible for charge noise are in strong thermal contact with the electrons in the SET, which can be at a much higher temperature than the substrate. We suggest that the noise is caused by electrons tunneling between the SET metal and nearby potential wells

Resonant and off-resonant microwave signal manipulation in coupled superconducting resonators

We present an experimental demonstration as well as a theoretical model of an integrated circuit designed for the manipulation of a microwave field down to the single-photon level. The device is made of a superconducting resonator coupled to a transmission line via a second frequency-tunable resonator. The tunable resonator can be used as a tunable coupler between the fixed resonator and the transmission line. Moreover, the manipulation of the microwave field between the two resonators is possible. In particular, we demonstrate the swapping of the field from one resonator to the other by pulsing the frequency detuning between the two resonators. The behavior of the system, which determines how the device can be operated, is analyzed as a function of one key parameter of the system, the damping ratio of the coupled resonators. We show a good agreement between experiments and simulations, realized by solving a set of coupled differential equations.Comment: 10 pages, 6 figure

Storage and on-demand release of microwaves using superconducting resonators with tunable coupling

We present a system which allows to tune the coupling between a superconducting resonator and a transmission line. This storage resonator is addressed through a second, coupling resonator, which is frequency-tunable and controlled by a magnetic flux applied to a superconducting quantum interference device (SQUID). We experimentally demonstrate that the lifetime of the storage resonator can be tuned by more than three orders of magnitude. A field can be stored for 18 {\mu}s when the coupling resonator is tuned off resonance and it can be released in 14 ns when the coupling resonator is tuned on resonance. The device allows capture, storage, and on-demand release of microwaves at a tunable rate.Comment: 5 pages, 3 figure

Period-tripling subharmonic oscillations in a driven superconducting resonator

We have observed period-tripling subharmonic oscillations, in a superconducting coplanar waveguide resonator operated in the quantum regime, $k_B T \ll \hbar\omega$. The resonator is terminated by a tunable inductance that provides a Kerr-type nonlinearity. We detected the output field quadratures at frequencies near the fundamental mode, $\omega/2\pi \sim 5\,$GHz, when the resonator was driven by a current at $3\omega$ with an amplitude exceeding an instability threshold. The output radiation was red-detuned from the fundamental mode. We observed three stable radiative states with equal amplitudes and phase-shifted by $120^\circ$. The downconversion from $3\omega$ to $\omega$ is strongly enhanced by resonant excitation of the second mode of the resonator, and the cross-Kerr effect. Our experimental results are in quantitative agreement with a model for the driven dynamics of two coupled modes

Simple, robust and on-demand generation of single and correlated photons

We propose two different setups to generate single photons on demand using an atom in front of a mirror, along with either a beam-splitter or a tunable coupling. We show that photon generation efficiency ~99% is straightforward to achieve. The proposed schemes are simple and easily tunable in frequency. The operation is relatively insensitive to dephasing and can be easily extended to generate correlated pairs of photons. They can also in principle be used to generate any photonic qubit of the form $\mu |0 \rangle + \nu |1\rangle$ in arbitrary wave-packets, making them very attractive for quantum communication applications.Comment: 10 pages, Added appendi