Coplanar resonators as computational elements in a superconducting qubit architecture
We are coupling a superconducting phase qubit, implemented using a current-biased Josephson junct... more We are coupling a superconducting phase qubit, implemented using a current-biased Josephson junction, to a high-Q coplanar waveguide resonator. The interaction between the phase qubit and the resonator can be controlled by tuning the qubit frequency into and out of resonance with the resonator, a tuning that can be achieved dynamically over times short compared to the Rabi time. By combining the quantum control flexibility of the phase qubit with the long coherence time and bosonic nature of the resonator, a number of interesting quantum operations can be explored, including long-term phase coherent quantum memory and two-qubit bus architectures. In this talk we will report on our recent progress with this experiment.
Clamping losses are a widely discussed damping mechanism in nanoelectromechanical systems, limiti... more Clamping losses are a widely discussed damping mechanism in nanoelectromechanical systems, limiting the performance of these devices. Here we present a method to investigate this dissipation channel. Using an atomic force microscope tip as a local perturbation in the clamping region of a nanoelectromechanical resonator, we increase the energy loss of its flexural modes by at least one order of magnitude. We explain this by a transfer of vibrational energy into the cantilever, which is theoretically described by a reduced mechanical impedance mismatch between the resonator and its environment. A theoretical model for this mismatch, in conjunction with finite element simulations of the evanescent strain field of the mechanical modes in the clamping region, allows us to quantitatively analyse data on position and force dependence of the tip-induced damping. Our experiments yield insights into the damping of nanoelectromechanical systems with the prospect of engineering the energy exchange in resonator networks.
Demonstration of quantum entanglement, a key resource in quantum computation arising from a noncl... more Demonstration of quantum entanglement, a key resource in quantum computation arising from a nonclassical correlation of states, requires complete measurement of all states in varying bases. By using simultaneous measurement and state tomography, we demonstrated entanglement between two solid-state qubits. Single qubit operations and capacitive coupling between two superconducting phase qubits were used to generate a Bell-type state. Full two-qubit
Confining a laser field between two high reflectivity mirrors of a high-finesse cavity can increa... more Confining a laser field between two high reflectivity mirrors of a high-finesse cavity can increase the probability of a given cavity photon to be scattered by an atom traversing the confined photon mode. This enhanced coupling between light and atoms is successfully employed in cavity quantum electrodynamics experiments and led to a very prolific research in quantum optics. The idea
Carbon nanofilaments with carbon beads grown on their surfaces were successfully synthesized repr... more Carbon nanofilaments with carbon beads grown on their surfaces were successfully synthesized reproducibly by a floating catalyst CVD method. The nanofilaments hosting the pearl-like structures typically show an average diameter of about 60 nm, which mostly consists of low-ordered graphite layers. The beads with diameter range 150-450 nm are composed of hundreds of crumpled and random graphite layers. The mechanism for the formation of these beaded nanofilaments is ascribed to two nucleation processes of the pyrolytic carbon deposition, arising from a temperature gradient between different parts of the reaction chamber. Furthermore, the Raman scattering properties of the beaded nanofilaments have been measured, as well as their confocal Raman G-line images. The Raman spectra reveal that that the trunks of the nanofilaments have better graphitic properties than the beads, which is consistent with the HRTEM analysis. The beaded nanofilaments are expected to have high potential applications in composites, which should exhibit both particleand fiber-reinforcing functions for the host matrixes.
Measurement is one of the fundamental building blocks of quantum-information processing systems. ... more Measurement is one of the fundamental building blocks of quantum-information processing systems. Partial measurement, where full wavefunction collapse is not the only outcome, provides a detailed test of the measurement process. We introduce quantum-state tomography in a superconducting qubit that exhibits high-fidelity single-shot measurement. For the two probabilistic outcomes of partial measurement, we find either a full collapse or a coherent yet nonunitary evolution of the state. This latter behavior explicitly confirms modern quantum-measurement theory and may prove important for error-correction algorithms in quantum computation.
An electron-phonon cavity consisting of a quantum dot embedded in a free-standing GaAs/AlGaAs mem... more An electron-phonon cavity consisting of a quantum dot embedded in a free-standing GaAs/AlGaAs membrane is characterized in Coulomb blockade measurements at low temperatures. We find a complete suppression of single electron tunneling around zero bias leading to the formation of an energy gap in the transport spectrum. The observed effect is induced by the excitation of a localized phonon mode confined in the cavity. This phonon blockade of transport is lifted at magnetic fields where higher electronic states with nonzero angular momentum are brought into resonance with the phonon energy.
We introduce a new design concept for superconducting quantum bits (qubits) in which we explicitl... more We introduce a new design concept for superconducting quantum bits (qubits) in which we explicitly separate the capacitive element from the Josephson tunnel junction for improved qubit performance. The number of two-level systems (TLS) that couple to the qubit is thereby reduced by an order of magnitude and the measurement fidelity improves to 90%. This improved design enables the first demonstration of quantum state tomography with superconducting qubits using single shot measurements.
We report an optomechanical near-field coupling detection scheme which enabled the first optical ... more We report an optomechanical near-field coupling detection scheme which enabled the first optical measurement of nanomechanical motion with an imprecision 3 dB below that at the standard quantum limit at room temperature.
Confining a laser field between two high reflectivity mirrors of a high-finesse cavity can increa... more Confining a laser field between two high reflectivity mirrors of a high-finesse cavity can increase the probability of a given cavity photon to be scattered by an atom traversing the confined photon mode 1 . This enhanced coupling between light and atoms is successfully employed in cavity quantum electrodynamics experiments and led to a very prolific research in quantum optics 2,3 . The idea of extending such experiments to sub-wavelength sized nanomechanical systems has been recently proposed 4 in the context of optical cavity cooling 5,6 . Here we present an experiment involving a single nanorod consisting of about 10 9 atoms precisely positioned to plunge into the confined mode of a miniature high finesse Fabry-Pérot cavity. We show that the optical transmission of the cavity is affected not only by the static position of the nanorod but also by its vibrational fluctuation. While an imprint of the vibration dynamics is directly detected in the optical transmission, back-action of the light field is also anticipated to quench the nanorod Brownian motion 4 . This experiment shows the first step towards optical cavity controlled dynamics of mechanical nanostructures and opens up new perspectives for sensing and manipulation of optomechanical nanosystems.
The single-electron transistor is the fastest and most sensitive electrometer available today 1,2... more The single-electron transistor is the fastest and most sensitive electrometer available today 1,2 . Single-electron pumps and turnstiles are also being explored as part of the global effort to redefine the ampere in terms of the fundamental physical constants 3-6 . However, the possibility of electrons tunnelling coherently through these devices, a phenomenon known as co-tunnelling 7 , imposes a fundamental limit on device performance. It has been predicted 8 that it should be possible to completely suppress co-tunnelling in mechanical versions of the single-electron transistor 9 , which would allow mechanical devices to outperform conventional single-electron transistors in many applications. However, the mechanical devices developed so far are fundamentally limited by unwanted interactions with the electrical mechanisms that are used to excite the devices 10-12 . Here we show that it is possible to overcome this problem by using ultrasonic waves rather than electrical currents as the excitation mechanism, which we demonstrate at low temperatures. This is a significant step towards the development of high-performance devices.
Any polarizable body placed in an inhomogeneous electric field experiences a dielectric force. Th... more Any polarizable body placed in an inhomogeneous electric field experiences a dielectric force. This phenomenon is well known from the macroscopic world: a water jet is deflected when approached by a charged object. This fundamental mechanism is exploited in a variety of contexts-for example, trapping microscopic particles in an optical tweezer 1 , where the trapping force is controlled via the intensity of a laser beam, or dielectrophoresis 2 , where electric fields are used to manipulate particles in liquids.
Spin systems and harmonic oscillators comprise two archetypes in quantum mechanics 1 . The spin-1... more Spin systems and harmonic oscillators comprise two archetypes in quantum mechanics 1 . The spin-1/2 system, with two quantum energy levels, is essentially the most nonlinear system found in nature, whereas the harmonic oscillator represents the most linear, with an infinite number of evenly spaced quantum levels. A significant difference between these systems is that a two-level spin can be prepared in an arbitrary quantum state using classical excitations, whereas classical excitations applied to an oscillator generate a coherent state, nearly indistinguishable from a classical state 2 . Quantum behaviour in an oscillator is most obvious in Fock states, which are states with specific numbers of energy quanta, but such states are hard to create 3-7 . Here we demonstrate the controlled generation of multi-photon Fock states in a solid-state system. We use a superconducting phase qubit 8 , which is a close approximation to a two-level spin system, coupled to a microwave resonator, which acts as a harmonic oscillator, to prepare and analyse pure Fock states with up to six photons. We contrast the Fock states with coherent states generated using classical pulses applied directly to the resonator.
The microwave performance of amorphous dielectric materials at very low temperatures and very low... more The microwave performance of amorphous dielectric materials at very low temperatures and very low excitation strengths displays significant excess loss. Here, we present the loss tangents of some common amorphous and crystalline dielectrics, measured at low temperatures (T < 100 mK) with near single-photon excitation energies, E/hω0 ∼ 1, using both coplanar waveguide (CPW) and lumped LC resonators. The loss can be understood using a two-level state (TLS) defect model. A circuit analysis of the half-wavelength resonators we used is outlined, and the energy dissipation of such a resonator on a multilayered dielectric substrate is considered theoretically.
Optical cavities with small mode volume are well-suited to detect the vibration of sub-wavelength... more Optical cavities with small mode volume are well-suited to detect the vibration of sub-wavelength sized objects. Here we employ a fiber-based, high-finesse optical microcavity to detect the Brownian motion of a freely suspended carbon nanotube at room temperature under vacuum. The optical detection resolves deflections of the oscillating tube down to 50 pm/Hz 1/2 . A full vibrational spectrum of the carbon nanotube is obtained and confirmed by characterization of the same device in a scanning electron microscope. Our work successfully extends the principles of high-sensitivity optomechanical detection to molecular scale nanomechanical systems.
Evolution and decay of a superconducting Josephson junction qubit due to partial measurement
Superconducting Josephson phase qubits have been shown to be a promising candidate for scalable q... more Superconducting Josephson phase qubits have been shown to be a promising candidate for scalable quantum computing. In many such quantum computing algorithms, partial measurement of the quantum state is used to project the system into a required subspace. We experimentally study the effect of a partial measurement on our Josephson phase qubit using state tomography and high fidelity measurement capabilities.
<p> This conference will be devoted to the recent scientific advances at the interface betw... more <p> This conference will be devoted to the recent scientific advances at the interface between photonics and quantum physics. During the last decades, the studies of fundamental issues in quantum.
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Papers by Eva Weig