A. Marx

University of Technology Munich, München, Bavaria, Germany

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Publications (86)201.13 Total impact

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    ABSTRACT: We report the observation of strong coupling between the exchange-coupled spins in a gallium-doped yttrium iron garnet and a superconducting coplanar microwave resonator made from Nb. The measured coupling rate of 450 MHz is proportional to the square root of the number of exchange-coupled spins and well exceeds the loss rate of 50 MHz of the spin system. This demonstrates that exchange-coupled systems are suitable for cavity quantum electrodynamics experiments, while allowing high integration densities due to their spin densities of the order of one Bohr magneton per atom. Our results furthermore show, that experiments with multiple exchange-coupled spin systems interacting via a single resonator are within reach.
    Physical Review Letters 09/2013; 111(12):127003. · 7.94 Impact Factor
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    ABSTRACT: We study quantum state tomography, entanglement detection, and channel noise reconstruction of propagating quantum microwaves via dual-path methods. The presented schemes make use of the following key elements: propagation channels, beam splitters, linear amplifiers, and field quadrature detectors. Remarkably, our methods are tolerant to the ubiquitous noise added to the signals by phase-insensitive microwave amplifiers. Furthermore, we analyze our techniques with numerical examples and experimental data. Our methods provide key toolbox components that may pave the way towards quantum microwave teleportation and communication protocols.
    08/2013;
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    ABSTRACT: Josephson parametric amplifiers (JPA) are promising devices for applications in circuit quantum electrodynamics (QED) and for studies on propagating quantum microwaves because of their good noise performance. In this work, we present a systematic characterization of a flux-driven JPA at millikelvin temperatures. In particular, we study in detail its squeezing properties by two different detection techniques. With the homodyne setup, we observe squeezing of vacuum fluctuations by superposing signal and idler bands. For a quantitative analysis we apply dual-path cross-correlation techniques to reconstruct the Wigner functions of various squeezed vacuum and thermal states. At 10 dB signal gain, we find 4.9+-0.2 dB squeezing below vacuum. In addition, we discuss the physics behind squeezed coherent microwave fields. Finally, we analyze the JPA noise temperature in the degenerate mode and find a value smaller than the standard quantum limit for phase-insensitive amplifiers.
    New Journal of Physics 07/2013; 15(12). · 4.06 Impact Factor
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    ABSTRACT: In the field of quantum information processing, superconducting circuits have become a well-established platform. In particular, systems consisting of a few qubits and/or harmonic oscillator circuits have been investigated. When scaling up these systems, it seems practical to aim for active guidance elements allowing for a directed transmission of quantum signals. One way to achieve this is by implementing switchable coupling between two microwave resonators. We show experimental progress on two superconducting transmission line resonators, where a superconducting flux qubit mediates a controllable coupling - the Quantum Switch. We show an experimental characterization of such a device and discuss spectroscopic evidence for the switching behavior.[4pt] We acknowledge support from the DFG via SFB 631, the German excellence initiative via NIM, and EU projects CCQED, SOLID and PROMISCE, the Basque Foundation for Science, Basque Government IT472-10, and Spanish MICINN FIS2009-12773-C02-01, DZ granted by ARAID
    03/2013;
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    ABSTRACT: Coupled superconducting transmission line resonators have applications in quantum information processing and fundamental quantum mechanics. A particular example is the realization of fast beam splitters, which however is hampered by two-mode squeezer terms. Here, we experimentally study superconducting microstrip resonators which are coupled over one third of their length. By varying the position of this coupling region we can tune the strength of the two-mode squeezer coupling from 2.4% to 12.9% of the resonance frequency of 5.44GHz. Nevertheless, the beam splitter coupling rate for maximally suppressed two-mode squeezing is 810MHz, enabling the construction of a fast and pure beam splitter.
    02/2013;
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    ABSTRACT: The parametric coupling of electromagnetic and mechanical degrees of freedom gives rise to a host of optomechanical phenomena. Examples include quantum-limited displacement measurements, sideband cooling or amplification of mechanical motion. Likewise, this interaction provides mechanically mediated functionality for the processing of electromagnetic signals, such as microwave amplification. Here, we couple a superconducting niobium coplanar waveguide cavity to a nanomechanical oscillator, and demonstrate all-microwave field-controlled tunable slowing and advancing of microwave signals, with millisecond distortion-free delay and negligible losses. This is realized by using electromechanically induced transparency, an effect analogous to electromagnetically induced transparency in atomic physics. Moreover, by temporally modulating the electromechanical coupling and correspondingly the transparency window, switching of microwave signals is demonstrated and its temporal dynamics investigated. The exquisite temporal control gained over the electromechanical coupling provides the basis for realizing advanced protocols for storage of both classical and quantum microwave signals.
    Nature Physics 01/2013; · 19.35 Impact Factor
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    ABSTRACT: Path entanglement constitutes an essential resource in quantum information and communication protocols. Here, we demonstrate frequency-degenerate entanglement between continuous-variable quantum microwaves propagating along two spatially separated paths. We combine a squeezed and a vacuum state using a microwave beam splitter. Via correlation measurements, we detect and quantify the path entanglement contained in the beam splitter output state. Our experiments open the avenue to quantum teleportation, quantum communication, or quantum radar with continuous variables at microwave frequencies.
    Physical Review Letters 12/2012; 109(25):250502. · 7.94 Impact Factor
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    ABSTRACT: For gradiometric three-Josephson-junction flux qubits, we perform a systematic study on the tuning of the minimal transition frequency, the so-called qubit gap. By replacing one of the qubit's Josephson junctions by a dc SQUID, the critical current of this SQUID and, in turn, the qubit gap can be tuned in situ by a control flux threading the SQUID loop. We present spectroscopic measurements demonstrating a well-defined controllability of the qubit gap between zero and more than 10 GHz. In the future, this enables one to tune the qubit into and out of resonance with other superconducting quantum circuits, while operating the qubit at its symmetry point with optimal dephasing properties. The experimental data agree very well with model calculations based on the full qubit Hamiltonian. From a numerical fit, we determine the Josephson coupling and the charging energies of the qubit junctions. The derived values agree well with those measured for other junctions fabricated on the same chip. We also demonstrate the biasing of gradiometric flux qubits near the symmetry point by trapping an odd number of flux quanta in the gradiometer loop. In this way, we study the effect of the significant kinetic inductance, thereby obtaining valuable information for the qubit design.
    New Journal of Physics 10/2012; 15(4). · 4.06 Impact Factor
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    ABSTRACT: In this work we show that a tunable coupling between microwave resonators can be engineered by means of simple Josephson junctions circuits, such as dc- and rf-SQUIDs. We show that by controlling the time dependence of the coupling it is possible to switch on and off and modulate the cross-talk, boost the interaction towards the ultrastrong regime, as well as to engineer red and blue sideband couplings, nonlinear photon hopping and classical gauge fields. We discuss how these dynamically tunable superconducting circuits enable key applications in the fields of all optical quantum computing, continuous variable quantum information and quantum simulation - all within the reach of state of the art in circuit-QED experiments.
    Physical review. B, Condensed matter 07/2012; 87(13).
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    ABSTRACT: Waveguide resonators are crucial elements in sensitive astrophysical detectors [1] and circuit quantum electrodynamics (cQED) [2]. Coupled to artificial atoms in the form of superconducting qubits [3, 4], they now provide a technologically promising and scalable platform for quantum information processing tasks [2, 5-8]. Coupling these circuits, in situ, to other quantum systems, such as molecules [9, 10], spin ensembles [11, 12], quantum dots [13] or mechanical oscillators [14, 15] has been explored to realize hybrid systems with extended functionality. Here, we couple a superconducting coplanar waveguide resonator to a nano-coshmechanical oscillator, and demonstrate all-microwave field controlled slowing, advancing and switching of microwave signals. This is enabled by utilizing electromechanically induced transparency [16-18], an effect analogous to electromagnetically induced transparency (EIT) in atomic physics [19]. The exquisite temporal control gained over this phenomenon provides a route towards realizing advanced protocols for storage of both classical and quantum microwave signals [20-22], extending the toolbox of control techniques of the microwave field.
    06/2012;
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    ABSTRACT: We investigate a network of coupled superconducting transmission line resonators, each of them made nonlinear with a capacitively shunted Josephson junction coupling to the odd flux modes of the resonator. The resulting eigenmode spectrum shows anticrossings between the plasma mode of the shunted junction and the odd resonator modes. Notably, we find that the combined device can inherit the complete nonlinearity of the junction, allowing for a description as a harmonic oscillator with a Kerr nonlinearity. Using a dc SQUID instead of a single junction, the nonlinearity can be tuned between 10 kHz and 4 MHz while maintaining resonance frequencies of a few gigahertz for realistic device parameters. An array of such nonlinear resonators can be considered a scalable superconducting quantum simulator for a Bose-Hubbard Hamiltonian. The device would be capable of accessing the strongly correlated regime and be particularly well suited for investigating quantum many-body dynamics of interacting particles under the influence of drive and dissipation.
    New Journal of Physics 02/2012; 14(7). · 4.06 Impact Factor
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    ABSTRACT: Superconducting microstrip resonators have several advantages when designing scalable circuit QED systems. Their simple geometry facilitates the implementation of additional circuit elements and control lines, and, most importantly, their spectrum tends to exhibit nearly no parasitic modes up to 20 GHz even for more complicated geometries. However, due to their specific field configuration they are not expected to yield high Q-factors at very low temperatures. We analyzed such resonators at Millikelvin temperatures and find experimentally useful quality factors of approximately 1500 even in the low temperature low power limit. Our analysis indicates that even ten times higher quality factors can be achieved straightforwardly by choosing substrates with better dielectric properties. Supported by the DFG via SFB 631 and by the German Excellence Initiative via NIM
    02/2012;
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    ABSTRACT: The recent evolution of circuit quantum electrodynamics with standing-wave microwave modes towards setups for propagating quantum microwaves has triggered the need for low-loss superconducting microwave beam splitters. Such a device should have ports obeying the coplanar geometry relevant for circuit QED and, at the same time, be compact for the sake of scalability. This combination presents a serious challenge. In this work, we present an experimental characterization of various compact superconducting coplanar microwave beam splitters. In addition, we briefly discuss efforts towards a tunable beam splitter.
    02/2012;
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    ABSTRACT: Using a low-mass (˜ 15,g), high-Q (> 100 000) nanomechanical oscillator coupled to a Nb superconducting quarter wave cavity, we realize a circuit nano-electromechanical system coupling microwaves to mechanical motion oscillating at 1.45,Hz. By exciting the system on the lower motional sideband with a strong drive tone, a transparency window for a probe field is created originating from the effect of optomechanically induced transparency (OMIT). This phenomenon, analogous to electromagnetically induced transparency in Atomic Physics, arises from the interference of different excitation pathways for an intracavity probe field. We utilize the transparency window to demonstrate slow microwave propagation. A tunable delay up to 4,s is demonstrated experimentally for a microwave pulse on resonance with the cavity. Furthermore, we systematically investigate the temporal dynamics of this transparency window when the drive tone is modulated, and the effect of the oscillator's Duffing nonlinearity on the OMIT window.
    02/2012;
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    ABSTRACT: The persistent current flux qubit is a Josephson junction based superconducting circuit exhibiting a strong anharmonicity in combination with excellent coherence times of more than 10,μs. However, quantum coherence decreases drastically away from an optimal point and a controlled design of the transition frequency at this point is demanding with respect to fabrication stability. Here, we present the spectroscopic analysis of a gradiometric flux qubit, where the tunnel coupling can be tuned from a few hundreds of Megahertz to several Gigahertz.
    02/2012;
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    ABSTRACT: Superconducting qubits acting as artificial two-level atoms allow for controlled variation of the symmetry properties which govern the selection rules for single and multiphoton excitation. We spectroscopically analyze a superconducting qubit-resonator system in the strong coupling regime under one- and two-photon driving. Our results provide clear experimental evidence for the controlled transition from an operating point governed by dipolar selection rules to a regime where one- and two-photon excitations of the artificial atom coexist. We find that the vacuum coupling between qubit and resonator can be straightforwardly extracted from the two-photon spectra where the detuned two-photon drive does not populate the relevant resonator mode significantly.
    arXiv. 07/2011;
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    ABSTRACT: For the detection of propagating quantum microwaves in circuit QED linear amplifiers are key ingredients. Phase sensitive amplifiers [e.g., Josephson parametric amplifiers (JPA)] in principle allow for the amplification of one signal quadrature without adding noise. In practice, however, internal losses often introduce a finite amount of noise. We have recently shown that, despite such a residual noise, signals on the quantum level can be fully characterized using two amplification chains and suitable correlations [E.P. Menzel et al., PRL 105, 100401 (2010)]. In this work, we characterize a flux-driven JPA. At 5.64,Hz the maximum degenerate gain is 25.5,B and the signal bandwidth is 1.8,Hz. Phase-insensitive measurements yield a noise temperature of 100±20,K, which is below the standard quantum limit of 135,K.
    03/2011;
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    ABSTRACT: Phase sensitive linear amplifiers receive increasing interest for applications in the field of circuit QED as they allow for the amplification of one signal quadrature without, in principle, adding noise. The flux-driven Josephson parametric amplifier characterized in this work is formed by a SQUID- terminated transmission line resonator with resonant frequency that can be varied by applying an ac magnetic flux signal through the SQUID. We have characterized two Josephson parametric amplifiers with different design parameters with respect to the center frequency and quality factor of the resonator, phase-dependent and phase-independent gains, as well as compression points and bandwidths.
    03/2011;
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    ABSTRACT: Experiments in the field of circuit QED require detection schemes for microwave signals on the single photon level. In particular, devices acting as microwave beam splitters are necessary. Using Nb thin films on silicon and sapphire substrates, we fabricated superconducting 180^o microstrip hybrid rings acting as beam splitters with center frequencies of about 6,Hz. For the magnitude of the coupling and isolation we find -3.5±0.5,B and at least -15,B, respectively, in a bandwidth of 2,Hz. We also investigate the effect of reflections at the contact between the superconducting hybrid ring and the normal conducting wiring using low temperature laser scanning microscopy. Our measurements indicate that our hybrid rings are well suited for on-chip applications in circuit QED experiments. We acknowledge financial support by the DFG via SFB 631, as well as support by and CFN, EU project SOLID and the German Excellence Initiative via NIM.
    01/2011;
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    ABSTRACT: Coupled superconducting transmission line resonators have potential applications in quantum information processing and fundamental quantum mechanics. Experimentally, high coupling strengths are desirable for a clear demonstration of quantum effects. We achieve coupling strengths of 10% of the resonator frequency (ultrastrong coupling) by distributed coupling. We find that, differently from the case of point-like coupling, the normal modes are no longer arranged symmetrically with respect to the single resonator frequency. Nevertheless, a detailed theoretical analysis shows that the system can still be described by a beam splitter Hamiltonian for two effective resonators. We expect that this result will allow for straightforward experimental access to exciting effects such as thermal entanglement in our samples.
    01/2011;

Publication Stats

297 Citations
201.13 Total Impact Points

Institutions

  • 2009–2012
    • University of Technology Munich
      • Faculty of Physics
      München, Bavaria, Germany
  • 2001–2012
    • Bavarian Academy of Sciences and Humanities
      Arching, Bavaria, Germany
  • 1997–1999
    • University of Cologne
      • Institute of Physics
      Köln, North Rhine-Westphalia, Germany
  • 1994–1995
    • University of Tuebingen
      • Institute of Physical and Theoretical Chemistry
      Tübingen, Baden-Wuerttemberg, Germany