Anthony J. Hoffman

University of Notre Dame, South Bend, Indiana, United States

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Publications (75)197.2 Total impact

  • Michael Harter, Yamac Dikmelik, Anthony J. Hoffman
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    ABSTRACT: The localization of electron wavefunctions due to interface roughness in a quantum cascade heterostructures is investigated by observing the electroluminescence spectra. Localization is more prominent in heterostructures with designed extended states.
    CLEO: Science and Innovations; 06/2014
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    ABSTRACT: A wide-bandwidth, highly efficient method of inter-chip waveguide coupling suitable for on-chip, mid-infrared sensing is discussed. Simulations and preliminary fabrication work on laser-to-waveguide coupling are presented, with losses predicted to be better than 6 dB.
    CLEO: Applications and Technology, San Jose, CA; 06/2014
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    ABSTRACT: We demonstrate excitation of surface phonon polaritons on patterned gallium phosphide surfaces. Control over the light-polariton coupling frequencies is demonstrated by changing the pattern periodicity and used to experimentally determine the gallium phosphide surface phonon polariton dispersion curve. Selective emission via out-coupling of thermally excited surface phonon polaritons is experimentally demonstrated. Samples are characterized experimentally by Fourier transform infrared reflection and emission spectroscopy, and modeled using finite element techniques and rigorous coupled wave analysis. The use of phonon resonances for control of emissivity and excitation of bound surface waves offers a potential tool for the exploration of long-wavelength Reststrahlen band frequencies.
    Applied Physics Letters 01/2014; 104(13):131105-131105-5. · 3.52 Impact Factor
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    ABSTRACT: We present Finite-Difference Time-Domain (FDTD) simulations to explore feasibility of chip-to-chip waveguide coupling via Optical Quilt Packaging (OQP). OQP is a newly proposed scheme for wide-bandwidth, highly-efficient waveguide coupling and is suitable for direct optical interconnect between semiconductor optical sources, optical waveguides, and detectors via waveguides. This approach leverages advances in quilt packaging (QP), an electronic packaging technique wherein contacts formed along the vertical faces are joined to form electrically-conductive and mechanically-stable chip-to-chip contacts. In OQP, waveguides of separate substrates are aligned with sub-micron accuracy by protruding lithographically-defined copper nodules on the side of a chip. With OQP, high efficiency chip-to-chip optical coupling can be achieved by aligning waveguides of separate chips with sub-micron accuracy and reducing chip-to-chip distance. We used MEEP (MIT Electromagnetic Equation Propagation) to investigate the feasibility of OQP by calculating the optical coupling loss between butt coupled waveguides. Transmission between a typical QCL ridge waveguide and a single-mode Ge-on-Si waveguide was calculated to exceed 65% when an interchip gap of 0.5 μm and to be no worse than 20% for a gap of less than 4 μm. These results compare favorably to conventional off-chip coupling. To further increase the coupling efficiency and reduce sensitivity to alignment, we used a horn-shaped Ge-on-Si waveguide and found a 13% increase in coupling efficiency when the horn is 1.5 times wider than the wavelength and 2 times longer than the wavelength. Also when the horizontal misalignment increases, coupling loss of the horn-shaped waveguide increases at a slower rate than a ridge waveguide.
    Proc SPIE 09/2013;
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    Yu Yao, Anthony J. Hoffman, Claire F. Gmachl
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    ABSTRACT: Mid-infrared quantum cascade lasers are semiconductor injection lasers whose active core implements a multiple-quantum-well structure. Relying on a designed staircase of intersubband transitions allows free choice of emission wavelength and, in contrast with diode lasers, a low transparency point that is similar to a classical, atomic four-level laser system. In recent years, this design flexibility has expanded the achievable wavelength range of quantum cascade lasers to similar to 3-25 mu m and the terahertz regime, and provided exemplary improvements in overall performance. Quantum cascade lasers are rapidly becoming practical mid-infrared sources for a variety of applications such as trace-chemical sensing, health monitoring and infrared countermeasures. In this Review we focus on the two major areas of recent improvement: power and power efficiency, and spectral performance.
    Nature Photonics 06/2012; 6(7):432-439. · 27.25 Impact Factor
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    ABSTRACT: We present a tunable coupling qubit (TCQ), which has independent and fast control over the qubit energy and the coupling strength to a superconducting microwave cavity in a circuit quantum electrodynamics architecture.
    05/2012;
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    ABSTRACT: We discuss various experimental approaches to improve the single shot measurement fidelity of a superconducting charge qubit. Dispersive readout is optimized on a transmon coupled to a superconducting coplanar waveguide resonator. Measurement parameters, such as microwave power and frequency are varied. Also control theory is adapted to construct a genetic algorithm which optimizes the shape of the drive pulse. Additionally, we attempt to reduce noise and increase SNR by employing a SLUG amplifier. Using these techniques, we discuss the feasibility of reaching the measurement fidelity needed for scalable quantum computation with superconducting circuits.
    02/2012;
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    ABSTRACT: We report measurements of the coupling between a superconducting microwave resonator and a transmon qubit fabricated on a separate chip and mounted to a three-dimensional cryogenic translation stage. The qubit-resonator system reached the strong coupling regime with a coupling strength in excess of 80 MHz. We use the translation stage to explore the position dependence of the coupling strength. With a scanning qubit stage, it is possible to measure many qubits in succession and study the statistics of the fabrication process. The system can also be used as a local probe of a large array of microwave cavities and superconducting qubits.
    02/2012;
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    ABSTRACT: Strong photon-qubit coupling in the circuit quantum electrodynamics architecture may lead to quantum phase transitions of light. Recent theoretical and experimental efforts have been made toward examining such quantum phase transitions in large systems; however, interesting crossovers may also exist in significantly smaller and more controllable systems. A sharp nonequilibrium self-trapping transition of light has been predicted in a system comprising two coupled resonators each containing a single qubit. A delocalized regime, where photons coherently oscillate between the two cavities, transitions via dissipation into a localized regime, where photons cannot tunnel. We realized this system experimentally using two capacitively coupled superconducting microwave coplanar waveguides each containing a single transmon qubit. We present our experimental investigation of the system using time and frequency domain measurements to probe its dynamics.
    02/2012;
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    ABSTRACT: It has been proposed that arrays of electromagnetic cavities, coupled to two level quantum systems can be used to realize quantum phase transitions of polaritons. One possible experimental realization is a circuit quantum electrodynamics architecture, in which transmon qubits are coupled to superconducting coplanar waveguide resonators (CPWRs); however, for this to be successful, arrays of resonators must be fabricated with low disorder. Results will be reported on characterization of an array of 12 niobium resonators on a sapphire substrate in a honeycomb pattern with the photonic lattice sites forming a Kagome star. These arrays were characterized by measuring many devices of the same design, and using statistical methods for analysis. Furthermore we investigate the origins of disorder, and its dependence on fluctuations in the CPWR geometry.
    02/2012;
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    ABSTRACT: We present results of time domain measurements on a tunable coupling qubit (TCQ) coupled to a superconducting coplanar waveguide resonator. The TCQ has the benefit of independently tunable qubit frequency and cavity-qubit coupling. We show that the TCQ's frequency and coupling can be dynamically controlled in tens of nanoseconds by using two on-chip flux control lines. Using this dynamic control, Rabi oscillations were measured at various coupling strengths showing that the coupling can be reduced by a factor greater than 1500. To measure qubit coherence at low coupling, the TCQ was tuned to a high coupling region, excited by a synchronized pi-pulse and then returned to the zero coupling region where the qubit state was measured. Coherence times of several microseconds were measured and are comparable to other superconducting qubits
    02/2012;
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    ABSTRACT: We demonstrate coherent control and measurement of a superconducting qubit coupled to a superconducting coplanar waveguide resonator with a dynamically tunable qubit-cavity coupling strength. Rabi oscillations are measured for several coupling strengths showing that the qubit transition can be turned off by a factor of more than 1500. We show how the qubit can still be accessed in the off state via fast flux pulses. We perform pulse delay measurements with synchronized fast flux pulses on the device and observe $T_1$ and $T_2$ times of 1.6 and 1.9 $\mu$s, respectively. This work demonstrates how this qubit can be incorporated into quantum computing architectures.
    Physical review. B, Condensed matter 08/2011; 84. · 3.66 Impact Factor
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    ABSTRACT: All-semiconductor, highly anisotropic metamaterials provide a straightforward path to negative refraction in the mid-infrared. However, their usefulness in applications is restricted by strong frequency dispersion and limited spectral bandwidth. In this work, we show that by stacking multiple metamaterials of varying thickness and doping into one compound metamaterial, bandwidth is increased by 27% over a single-stack metamaterial, and dispersion is reduced.
    Optics Express 08/2011; 19(16):14990-8. · 3.55 Impact Factor
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    ABSTRACT: Mediated photon-photon interactions are realized in a superconducting coplanar waveguide cavity coupled to a superconducting charge qubit. These nonresonant interactions blockade the transmission of photons through the cavity. This so-called dispersive photon blockade is characterized by measuring the total transmitted power while varying the energy spectrum of the photons incident on the cavity. A staircase with four distinct steps is observed and can be understood in an analogy with electron transport and the Coulomb blockade in quantum dots. This work differs from previous efforts in that the cavity-qubit excitations retain a photonic nature rather than a hybridization of qubit and photon and provides the needed tolerance to disorder for future condensed matter experiments.
    Physical Review Letters 07/2011; 107(5):053602. · 7.73 Impact Factor
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    ABSTRACT: We perform time-domain experiments on a superconducting qubit with a V-level energy structure coupled to a superconducting, coplanar waveguide resonator. Quantum interference and the V-level energy scheme allow independent control of the qubit energy and dipole via two on-chip fast flux bias lines [1]. The tunable dipole is predicted to protect the qubit from cavity-induced spontaneous emission. We probe this "Purcell protection" by measuring the qubit lifetime at constant cavity-qubit detuning for a range of coupling strengths. We also show how the coupled cavity-qubit energy spectrum allows for a cycling-type measurement that is predicted to improve the signal to noise ratio of qubit state readout by as much as an order of magnitude.[4pt] [1] J.M. Gambetta et al., arXiv:1009.4470v1
    03/2011;
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    ABSTRACT: Recently, quantum phase transitions of light have been the focus of much theoretical attention. One possible experimental realization relies upon the circuit quantum electrodynamics architecture (cQED); however, in order for this to be successful, coupled arrays of superconducting resonators must first be realized with low disorder. Here we fabricate and characterize an array with low disorder consisting of 12 niobium resonators on a sapphire substrate in a honeycomb pattern with the photonic lattice sites forming a Kagome star. The structure is characterized by measuring transmission through different input-output port pairs and by varying the hopping rate between resonators. A family of resonant peaks corresponding to the various modes of the coupled array is identifiable and agrees well with both a tight-binding Hamiltonian and simulations from a commercial microwave software package. These experiments are an important step in realizing strongly correlated interactions in cQED.
    03/2011;
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    ABSTRACT: We demonstrate a new superconducting charge qubit that realizes a V-shaped energy level spectrum, enabling tunable coupling between the qubit and a superconducting cavity while retaining all of the advantages, including charge noise insensitivity, common to other charge qubits such as the transmon. Tunable coupling is achieved with quantum interference between the two excited states of the qubit. We report measurements of the vacuum Rabi splitting, showing that the coupling strength can be tuned from greater than 40 MHz to less than 200 kHz using fast flux bias lines. This dynamically tunable coupling is an intrinsic property of the qubit and requires no additional coupling circuit elements. This new qubit design shows great promise for future quantum information processing and quantum optics experiments.
    03/2011;
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    ABSTRACT: We introduce a new type of superconducting charge qubit that has a V-shaped energy spectrum and uses quantum interference to provide independently tunable qubit energy and coherent coupling to a superconducting cavity. Dynamic access to the strong coupling regime is demonstrated by tuning the coupling strength from less than 200 kHz to greater than 40 MHz. This tunable coupling can be used to protect the qubit from cavity-induced relaxation and avoid unwanted qubit-qubit interactions in a multiqubit system.
    Physical Review Letters 02/2011; 106(8):083601. · 7.73 Impact Factor
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    ABSTRACT: We develop an analytical technique for retrieving the size and shape of subwavelength objects using far-field measurements. The approach relies on subwavelength diffraction gratings scattering evanescent information into the far field along with a numerical algorithm that is capable of deconvoluting this information based on the far-field intensity measurements. Several examples are presented, demonstrating resolution on the order of λ0/20. The developed method can be used at any frequency range, and may become a practical alternative to scanning near-field microscopy.
    Applied Physics Letters 09/2010; 97(10). · 3.52 Impact Factor
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    ABSTRACT: The choice of polarity of operation in a quantum cascade (QC) laser is made at the beginning of every QC laser design and growth, yet little work has been done to ascertain any performance benefits of one polarity versus the other. In this paper, we compare two QC lasers of the same design, differentiated only by the reversing of the growth order of the heterostructure layers in the laser core, which results in opposite polarities of operation. Analysis is performed through continuous wave (CW) and pulsed threshold current measurements to observe the change in active core temperature with input power. A thermoelectric effect is observed, where the direction of current flow improves thermal transport in negative polarity lasers (electron flow toward the heat sink) over positive polarity (electron flow away from the heat sink), leading to a maximum observed reduction in laser core heating of 10.0 ± 5.5 K for a thermal load of 7.2 kW/cm<sup>2</sup> in CW operation.
    IEEE Photonics Journal 07/2010; · 2.36 Impact Factor

Publication Stats

475 Citations
197.20 Total Impact Points

Institutions

  • 2014
    • University of Notre Dame
      • Department of Electrical Engineering
      South Bend, Indiana, United States
  • 2007–2010
    • Princeton University
      • Department of Electrical Engineering
      Princeton, NJ, United States
  • 2009
    • Johns Hopkins University
      • Department of Electrical and Computer Engineering
      Baltimore, MD, United States