Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session K51: Decoherence and Defects in Superconducting CircuitsFocus

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Sponsoring Units: GQI Chair: Kevin Osborn, Laboratory for Physical Sciences Room: 398 
Wednesday, March 15, 2017 8:00AM  8:36AM 
K51.00001: RandomDefect Laser: Manipulating Lossy TwoLevel Systems to Produce a Circuit with Coherent Gain Invited Speaker: Yaniv Rosen Random twolevel system (TLS) defects in dielectrics absorb energy and limit the coherence of superconducting quantum devices including qubits and resonators used in quantum computing applications. So far attempts to reduce this loss have been confined to device design and material optimization. In the present work we demonstrate the ability to control the loss of a dielectric by directly manipulating the population of its TLSs using a uniform swept dc electric field and two AC pump fields. The swept field shifts the TLS energies through a fixedfrequency pump field resulting in an inversion of the TLS population. After the sweep, the TLSs are brought into degeneracy with the resonator where they emit photons. The emission is found to be dependent on individual cavityTLS interactions, and the narrowing linewidth at increasing photon occupancy indicates stimulated emission. Characterization with a microwave probe shows a transition from ordinary defect loss to negligible microwave absorption, and ultimately to coherent amplification. Thus, instead of absorbing microwave energy, the TLSs can be tuned to reduce loss and even amplify signals. [Preview Abstract] 
Wednesday, March 15, 2017 8:36AM  8:48AM 
K51.00002: A compact qubit with tuned protection from sparse defects Neda Forouzani, Tim Kohler, Bahman Sarabi, Alexander L. Burin, Kevin D. Osborn Tunneling twolevel systems (TLSs), which are present in imperfect insulating regions and Josephson junction barriers of superconducting qubits, are known to be a source of decoherence. Fortunately, qubits that are made with a sufficiently large area allow small influence from the imperfect regions due to the large capacitance from ideal regions, namely the crystalline substrate below and vacuum above its superconducting metal. On the other hand, it is desirable to have compact qubits for multiqubit architectures, as this can enable a low residual qubit to qubit coupling. Although a compact qubit may have strong coupling to the defects and the possibility of added decoherence, it is also possible to take advantage of the finite number of TLSs in a small volume limit and thus to protect the qubit from the deleterious effects of TLSs. This is especially allowed if the defects can be tuned in energy. We present a compact design where the qubit and cavity share a piece of a deposited dielectric. While the qubit is coupled to TLSs located within the dielectric we can apply an electric field to bias the sparsely spaced TLSs out of the frequency range of the qubit. Additionally, the qubit state can be controlled independently from the nearresonance TLSs through microwave pulses. [Preview Abstract] 
Wednesday, March 15, 2017 8:48AM  9:00AM 
K51.00003: Intrinsic capacitance noise from twolevel systems in dielectrics Igor Diniz, Matthias le Dall, Rogerio de Sousa Resonant twolevel systems (TLSs) are believed to be the main source of dielectric loss in superconducting qubits at subKelvin temperatures. The presence of TLSs in the capacitors and Josephson junctions forming the qubits inevitably lead to qubit energy relaxation followed by photon emission with rate $1/T_1$. We show that in addition to the broadly recognized single photon contribution to $1/T_1$, inelastic multiphoton processes may also give an important contribution. Two and threephoton processes are related to capacitance noise which, in contrast to single photon loss, gives a broadband contribution to relaxation and dephasing. Dielectric loss occurs even when there are no TLSs resonant with the qubit excitation frequency. We predict that qubits made of high quality dielectrics with a low density of TLSs will have their $T_1$ and $T_2$ times dominated by capacitance noise. [Preview Abstract] 
Wednesday, March 15, 2017 9:00AM  9:12AM 
K51.00004: Characterization of the dominant loss mechanisms in superconducting coplanar waveguide resonators Greg Calusine, Alexander Melville, Wayne Woods, David K. Kim, Xhovalin Miloshi, Arjan Sevi, Jonilyn Yoder, William D. Oliver The characterization of losses in superconducting coplanar waveguide (CPW) resonators is commonly used as a surrogate means to probe relaxation in superconducting qubit capacitor structures. However, this method is complicated by devicetodevice variations that result from a sensitivity to variations in fabrication processes, packaging, and measurement methods. We present results on characterizing ensembles of aluminum, niobium, and titanium nitride superconducting CPW resonators to determine the statistical significance of the effects of fabrication process changes on resonator intrinsic quality factor. Furthermore, we report progress on experiments aimed at determining the impact of other competing loss mechanisms such as vortex trapping, package coupling, and substrate loss. These results are then applied to the study of relaxation in superconducting qubits and investigations into the microscopic origins of surface losses. This research was funded in part by the Intelligence Advanced Research Projects Activity (IARPA). The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of IARPA or the US Government. [Preview Abstract] 
Wednesday, March 15, 2017 9:12AM  9:24AM 
K51.00005: Addressing surfaceinduced loss and decoherence in superconducting quantum circuits Andreas Fuhrer, Peter Mueller, Andreas Kuhlmann, Stefan Filipp, Veeresh Deshpande, Ute Drechsler Many of the advances in coherence and fidelity of superconducting qubits have been made possible by clever engineering of the coupling to the environment and operation at noiseinsensitive sweet spots. However, this leads to a compromise in experimental flexibility and device tunability, which can become inhibitive as the system size is scaled up. Material and interface related degrees of freedoms are harder to mitigate and are expected to become increasingly important in more complex systems. They impose limits both on coherence (fluxnoise) and lifetimes (surface loss) of superconducting qubits. To study and eliminate these effects we have constructed a reusable UHVcompatible sample enclosure that enables us to perform various surface passivation steps before cooling superconducting devices to cryogenic temperatures. The enclosure can accommodate large chips with up to 18 microwave ports and can be vacuum sealed at pressures below 8e10 mbar. We discuss its operation principle and present first measurement results of superconducting CPW resonators and qubit devices with and without prior surface treatments. [Preview Abstract] 
Wednesday, March 15, 2017 9:24AM  9:36AM 
K51.00006: Measuring the Effects of SQUID Geometry on Transmon Qubit Coherence A. Dunsworth, Z. Chen, C. Quintana, B. Campbell, B. Chiaro, C. Neill, J. Wenner, J.M. Martinis Superconducting qubits have energy relaxation times that are commonly limited by lossy dielectrics near superconducting surfaces. Furthermore, flux tunable qubits often show 'holes' in their energy relaxation spectrum due to resonant coupling with two level states (TLS's) present in these amorphous dielectrics. There are also evidences that fluctuating surface spins near the SQUID loop are a dominant source of phase noise. All three of these features are intimately tied to the distribution of electric and magnetic fields in these circuits which in turn depend on their geometry . Using Xmon transmon qubits we investigated the effect of changing the SQUID loop geometry on phase noise and TLS resonances. Using various noise measurement techniques we extract phase noise spectra nearly continuously over nine orders of magnitude in frequency, while $T_{1}$ spectra are measured in the qubits' transition frequency range of 3 to 6 GHz. [Preview Abstract] 
Wednesday, March 15, 2017 9:36AM  9:48AM 
K51.00007: Trenched TiN superconducting coplanar waveguide resonators for determining interfacial losses Alexander Melville, Greg Calusine, Wayne Woods, David Kim, Xhovalin Miloshi, Arjan Sevi, Jonilyn Yoder, William Oliver Twolevel systems at metalsubstrate, metalair, and substrateair interfaces are a significant contributor to loss in superconducting resonators probed at the singlephoton limit. Electromagnetic simulation tools can be used to determine the relative electricfield participation of these various interfaces for a given resonator geometry. The challenge is to identify a set of geometries that sufficiently alters the relative participation values in order to deconvolve the loss factors of different interfaces from the measured resonator quality factors. In this talk, we summarize measurements of a matrix of titanium nitride resonator geometries with varying etch parameters to perform this extraction, and we use this information to improve resonator quality factors in the singlephoton limit. This research was funded in part by the Intelligence Advanced Research Projects Activity (IARPA). The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of IARPA or the US Government. [Preview Abstract] 
Wednesday, March 15, 2017 9:48AM  10:00AM 
K51.00008: Ar/SF$_{\mathrm{6\thinspace }}$plasma functionalization of superconducting TiN resonators for favorable Q factors Evgeniya Lock, Peng Xu, Tim Kohler, Yaniv Rosen, Aruna Ramanayaka, Kevin Osborn Charged tunneling defects at the vacuumdielectric interfaces are known to be deleterious to quantum bits in superconducting quantum computing. These tunneling defects are believed to be charged atoms or groups of atoms which cause qubit decoherence through shared electricfield modes. In this work we study the performance of titanium nitride resonators on silicon substrates.In particular, we investigate the effects of plasma functionalization in Ar/SF$_{\mathrm{\mathbf{6}}}$\textbf environment on the resonators' intrinsic quality factors. We correlate the change in Q factors with the structural modification of the sidewall below the edge of the TiN. We also report on the related chemical and morphological modifications of the Sivacuum surface. [Preview Abstract] 
Wednesday, March 15, 2017 10:00AM  10:12AM 
K51.00009: Benchmarking Lifetimes in Hybrid Transmons William Livingston, Allison Dove, Irfan Siddiqi High quality titanium nitride, aluminum, and niobium films deposited on silicon have resulted in single photon, lowtemperature resonator quality factors exceeding one million. These materials have the potential to enhance long qubit lifetimes when combined with subsequent shadow evaporation to deposit Josephson junctions with polycrystalline aluminum electrodes and an amorphous aluminum oxide tunnel barrier. We investigate the performance of threedimensional transmon qubits with capacitive elements produced in a subtractive process that yields high quality factor linear resonators. We report the coherence times of such hybrid qubits made with these three capacitor materials and compare them to devices fabricated in a conventional single step, lithographic process. We also investigate the potential role of quasiparticle confinement induced by the gap gradient between the junction electrodes and the capacitors by way of temperature dependent measurements and operation in different configurations of electromagnetic shielding. [Preview Abstract] 
Wednesday, March 15, 2017 10:12AM  10:24AM 
K51.00010: Tunable Superconducting Qubits with Reduced Sensitivity to 1/f flux noise M. D. Hutchings, J. B. Hertzberg, Y. Liu, J. M Chow, B. L. T. Plourde Superconducting qubits are a leading candidate in the pursuit of creating a fault tolerant quantum computer. However, as these devices scale in complexity, significant challenges must be overcome. Qubits that are both fluxtunable and show less sensitivity to flux noise have the potential to assist scalability. Splitting the junction of a transmon qubit creates a SQUID (Superconducting QUantum Interference Device) loop that allows for tuning of the qubit energy level with magnetic flux. However, this tunability can lead to excess dephasing due to flux noise. By creating asymmetry between the junctions of the SQUID loop, the level of qubit frequency tunability can be adjusted. We compare coherence from qubits with a range of junction asymmetries. We will report on how, in these qubits, the sensitivity to dephasing by flux noise scales with junction asymmetry. Furthermore, we use this understanding to fabricate a qubit where the level of dephasing due to flux noise has been reduced below the background set by other, nonflux dependent dephasing sources. This is achieved whilst still maintaining a useful level of frequency tunability. [Preview Abstract] 
Wednesday, March 15, 2017 10:24AM  10:36AM 
K51.00011: Fluxnoise insensitive and fluxtunable superconducting qubit Eyob Sete, Matthew Reagor, Nicolas Didier, Chad Rigetti Fast highfidelity twoqubit gates are an essential component of a universal quantum computer. Tunable qubits are promising candidates to realize such gates. However, tunability often comes at the expense of increased noise sensitivity for a qubit, thus degrading gate performance. We propose a superconducting circuit that mitigates a dominant noise source for a class of tunable qubits. The circuit consists of a SQUID with asymmetric junctions and shunted using a superinductor. We show that flux ‘sweet spots’ can be engineered at the frequency of operation by varying the junction asymmetry and the applied magnetic flux. This device coupled with a fixed frequency qubit allows a realization of fast highfidelity twoqubit gates. [Preview Abstract] 
Wednesday, March 15, 2017 10:36AM  10:48AM 
K51.00012: Convergent expressions for Purcell rate and Lamb shift of superconducting qubit in an open multimode resonator Alexandru Petrescu, Moein Malekakhlagh, Hakan Tureci The accurate calculation of the Purcell decay rate and the Lamb shift of an atom in an open multimode environment is a longstanding problem. This problem attained a more immediate and practical significance with the advent of quantum information processing with superconducting quantum circuits. It is now recognized that any attempt at the more accurate calculation of circuit QED quantities gives rise to divergent results [1,2] unless high frequency cutoffs are introduced. We resolve this problem by presenting convergent expressions for the Lamb shift and Purcell decay rate of a superconducting qubit coupled to an open multimode resonator. Our calculation is based on a formulation of subgap superconducting quantum electrodynamics using a HeisenbergLangevin approach [3]. [1] A. A. Houck \textit{et al.}, Phys. Rev. Lett. \textbf{101}, 080502 (2008). [2] S. Filipp, \textit{et al.}, Phys. Rev. A \textbf{83}, 063827 (2011). [3] Moein Malekakhlagh, Alexandru Petrescu, Hakan E. T\"{u}reci, arXiv:1609.00359 [Preview Abstract] 
Wednesday, March 15, 2017 10:48AM  11:00AM 
K51.00013: NonMarkovian dynamics of superconducting qubit in open multimode resonator Moein Malekakhlagh, Alexandru Petrescu, Hakan Tureci We study the dynamics of a transmon qubit that is capacitively coupled to an open multimode superconducting resonator. Our effective equations are derived by eliminating resonator degrees of freedom while encoding their effect in the Green’s function of the electromagnetic background. We account for the dissipation of the resonator exactly by employing a spectral representation for the Green’s function in terms of a set of nonHermitian modes and show that it is possible to derive effective HeisenbergLangevin equations without resorting to the rotating wave, two level, Born or Markov approximations. A wellbehaved time domain perturbation theory is derived to systematically account for the nonlinearity of the transmon. We apply this method to the problem of spontaneous emission, capturing accurately the nonMarkovian features of the multimode qubit dynamics valid for any qubitresonator coupling strength. References: Moein Malekakhlagh, Alexandru Petrescu, Hakan E. T\"{u}reci, arXiv:1609.00359 [Preview Abstract] 
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