Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session Y39: Superconducting Circuits: Fabrication and Materials II |
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Sponsoring Units: DQI Chair: David Pappas, NIST Room: LACC 501B |
Friday, March 9, 2018 11:15AM - 11:27AM |
Y39.00001: Characterizing electromechanical coupling of dielectric materials for superconducting circuits Taekwan Yoon, Yiwen Chu, Prashanta Kharel, William Renninger, Luigi Frunzio, Peter Rakich, Robert Schoelkopf To further improve the coherence time of superconducting qubits, one needs to consider many different sources of loss. Among those, dielectric loss has been studied in depth, and one contributor of this loss can originate from the electromechanical coupling between the superconducting circuit and the dielectric. In my talk, I will present a highly sensitive technique for measuring electromechanical coupling in a material at cryogenic temperatures and in the GHz regime. The measurement is based on RF driving of phonons through electromechanical coupling and optical readout through Brillouin scattering. I will show measurements done on various materials, including the observation of effective piezoelectricity in inversion symmetric materials, and discuss its implications for qubit coherence times. |
Friday, March 9, 2018 11:27AM - 11:39AM |
Y39.00002: Fully Superconducting Multi-Chip Module Process with Controlled Interchip Spacing for Quantum Integrated Circuits Daniel Yohannes, Denis Amparo, John Vivalda, Mario Renzullo, Jason Walter, Mykola Chernyashevskyy, Andrei Talalaevskii, Oleksandr Chernyashevskyy, Igor Vernik, Oleg Mukhanov We present a fabrication process for a fully superconducting multichip module (MCM), compatible with superconducting qubit technology, to allow a 3D integration of quantum circuits while preserving qubit coherence. In particular, this MCM can be used to couple a Nb-based Single Flux Quantum classical coprocessor for the integrated control and readout of large-scale superconducting multiqubit integrated circuits. We report on the development of superconducting indium-bump MCM technology for the connection of a multi-layer Nb circuit to a qubit chip by integrating indium into the HYPRES fabrication process. The MCM assembly becomes fully superconducting at the transition temperature of 3.4 K, as determined by indium. For the dense MCM bumps, soft malleability of indium may result in poor control of the interchip spacing, which can lead to shorts between bumps. To address this problem, we devised a mechanism in where a more rigid metal supports the indium bumps. This limits the deformation while allowing cold welding and providing a controlled separation between flip-chip and MCM carrier. We characterize the performance of the MCM by measuring the bump array critical current, critical current dependence vs temperature and its mechanical stability over multiple thermal cycles. |
Friday, March 9, 2018 11:39AM - 11:51AM |
Y39.00003: Broadband Two-Level System Defects Inversion Measured Through Dressed Resonator Frequency and Dielectric Loss Samaresh Guchhait, Yaniv Rosen, Alexander Burin, Kevin Osborn Two-level system (TLS) defects are known to cause performance limiting decoherence in qubits and noise in photon detectors. We have achieved a broadband TLS inversion in the GHz regime, as monitored by a resonator. The TLS inversion is caused and controlled by application of a strong pump field and a swept electric field bias. The resonator responds to the TLS inversion through changes in its resonance frequency and internal loss. For the lowest bias rates, inverted TLSs are confined in a narrow band near the pump frequency due to relaxation processes. In this regime the frequency and internal loss tangent are nearly unchanged. With increasing bias rates, a large fraction of TLSs are inverted and distributed in energy below the resonance frequency. This causes large change of resonance frequency and loss up to some maximum value. For even larger bias rates, the probability of TLS inversion is lower when interacting with the pump field due a larger probability of Landau-Zener tunneling. As a result the shifts in resonance frequency and loss lower from their respective maximum values. Numerical simulations of permittivity change agree with experimental results, and give strong evidence for broadband TLS inversion. |
Friday, March 9, 2018 11:51AM - 12:03PM |
Y39.00004: Physical and Microwave Characterization of Superconducting Indium and Aluminium/Indium Thin Films Corey Rae McRae, Jérémy Béjanin, Carolyn Earnest, Adel Abdallah, Thomas McConkey, John Rinehart, Christopher Deimert, Joseph Thomas, Zbigniew Wasilewski, Matteo Mariantoni In a superconducting quantum circuit, understanding the microwave losses in each implemented material is crucial to the optimization of device performance and ultimately the scalability of the system. Indium, a low temperature superconductor and common solder, has become a recent addition to the superconducting quantum computing toolbox. In this work, we characterize microwave loss in thin film indium and aluminium/indium by measuring superconducting microwave resonators and performing quality factor fitting at a variety of powers down to the sub-photon regime and a variety of temperatures down to 10 mK. We also fit the two level system (TLS) loss model to this data, as well as perform surface metrology. We compare films deposited by thermal evaporation, sputtering, and molecular beam epitaxy to capped and uncapped aluminium/indium films, as well as films processed in various ways. We find unprocessed thermally evaporated indium films to be the highest performing, with a high power quality factor of 3,600,000. |
Friday, March 9, 2018 12:03PM - 12:15PM |
Y39.00005: Noise and loss from the dielectric environment of planar superconducting circuits Jonathan Burnett, Andreas Bengtsson, Per Delsing, Jonas Bylander Understanding of loss mechanisms in superconducting resonators has helped to produce long-lived superconducting qubits. Most notably, for identical fabrication processes, the single-photon quality factor has become a reliable indicator for qubit relaxation time. While this understanding of loss has seen typical T1 approach ≈100μs, the dephasing times often remain far below the relaxation limit. Superconducting resonators can provide information on the time-varying properties of these decoherence mechanisms. We show measurements on a variety of superconducting resonators and qubits, showing that our mean qubit T1 (75μs) is consistent with our mean resonator single photon Qi (106). We then study the temporal properties of the resonator in the framework of interacting two-level systems and apply this understanding to fluctuations in qubit parameters. |
Friday, March 9, 2018 12:15PM - 12:27PM |
Y39.00006: Fabrication of high-impedance Josephson junction chains for novel quantum devices Raymond Mencia, Nicholas Grabon, Aaron Somoroff, Natalia Pankratova, Vladimir Manucharyan High-impedance superconducting transmission lines and high-inductance lumped circuit elements are useful for a range of quantum information experiments, from improving the coherence of superconducting qubits to simulating exotic many-body Hamiltonians. Conventional electromagnetic structures are generally limited by the vacuum impedance with a relatively low value of 377.0 Ohm. By contrast, wires made of Josephson tunnel junction chains have a large kinetic inductance associated with Cooper pair tunneling and thus are not limited by the properties of the low-impedance vacuum. Here we present fabrication of compact linear chains involving up to 30,000+ Al/AlOx/Al tunnel junctions utilizing the Dolan bridge technique and double-angle shadow evaporation. Shorter chains are used to create conventional fluxonium qubits with coherence times exceeding 100 microseconds. Longer chains demonstrate propagating microwaves with a wave impedance exceeding the value of von Klitzing resistance, 26.5 kOhm. Ultra-tight junction packing is demonstrated to achieve inductances above 1 uH with high self-resonance frequencies, useful for exploring protected qubit designs. |
Friday, March 9, 2018 12:27PM - 12:39PM |
Y39.00007: High-Impedance Titanium Nitride Resonators via Atomic Layer Deposition Abigail Shearrow, Nathan Earnest, Helin Zhang, Samuel Whiteley, Srivatsan Chakram, Erik Shirokoff, David Awschalom, David Schuster High-impedance resonators provide the opportunity to achieve ultra-strong coupling in superconducting qubits, strong coupling in hybrid systems, and highly compact multi-modal systems. The use of materials with a large kinetic inductance, like titanium nitride (TiN), allow one to realize high-impedance resonators using thin films. Atomic Layer Deposition (ALD) allows for atomic precision in film thickness on a wafer scale, a necessity for precise device fabrication in thin films. Here we present results of using ALD of TiN to realize thin films (thicknesses ∼ 8 nm) and high-impedance resonators (Zchar ≥ 1kΩ). We will discuss quality factor measurements and other applications of these highly inductive TiN thin films. |
Friday, March 9, 2018 12:39PM - 12:51PM |
Y39.00008: Study of two-level defects in a thin amorphous material by electrical capacitor bridge Chih-Chiao Hung, Bahman Sarabi, Neda Forouzani, Kevin Osborn In superconducting qubits, thermally grown aluminum oxide is usually used as a barrier between two superconducting electrodes to form an S-I-S Josephson junction (JJ). This leads to an inevitable source of decoherence in superconducting qubits. Here, we present a study of TLSs in parallel plate capacitors mimicking JJ (Al/AlOx/Al) due to the use of a thin AlOx film. These will be as thin as 10 nm,and therefore within an order of magnitude of the JJ barrier thickness. Using a four-arm bridge capacitor design, we can tune the frequency of TLSs by applying a DC electric field, and observe individual TLSs reach degeneracy with resonator in a demonstration of cavity QED with TLS. The transmission versus applied electric field will also give us the TLS dipole moment’s z-component. We will study these resonators over a range of thicknesses and determine if the TLSs are affected by proximity to the electrodes which is less than the superconducting coherence length. |
Friday, March 9, 2018 12:51PM - 1:03PM |
Y39.00009: Low Loss Multi-Layer Wiring for Superconducting Qubits Andrew Dunsworth, Brooks Campbell, Zijun Chen, Ben Chiaro, Charles Neill, James Wenner, John Martinis Complex integrated circuits require multiple wiring layers. In complementary metal-oxide-semiconductor (CMOS) processing, these layers are robustly separated by amorphous dielectrics. These dielectrics would dominate energy loss in superconducting integrated circuits. I will discuss a procedure that capitalizes on the structural benefits of inter-layer dielectrics during fabrication and mitigates the added loss. We separate and support multiple wiring layers throughout fabrication using silicon oxide scaffolding, then remove it post-fabrication. We use this technique to create freestanding aluminum vacuum gap crossovers (airbridges). We characterize the added capacitive loss of these airbridges by connecting ground planes over microwave frequency coplanar waveguide resonators and measuring resonator loss. We measure a low power resonator loss of 3.9E-8 per bridge, which is 100 times lower than dielectric supported bridges. We further characterize these airbridges as crossovers, control line jumpers, and as part of a coupling networks in nine and fifteen gmon linear chains, as well as fluxmon qubits. We measure gmon characteristic lifetimes (T1's) in excess of 30 microseconds. |
Friday, March 9, 2018 1:03PM - 1:15PM |
Y39.00010: Correlational Study of Structure and Superconducting Resonator Loss in Epitaxial Al on Si Ashish Alexander, Christopher Weddle, Christopher Richardson One limiting mechanism of the internal quality factor of the superconducting resonators used in circuit quantum electrodynamics is the resonant interaction of two-level system (TLS) defects. The superconductor/substrate interface experiences the highest electric field energy in a coplanar waveguide and has been shown to be the most critical boundary for TLS loss. |
Friday, March 9, 2018 1:15PM - 1:27PM |
Y39.00011: Microwave Response of Vortices in Superconducting Resonators with High Kinetic Inductance Kenneth Dodge, JJ Nelson, Michael Senatore, Peng Xu, Kevin Osborn, David Pappas, Britton Plourde Magnetic flux vortices driven by microwave currents are an important loss mechanism in superconducting resonators and qubits. High kinetic inductance superconductors are important for a variety of detector technologies that require large inductances in a compact form factor. Prior work has focused on the behavior of vortices trapped in aluminum thin films. Here we present measurements of vortices trapped in coplanar waveguide resonators fabricated from superconducting thin films with a large amount of kinetic inductance, including TiN and NbTiN. Field cooling of multiple resonators with different parameters is used to study the magnetic field, frequency, and temperature dependence of the microwave vortex response. |
Friday, March 9, 2018 1:27PM - 1:39PM |
Y39.00012: Process Monitoring to Improve Reliability and Yield for Superconducting Circuits Alexandra Day, Danna Rosenberg, David Kim, Jonilyn Yoder, William Oliver As the scale and complexity of superconducting circuits increases, characterizing and monitoring the underlying processes will become increasingly important. MIT Lincoln Laboratory uses process control structures to characterize devices, track key parameters, and support the development of more complex designs. We will describe the development of software and database capabilities that allow us to visualize cross-wafer patterns, examine trends, and monitor process stability. |
Friday, March 9, 2018 1:39PM - 1:51PM |
Y39.00013: Thermal Desorption and Substrate Cleaning of Aluminum Superconducting Resonators Carolyn Earnest, Jérémy Béjanin, Thomas McConkey, Corey Rae McRae, Evan Peters, John Rinehart, Matteo Mariantoni High quality resonators are critical to superconducting quantum computing architectures. We fabricate aluminum-on-silicon coplanar waveguide (CPW) resonators by means of e-beam lithography in ultra-high vacuum (UHV) conditions. We investigate the relative and cumulative effects of UHV thermal desorption and hydrofluoric acid (HF) cleaning of the silicon substrate prior to aluminum deposition. We characterize various resonators by measuring their internal quality factor as a function of power, temperature, and time. Our results indicate that e-beam deposited aluminum in UHV is competitive with molecular beam deposited aluminum, thus showing that long-lived superconducting quantum bits (qubits) can be fabricated using a single deposition tool. |
Friday, March 9, 2018 1:51PM - 2:03PM |
Y39.00014: Carbon-Nanotube-Based Microwave Filters for Superconducting Qubit Measurements Mehran Vahdani Moghaddam, C. W. S. Chang, Ibrahim Nsanzineza, A. Vadiraj, C.M. Wilson Qubit measurements suffer from stray photons. Existing solution are not ideal because their performance is limited in temperature and frequency. Here, we report on the design, fabrication and characterization of two types of lossy transmission line filters. The characteristic impedance of both filters is nominally 50Ω and they are fabricated from multiwalled carbon nanotubes embedded in stainless-steel powder and Stycast. We show that the scattering parameters remain unchanged down to 10mK over a wide range of frequencies spanning from 10MHz to 50GHz. The cutoff frequency of the filter is easily tuned by varying the concentration of carbon nanotubes in the mixture. The small dimension of these filters together with their flat return loss over a wide band make them a good candidate for cryogenic applications including qubit measurements. |
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