Bulletin of the American Physical Society
2024 APS March Meeting
Monday–Friday, March 4–8, 2024; Minneapolis & Virtual
Session K47: Superconducting Loss CharacterizationFocus
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Sponsoring Units: DQI DCMP DMP Chair: Ziwen Huang, Fermilab Room: 200CD |
Tuesday, March 5, 2024 3:00PM - 3:36PM |
K47.00001: The dielectric dipper: experimental comparisons of common dielectric substrates in isolation Invited Speaker: Alexander P Read We have developed a differential probe of bulk dielectric loss with a sensitivity on the order of 10-8. The method uses a 3D cavity to probe the low-power behavior of dielectrics at cryogenic temperatures without the need for lithographic processing. Using this technique, we investigate various substrate materials and processes with the aim to better quantify dielectric loss, understand its origin, and determine how it can be mitigated in fabrication of superconducting quantum devices. So far, the method has identified bulk loss as a major source of decoherence of transmon qubits on EFG sapphire. With a bulk loss tangent of 6*10-8, bulk loss in EFG sapphire places a limit on transmon lifetime of about 800 us. We have also identified HEM sapphire and Float-zone silicon as promising materials for use as substrates to further extend superconducting qubit lifetimes. |
Tuesday, March 5, 2024 3:36PM - 3:48PM |
K47.00002: Extracting Contributions to Qubit Loss from Superconducting Microwave Resonators. Part 1 Shravan Patel, Elam J Blackwell, Spencer Weeden, David C Harrison, Francisco Schlenker, Matthew Snyder, Gabriel Spahn, Abigail Shearrow, JT Paustian, K Okubo, B.L.T Plourde, Robert McDermott Superconducting coplanar waveguide resonators play a critical role in information storage and qubit state measurement in superconducting quantum information processing. At the same time, these resonators are a versatile testbed for characterizing the various contributions to qubit loss. Ideally, the internal quality factors Qi of these resonators should reach ten million or higher, limited only by the loss tangent of the silicon or sapphire substrate. However, in real devices, Qi is limited by the presence of various loss channels, including two-level state (TLS) defects at amorphous interfaces, trapped magnetic flux vortices, and nonequilibrium quasiparticles. In this work, we measure Qi as a function of photon occupation in Al and Nb thin-film microwave resonators. We present data from devices that involve systematic variation of geometry in order to modify the participation ratios of lossy interfaces and determine the dominant contributors to energy relaxation. |
Tuesday, March 5, 2024 3:48PM - 4:00PM |
K47.00003: Extracting Contributions to Qubit Loss from Superconducting Microwave Resonators. Part 2 Elam J Blackwell, Shravan Patel, Spencer Weeden, David C Harrison, Francisco Schlenker, Matthew Snyder, Gabriel Spahn, Abigail Shearrow, JT Paustian, K Okubo, B.L.T Plourde, Robert McDermott Superconducting coplanar waveguide resonators play a critical role in information storage and qubit state measurement in superconducting quantum information processing. At the same time, these resonators are a versatile testbed for characterizing the various contributions to qubit loss. Ideally, the internal quality factors of these resonators should reach ten million or higher, limited only by the loss tangent of the silicon or sapphire substrate. For practical devices, however, dielectric loss at interfaces is a dominant contributor to relaxation. In this work, we explore the dependence of interface losses on the details of device fabrication process. We investigate the impact on resonator quality factor of various silicon wafer treatments prior to metal deposition and various approaches to deposition and etch of the superconducting metals. In addition, we explore the dependence of metal-substrate and substrate-air losses on ion beam bombardment over a range of ion energies. These results will guide the optimization of fabrication processes for improved qubits. |
Tuesday, March 5, 2024 4:00PM - 4:12PM |
K47.00004: Origin of bulk two-level system loss in superconducting circuits on silicon: Part I Experiment Zihuai Zhang, Kadircan Godeneli, Srujan Meesala, Alp Sipahigil Two-level systems (TLS) in amorphous materials are the dominant loss mechanism for superconducting quantum circuits. While the microscopic origin of amorphous TLSs is still unknown, recent efforts enabled reduction in interface TLS losses via reduced surface participation and improved surface treatment. With these improvements, experiments are now approaching a regime where bulk TLS losses may have a significant contribution to qubit loss. In this work, we present the first microscopic identification of a TLS bath for superconducting circuits based on defects in bulk silicon. We use low surface-participation microwave resonators to study the impact of common acceptors and donors on microwave losses. We show that spin-orbit acceptor systems lead to a large microwave loss tangent at the single-photon level. In this talk, we describe experiments where we study two-level system loss as a function of dopant concentration, temperature, and magnetic field, and demonstrate the spin-orbit nature of the bulk TLS bath in silicon. |
Tuesday, March 5, 2024 4:12PM - 4:24PM |
K47.00005: Origin of bulk two-level system loss in superconducting circuits on silicon: Part II Theory Kadircan Godeneli, Zihuai Zhang, Srujan Meesala, Alp Sipahigil Two-level systems (TLS) in amorphous materials are the dominant loss mechanism for superconducting quantum circuits. While the microscopic origin of amorphous TLSs is still unknown, recent efforts enabled reduction in interface TLS losses via reduced surface participation and improved surface treatment. With these improvements, experiments are now approaching a regime where bulk TLS losses may have a significant contribution to qubit loss. In this work, we present the first microscopic identification of a TLS bath for superconducting circuits based on defects in bulk silicon. We use low surface-participation microwave resonators to study the impact of common acceptors and donors on microwave losses. We show that spin-orbit acceptor systems lead to a large microwave loss tangent at the single-photon level. In this talk, we describe a microscopic model to explain the experimental observations. We discuss the impact of these observations for low-loss superconducting circuits on silicon, and the prospect of using acceptor spins for building hybrid quantum systems in silicon. |
Tuesday, March 5, 2024 4:24PM - 4:36PM |
K47.00006: Annealing reduces silicon nitride microwave-frequency two-level systems loss Kazemi Adachi, Sarang Mittal, Alec L Emser, Cyril Metzger, Nicholas E Frattini, Maxwell D Urmey, Sheng-Xiang Lin, Luca G Talamo, Sarah Dickson, David Carlson, Scott Papp, Cindy A Regal, Konrad W Lehnert Silicon nitride Si3N4 is used in a wide variety of quantum applications at microwave-frequencies for its dielectric, thermal, and mechanical properties. We report measurements of the dielectric loss of stoichiometric Si3N4 with superconducting circuits, and explain the material’s behavior with the resonant and relaxation components of the two-level systems (TLS) model. Post-deposition annealing of the Si3N4 film drastically reduces the loss compared to as-deposited films. Infrared spectroscopy points to the presence of hydrogen impurities in the as-deposited film as the origin of the TLS loss. |
Tuesday, March 5, 2024 4:36PM - 4:48PM |
K47.00007: Probing defect densities at the edges and inside Josephson junctions of superconducting qubits Alexander Bilmes Tunneling defects in disordered materials form spurious two-level systems which are a major source of decoherence for micro-fabricated quantum devices. For superconducting qubits, defects in tunnel barriers of submicrometer-sized Josephson junctions couple strongest to the qubit, which necessitates optimization of the junction fabrication to mitigate defect formation. Here, we investigate whether defects appear predominantly at the edges or deep within the amorphous tunnel barrier of a junction. For this, we compare defect densities in differently shaped Al/AlOx/Al Josephson junctions that are part of a Transmon qubit. We observe that the number of detectable junction-defects is proportional to the junction area, and does not significantly scale with the junction’s circumference, which proposes that defects are evenly distributed inside the tunnel barrier. Moreover, we find very similar defect densities in thermally grown tunnel barriers that were formed either directly after the base electrode was deposited, or in a separate deposition step after removal of native oxide by Argon ion milling. |
Tuesday, March 5, 2024 4:48PM - 5:00PM |
K47.00008: Intermodulation spectroscopy of the nonlinear response due to two-level systems in superconducting resonators Gustav Andersson, Janka Biznárová, Juan Carlos Rivera Hernández, Daniel Forchheimer, Jonas Bylander, David B Haviland Two-level system (TLS) loss is typically limiting the coherence of high-quality superconducting quantum circuits. The loss induced by TLS defects is nonlinear, resulting in quality factors with a strong dependence on the circulating microwave power. We observe frequency mixing due to this nonlinearity by applying a two-tone drive to a coplanar waveguide resonator fabricated from aluminum on a silicon substrate. The amplitude and phase of the resulting intermodulation products are measured using a multifrequency lock-in technique without analog up- or downconversion of microwave signals, potentially providing an efficient route to characterizing the dielectric loss due to TLS. We present intermodulation spectroscopy results and discuss progress towards reconstructing the TLS-induced nonlinear response using different reconstruction algorithms. |
Tuesday, March 5, 2024 5:00PM - 5:12PM |
K47.00009: Towards individual TLS detection by scanning gate microscopy Riju Banerjee, Marius Hegedus, Andrew Hutcheson, Sebastian Graaf Superconducting circuits have emerged as a promising platform for quantum computations and quantum sensing applications. However, superconducting circuits are persistently plagued by challenges related to decoherence and losses caused by two-level system (TLS) defects. Hence, understanding their origin, properties, location and ultimately mitigating these defects will play a central role in realizing superconducting circuits capable of long-term storage and manipulation of quantum states. In this talk, we will present a novel technique to detect, measure and image individual TLS defects in operational superconducting quantum circuits. We will first briefly discuss the challenges and complexities of incorporating a custom-built scanning probe microscope in a mK dry fridge while maintaining a coherent environment for operating superconducting circuits, followed by our protocol for TLS detection with a sharp AFM tip. Our results open a new avenue for a systematic exploration of individual TLS defects in superconducting circuits and demonstrates a path forward to study their spatial distribution, physical structures, and chemical compositions. |
Tuesday, March 5, 2024 5:12PM - 5:24PM |
K47.00010: Dynamical disorder of two-level defects in quantum devices Moshe Schechter Quantum defects in the form of two-level systems (TLSs) are a dominant source of noise affecting relaxation and coherence of superconducting qubits. Long time modifications of the spectral properties are especially deleterious to scalability, since sweet spots frequencies for qubit coherence and relaxation vary in time and space. In this talk we present new experimental evidence for the existence of two types of TLSs, interacting weakly and strongly with the strain. We then argue that it is the dynamic disorder introduced by the strongly interacting TLSs that is responsible for the significant changes in the spectral properties of the TLS bath over thermal cycling, as well as over long times at low temperatures. |
Tuesday, March 5, 2024 5:24PM - 5:36PM |
K47.00011: Spectroscopic Study of Two-Level System Defects in Transmon Qubits Christopher Förbom, Elsa Mannila, Mikael Kervinen, Visa Vesterinen, Jorden Senior, Debopam Datta, Patrik Eskelinen, Joonas Govenius A major source of decoherence in superconducting qubits is believed to be due to defects that manifest as two-level systems (TLS) present in qubit materials. In this work, we have investigated TLS defects in planar transmon qubits. We probed TLS defects in our qubits spectroscopically by utilizing microwave pulse sequences that shift the qubit g-e transition into resonance with the defects. We compared the obtained spectral data between the differently fabricated qubits. In our spectroscopic experiments we found spectral features of varying lineshapes and temporal dynamics attributable to TLS defects. The resolved TLS defects exhibited telegraphic and diffusive dynamics, ranging from sub-minute-scale jumps to hour-scale drifts. We also found connections between the presence of TLS defects and the parameter fluctuations of our qubits. Notably, the fluctuations were present as qubit-TLS interactions that could affect the qubit T1 and T2 times by an order of magnitude while also causing instabilities in the qubit g-e transition frequency. Our results provide an insight into the relevance of optimizing fabrication when considering the prevalence of TLS defects and their impact on qubit coherence characteristics. |
Tuesday, March 5, 2024 5:36PM - 5:48PM |
K47.00012: More is better - High throughput of qubit measurements for materials study Ella O Lachman, Eric Bhada, Mehrnoosh Vahidpour, Daniel Setiawan, David Snow, Brian McVey, Mike Espitia, Cameron J Kopas, Josh Y Mutus Despite great advances and improvements in the performance of superconducting qubits through changes in device design and geometry, significant improvements in materials remain elusive. Qubits performance varies significantly across devices, wafers and cooldowns and so more trials are needed to get a true estimate of performance. When testing the improvement of various new materials or fabrication processes, it is important to do true A/B testing with enough statistics from multiple qubits, chips and wafers. This creates a bottleneck, as dilution refrigerators’ cycle time adds days to the measurement time. |
Tuesday, March 5, 2024 5:48PM - 6:00PM |
K47.00013: Low-power Pound-Drever-Hall measurement of superconductiong resonators John Pitten, Jim Phillips, Josh Y Mutus, Corey Rae H McRae As superconducting qubit coherence times reach an upper bound set by coupling to two-level system defects in amorphous dielectric substrates, rapid material loss characterization is needed to achieve significant improvement. Power sweeps of superconducting microwave resonators are commonly used to distinguish between loss channels induced in superconducting qubits, but the standard method of measurement using a simple vector network analyzer frequency sweep is inefficient and slow --- a full characterization of a single device takes days due to the very low power levels required. The Pound-Drever-Hall (PDH) technique, however, can extract the relevant resonator parameters in a fraction of the time. This technique generates a frequency modulated signal and locks the carrier frequency to resonance, allowing more efficient measurement of Q by varying the modulation frequency. Previous implementations relied on a microwave power diode for square-law detection of the transmitted signal, limiting the dynamic range of the technique. Here, we report on progress towards a low power implementation of PDH with direct, independent detection of the carrier and sidebands, for the goal of faster superconductiong resonator measurements. |
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