Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session X33: Superconducting Qubits: Coherence |
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Sponsoring Units: DQI Chair: Julian Kelly Room: LACC 408B |
Friday, March 9, 2018 8:00AM - 8:12AM |
X33.00001: Temporal Fluctuations in the Coherence Parameters of Planar Single-Junction Transmons Andreas Bengtsson, Jonathan Burnett, Per Delsing, Jonas Bylander Superconducting qubits have seen improvements in their coherence times to beyond a hundred microseconds. However, reliably reaching the relaxation limit T2=2T1 remains challenging. The decoherence rates vary in time, which makes it difficult to evaluate the device-to-device performance and identify the factors that prevent us from reaching this limit. We experimentally investigate the temporal variations in the relaxation and dephasing rates of a single-Josephson-junction transmon qubit with 〈T1〉≈〈T2〉≈ 75 µs. We gather statistics of the fluctuations of the coherence metrics for different qubit temperatures. The results are analyzed in terms of several known decoherence mechanisms, such as quasiparticles, thermally induced noise via the readout resonator, and interacting two-level systems. |
Friday, March 9, 2018 8:12AM - 8:24AM |
X33.00002: Investigation of coupling of mechanical vibrational modes to superconducting qubits Salvatore Olivadese, Jay Gambetta, Jerry Chow, Markus Brink, Pat Gumann Investigation of the coupling of mechanical vibrational modes to superconducting qubits in a cryogenic refrigeration system motivates more systematic study. We have carried out a series of low frequency experiments, monitoring the performance of our superconducting transmon qubits. The unique design of the entire setup allows observations of the qubit as well as the readout resonator parameters when an external low frequency drive is applied. Initial results indicate potentially different mechanisms from the low frequency drive affecting the qubit and the readout resonator. |
Friday, March 9, 2018 8:24AM - 8:36AM |
X33.00003: Temperature Dependence of Transmon Relaxation Due to Non-Equilibrium Quasiparticles: Experiment and Analysis Rui Zhang, Cody Ballard, Jen-Hao Yeh, Sudeep Dutta, Shavindra Premaratne, Christopher Lobb, Frederick Wellstood, Benjamin Palmer Understanding and decreasing the energy relaxation rate of transmon qubits is important for their use in quantum computing. We measured the relaxation time T1 of multiple Al/AlOx/Al transmons. The devices were fabricated using different processes, mounted in different 3D aluminum cavities, and measured in two different dilution refrigerators. In some devices we found that T1 increased by as much as a factor of two (up to 90 µs) when increasing the temperature of the device from 15 mK to 100 mK. We will discuss our results and compare them to a model based on the behavior of non-equilibrium quasiparticles residing in regions of the transmon with slightly different superconducting energy gaps. Fits to the model allow us to extract key parameters such as the values of the superconducting energy gap and density of non-equilibrium quasiparticles in each region. |
Friday, March 9, 2018 8:36AM - 8:48AM |
X33.00004: Dependence of 3D Cavity Lifetime on Magnetic Field Kaicheng Li, Christopher Axline, Teresa Brecht, Luigi Frunzio, Leonid Glazman, Robert Schoelkopf In 3D circuit quantum electrodynamics, 3D cavities are often used as storage for quantum information for their relatively long lifetimes. Nonetheless, it would be desirable to further extend the cavity lifetime by determining which of the various loss phenomena limits the cavity performance. One loss mechanism known to contribute loss to some systems [1] arises from trapped magnetic flux in the form of superconducting vortices. Indirect evidence from past experiments suggests that ambient magnetic field near the cavity suppresses its superconductivity and reduces its lifetime. To determine whether ambient magnetic field is limiting the 3D cavity lifetime, we study the dependence of 3D cavity lifetime on an externally applied magnetic field; additionally, we compare the performance of a coaxial stub 3D cavity with that of a rectangular 3D cavity. [1] C. Song, T.W. Heitmann, M.P. Defeo, K. Yu, R. Mcdermott, M. Neeley, J.M. Martinis, and B.L.T. Plourde, Physical Review B 79, (2009). |
Friday, March 9, 2018 8:48AM - 9:00AM |
X33.00005: Two-qubit quantum bath engineering in near-resonance regime Zhaoqi Leng, Gengyan Zhang, Basil Smitham, Andrew Houck Quantum bath engineering harnesses carefully-designed environmental dissipation to stabilize long-lived quantum states. Recent experiments have demonstrated entanglement stabilization by dispersively coupling two qubits to one or multiple resonators. Here, we explore an alternative scheme to stabilize an entangled state of two qubits by hybridizing them with a very lossy resonator. Experimental results on resonator induced correlated decay and stabilization of an entangled qubit state will be presented. |
Friday, March 9, 2018 9:00AM - 9:12AM |
X33.00006: Dephasing in tunable Xmon transmon qubits Rami Barends, Zijun Chen, Paul Klimov, Julian Kelly, Andrew Dunsworth, Ben Chiaro, Anthony Megrant, Amit Vainsencher, Evan Jeffrey, John Martinis One of the key challenges in superconducting qubits is maintaining coherent control during operation. For our frequency-tunable architecture, the environment at a broad range of frequencies becomes relevant for the success rate of quantum algorithms. I will present Carr-Purcell-Meiboom-Gill measurements aimed at identifying phase noise and its contribution to gate error away from the flux-insensitive point. |
Friday, March 9, 2018 9:12AM - 9:24AM |
X33.00007: Broadband Cryogenic milli-Kelvin Attenuators: Cooling Power Improvements Jen-Hao Yeh, Rui Zhang, Shavindra Premaratne, Jay LeFebvre, Frederick Wellstood, Benjamin Palmer Dephasing due to fluctuations in the number of photons in a read-out cavity resonator can cause significant decoherence in superconducting transmon qubits.[1] To reduce the number of thermal photons injected into a read-out cavity by an input line that goes to higher temperature stages and is subjected to heating from applied signals, we have designed cryogenic microwave attenuators for operation below 100 mK and up to 10 GHz. To improve the transfer of heat from the dissipative elements in our attenuator we have interleaved thick conducting heat sinks with distributed dissipative NiCr elements. Our 20 dB attenuators have a minimum noise temperature Tn < 50 mK with no applied power.[2] When the dissipated power Pd is increased, this design shows an improvement in the cooling power proportional to the thickness t of the heat sink, and we find Tn ∝ (Pd/t)1/5. With thickness t = 10 μm, the attenuator can dissipate up to Pd = 100 nW and keep Tn ~ 100 mK. |
Friday, March 9, 2018 9:24AM - 9:36AM |
X33.00008: Transmon Relaxation Due to Non-Equilibrium Quasiparticles: Modelling Temperature Dependence Cody Ballard, Rui Zhang, Jen-Hao Yeh, Sudeep Dutta, Shavindra Premaratne, Christopher Lobb, Frederick Wellstood, Benjamin Palmer We have measured the relaxation time T1 of Al/AlOx/Al transmon qubits mounted in 3D aluminum cavities and have found that T1 can increase by as much as a factor of two (from 45 µs to 90 µs in one device) as the temperature increased from 15 mK to 100 mK. We present a model of this phenomenon that is based on the behavior of non-equilibrium quasiparticles in the transmon’s superconducting junction, which has two constituent layers with different volumes and slightly different superconducting energy gaps. At low temperatures, the non-equilibrium quasiparticles concentrate in the low-gap small-volume region producing a higher density near the junction. This higher quasiparticle density causes an increase in the relaxation rate of the transmon. As the temperature is increased, these quasiparticles are liberated from the low-gap region into the large-gap, large-volume region, leading to a reduction in the density of quasiparticles at the junction and an increase in the relaxation time. |
Friday, March 9, 2018 9:36AM - 9:48AM |
X33.00009: Reducing qubit decoherence in 3D circuit quantum electrodynamics with cold cavity attenuators Zhixin Wang, Shyam Shankar, Zlatko Minev, Philippe Campagne-Ibarcq, A. Narla, Michel Devoret Dephasing induced by residual thermal photons in the readout resonator is one of the leading factors limiting the coherence time of transmon qubits in the circuit QED architecture. This residual thermal population of the order of 10-1-10-3 is suspected to arise from noise impinging on the resonator from the input and output transmission lines. We have designed and tested a new type of narrowband microwave attenuator made of a brass or OFHC copper cavity that is well thermalized to the mixing chamber stage of a dilution refrigerator. By adding such a cavity attenuator inline with a 3D superconducting cavity housing a transmon qubit, we have reproducibly measured increased qubit coherence times below 100 mK. At base temperature, through Hahn echo experiment, we measured T2E / 2T1 = 0.98(+0.02/-0.13). We thus obtained a upper bound on the residual photon population close to 10-4 in the fundamental mode of the readout cavity, which to our knowledge is the lowest value reported so far. |
Friday, March 9, 2018 9:48AM - 10:00AM |
X33.00010: Ultra-high Q > 1010 superconducting 3D cavities for quantum computing and quantum memory Alexander Romanenko, Mohamed Hassan, Roman Pilipenko, Mattia Checchin, Dmitri Sergatskov, Saravan Chandrasekaran, Chris Wilen, Robert McDermott 3D superconducting radio frequency (SRF) cavities with very high Q factors of 1010-1011 are routinely used in modern particle accelerators. These are cavities made of bulk niobium and prepared with specialized surface processing techniques based on the detailed understanding of various Q-limiting mechanism developed in SRF over the past several decades. We have merged the advantages of high Q SRF with state-of-the-art transmon qubit, and in this contribution we report the first 'quantum' regime (T<20 mK, N<~10 photons) measurements of high Q bare SRF cavities, as well as SRF cavity-transmon qubit system performance (coherence time etc), demonstrating the high potential of this approach for both 3D cQED quantum computation and quantum memory. |
Friday, March 9, 2018 10:00AM - 10:12AM |
X33.00011: Probing non-equilibrium quasiparticle dynamics in transmon qubits: Part 1 Max Hays, Kyle Serniak, Gijs De Lange, Shyam Shankar, Spencer Diamond, Uri Vool, Luke Burkhart, Ioan-Mihai Pop, Luigi Frunzio, Robert Schoelkopf, Manuel Houzet, Leonid Glazman, Michel Devoret Non-equilibrium quasiparticle excitations negatively affect the performance of superconducting devices. Although the source of non-equilibrium quasiparticles is not well understood, the severity of their effect can be mitigated by improved filtering and device design. We implement Ti-proximitized Al quasiparticle traps aimed at reducing the density of quasiparticles in sensitive regions of superconducting devices. To characterize these traps, we detect individual quasiparticle tunneling events in an offset-charge-sensitive 3D transmon qubit, and correlate these events with qubit transitions. These measurements probe the limit on transmon coherence imposed by quasiparticles, and offer insight into the quasiparticle energy distribution. This talk will focus on experimental design and measurement methods. |
Friday, March 9, 2018 10:12AM - 10:24AM |
X33.00012: Probing non-equilibrium quasiparticle dynamics in transmon qubits: Part 2 Kyle Serniak, Max Hays, Gijs De Lange, Shyam Shankar, Spencer Diamond, Uri Vool, Luke Burkhart, Ioan-Mihai Pop, Luigi Frunzio, Robert Schoelkopf, Manuel Houzet, Leonid Glazman, Michel Devoret Non-equilibrium quasiparticle excitations negatively affect the performance of superconducting devices. Although the source of non-equilibrium quasiparticles is not well understood, the severity of their effect can be mitigated by improved filtering and device design. We implement Ti-proximitized Al quasiparticle traps aimed at reducing the density of quasiparticles in sensitive regions of superconducting devices. To characterize these traps, we detect individual quasiparticle tunneling events in an offset-charge-sensitive 3D transmon qubit, and correlate these events with qubit transitions. These measurements probe the limit on transmon coherence imposed by quasiparticles, and offer insight into the quasiparticle energy distribution. This talk will focus on experimental data and analysis. |
Friday, March 9, 2018 10:24AM - 10:36AM |
X33.00013: Dynamics of a qubit while simultaneously monitoring its relaxation and dephasing Quentin FICHEUX, Sébastien Jezouin, Zaki Leghtas, Benjamin Huard Measuring a spin-1/2 along one direction projectively maximally randomizes the outcome of a following measurement along a perpendicular direction. Here, using either projective or weak measurements, we explore the dynamics of a superconducting qubit for which we measure simultaneously the three components x, y and z of the Bloch vector. |
Friday, March 9, 2018 10:36AM - 10:48AM |
X33.00014: Measurement of the conversion between coherence and quantum correlations in a superconducting circuit Yuwei Ma, Weiting Wang, Jiajun Ma, Hu Ling, Yuan Xu, Haiyan Wang, Xiongfeng Ma, Mile Gu, Luyan Sun Coherence and quantum correlations are important resources in quantum information processing. Recently, it is shown that these two quantum resources can be converted to each other by appropriate means. In this talk, based on a superconducting quantum computing platform, we demonstrate such a resource conversion procedure in the model of deterministic quantum computation with one qubit (DQC1). By observing that the yield of the quantum correlation is upper bounded by the coherence consumption during the computation procedure, we confirm that the resource conversion process is exploited in quantum computation. |
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