Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session A29: Superconducting Circuits: New Qubit Technologies and Design IFocus

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Sponsoring Units: DQI Chair: Raymond Simmonds, National Institute of Standards and Technology Boulder Room: BCEC 162A 
Monday, March 4, 2019 8:00AM  8:12AM 
A29.00001: Weakly Tunable Qubit: Part I Baleegh Abdo, José ChavezGarcia, Jared B Hertzberg, Firat Solgun, Oblesh Jinka, Easwar M Magesan, Markus Brink, Jay Gambetta, Jerry M. Chow Performing fast, highfidelity twoqubit gates is an important requirement of quantum computers. Crossresonance gates applied to pairs of transmons satisfy this requirement with the added advantage of being fully controlled by microwave signals. However, applying these gates to a large transmon lattice is quite challenging. This is because crossresonance gates set stringent requirements on the frequency landscape of neighboring qubits, which are difficult to satisfy with fixedfrequency transmons due to their relatively large frequency spread. To solve this problem, we realize a new fluxtunable qubit, compatible with crossresonance gates, which can be tuned by less than 150 MHz. Such a weakly tunable qubit is useful for avoiding frequency collisions in a large lattice while limiting its susceptibility to flux noise. In this talk, we will introduce the qubit circuit and theory. 
Monday, March 4, 2019 8:12AM  8:24AM 
A29.00002: Weakly Tunable Qubit: Part II Jose Chavez, Baleegh Abdo, Jared B Hertzberg, Firat Solgun, Oblesh Jinka, Easwar M Magesan, Markus Brink, Jay Gambetta, Jerry M. Chow Performing fast, highfidelity twoqubit gates is an important requirement of quantum computers. Crossresonance gates applied to pairs of transmons satisfy this requirement with the added advantage of being fully controlled by microwave signals. However, applying these gates to a large transmon lattice is quite challenging. This is because crossresonance gates set stringent requirements on the frequency landscape of neighboring qubits, which are difficult to satisfy with fixedfrequency transmons due to their relatively large frequency spread. To solve this problem, we realize a new fluxtunable qubit, compatible with crossresonance gates, which can be tuned by less than 150 MHz. Such a weakly tunable qubit is useful for avoiding frequency collisions in a large lattice while limiting its susceptibility to flux noise. In this talk, we will present some preliminary measurement results of these weakly tunable qubits. 
Monday, March 4, 2019 8:24AM  8:36AM 
A29.00003: Phase slips in voltagebiased superconducting rings: a qubit proposal? Ahmed Kenawy, Wim Magnus, Bart Soree The quantization of magnetic flux in superconductors lies at the heart of realizing qubits using superconducting circuits. To construct a flux qubit, one must coherently couple the flux states of a superconducting ring, which requires breaking the rotational symmetry of the ring to introduce a phaseslip center. Established implementations guarantee an inherently broken symmetry by interrupting the ring with an insulating barrier; hence, forming a Josephson junction, giving rise to the flux qubit. However, these implementations are plagued by the variablitly of the fabrication process of the junction. Here, we theoretically propose a junctionless flux qubit consisting of a voltagebiased superconducting ring. The inplane electric field, arising from the bias voltage, locally suppresses the density of superconducting electrons; hence, imitating the effect of interrupting the superconductor with an insulator. Furthermore, the proposed qubit could allow for electrictunability of the transition frequency, a desired feature for multiqubit systems since it renders the circuit less sensitive to magnetic noise. Realizing electricallytunable flux qubits with long coherence times paves the road towards scaling up superconducting quantum computers. 
Monday, March 4, 2019 8:36AM  8:48AM 
A29.00004: The Null Flux Qubit Raina Olsen, Mohammadreza Rezaee We discuss a superconducting flux qubit whose sensitivity to low frequency flux is reduced through a null flux geometry consisting of one closed wire loop twisted into a `figure eight' shape composed of two open loops, each with opposite circulation. This protects against global flux noise in a similar way as the gradiometer qubit, but with only one loop degree of freedom instead of the two DOF of the gradiometer. This important difference opens several possibilities for dealing with the local flux noise that causes decoherence in the gradiometer qubit. We further show that the null flux shape increases the sensitivity to high frequency flux, making the system potentially useful for superconducting transduction. 
Monday, March 4, 2019 8:48AM  9:00AM 
A29.00005: Symmetry Protected Qubits Through Fluxon Pairing Wen Ting Hsieh, Matthew Bell, WenSen Lu, Wenyuan Zhang, Plamen Kamenov, Konstantin Kalashnikov, Michael Gershenson We present timedomain experiments with symmetryprotected qubits whose quantum states are encoded in the parity of fluxons in a superconducting loop. The qubit is composed of a Cooper pair box and a superinductor arranged in a superconducting loop [1]. The lowestenergy states of this qubit correspond to even/odd parity of fluxons in the loop. We will discuss proofofconcept experiments that show how the qubit can be placed into a state protected against energy relaxation where the fluxon parity is preserved through the AharonovCasher interference. We will show that using fast gate pulses, the fluxonpairing qubit can by adiabatically switched between the protected and unprotected states for quantum state initialization and readout. We will also discuss preliminary results on the implementation of fluxonpairing qubits with superinductors fabricated from meandered nanowires made of strongly disordered Aluminum [2]. 
Monday, March 4, 2019 9:00AM  9:12AM 
A29.00006: Breaking the tradeoff between gate and relaxation times of a superconducting qubit with a Josephson quantum filter: Theory Kazuki Koshino, Shingo Kono, Yutaka Tabuchi, Atsushi Noguchi, Dany LachanceQuirion, Yasunobu Nakamura When we couple a superconducting qubit strongly to a transmission line, the qubit completely reflects a weak resonant microwave field propagating through the line. On the other hand, the qubit becomes transparent to a stronger field due to the absorption saturation. This implies that a superconducting qubit functions as a nonlinear mirror, which we call a Josephson quantum filter (JQF). We theoretically investigate a setup in which a qubit (data qubit) to be controlled is coupled to an end of a semiinfinite control line and a JQF is placed at a distance of the order of the resonance wavelength of the qubit. An effective cavity is formed by the termination point and the JQF, and the radiative decay of the data qubit is suppressed if this effective cavity has a large detuning from the data qubit. Nearly complete suppression is achieved under the following conditions: the radiative decay rate of JQF is much larger than that of the data qubit, and the distance between the data qubit and the JQF is close to the half of the resonance wavelength. 
Monday, March 4, 2019 9:12AM  9:24AM 
A29.00007: Breaking the tradeoff between gate and relaxation times of a superconducting qubit with a Josephson quantum filter: Experiment Shingo Kono, Kazuki Koshino, Yutaka Tabuchi, Atsushi Noguchi, Dany LachanceQuirion, Yasunobu Nakamura The rapid development in designs and fabrication techniques of superconducting qubits is making coherence times of qubits longer and longer. In the near future, however, the radiation decay of the qubit (data qubit) into the control line will be a fundamental limitation, imposing a tradeoff between the gate and relaxation times. Here, we successfully break the tradeoff by strongly coupling another superconducting qubit, or a Josephson quantum filter (JQF), along the control line connected to the data qubit. The JQF prevents the data qubit from emitting a microwave photon and thus suppresses the relaxation, while transmitting faithfully largeamplitude control microwave pulses through the saturation of the quantum filter. In this talk, we will present the circuit design and demonstrate the effect of JQF on the qubit coherence. 
Monday, March 4, 2019 9:24AM  9:36AM 
A29.00008: Theoretical analysis on the composite qubit approach to superconducting quantum computing YunPil Shim, Daniel Campbell, Bharath Kannan, Roni Winik, David K Kim, Alexander Melville, Bethany M Niedzielski, Jonilyn L Yoder, Terry Philip Orlando, Simon Gustavsson, William D Oliver, Charles Tahan Encoded qubit scheme [1] for superconducting qubit architecture could be a useful alternative to the conventional approach for simpler control of qubits. The composite qubit (CQB) defined in a twodimensional subspace of a system of two physical transmon qubits can be manipulated using only base band control of each transmon qubit frequencies, allowing for microwavefree single and twoqubit quantum gates. We present theoretical analysis of the optimal operating point (sweet spot) and the gate operations in a real device in the presence of various sources of noise and errors. The CQB computational subspace consists of excited states of the system and relaxation to the physical ground state is one of the main sources of error. We discuss the randomized benchmarking (RB) protocol in the presence of this leakage error. 
Monday, March 4, 2019 9:36AM  9:48AM 
A29.00009: Hamltonian Quantum Computing with transmon qubits Alessandro Ciani, Barbara Terhal, David Peter DiVincenzo We discuss a possible implementation based on superconducting transmon qubits of the scheme for Hamiltonian 
Monday, March 4, 2019 9:48AM  10:00AM 
A29.00010: LowLoss Dielectric Materials and the Merged Element Transmon Corey Rae McRae, Anthony McFadden, Mustafa Bal, Xian Wu, Junling Long, HsiangSheng Ku, Jianguo Wen, Jie Wang, Ilke Arslan, Chris Palmstrom, David Pappas, Russell Lake Josephson junctions, a crucial component in quantum bits (qubits), are commonly composed of a pair of superconducting aluminum films separated by a thin layer of amorphous aluminum oxide. In order to avoid the high density of twolevel states (TLS) in amorphous oxides, Josephson junctions are designed to be small, thus reducing the participation of the lossy material. However, the persistence of lossy materials in qubits leads to diminishing returns with this strategy. An alternative approach to TLS loss minimization in a transmon qubit junction is to combine the qubit’s nonlinear inductance and capacitance into a single trilayer junction with extremely low dielectric loss, i.e., a mergedelement transmon. In this work, we characterize dielectric thin films within lumpedelement resonators to determine microwave losses in the singlephoton regime and to identify dielectric barrier materials for a merged element transmon. In addition to amorphous solid barriers, we measure microwavefrequency loss of singlecrystal epitaxial superconductorinsulatorsuperconductor trilayers. 
Monday, March 4, 2019 10:00AM  10:12AM 
A29.00011: Capacitively Shunted Flux Qubit with High Anharmonicity and Long Coherence Times Jiahao Shi, Muhammet Ali Yurtalan, Graydon JK Flatt, Adrian Lupascu We present experiments on a three Josephson junction flux qubit with all junctions shunted by large capacitive pads. The qubit is capacitively coupled to a coplanar waveguide resonator for dispersive readout and capacitively coupled to a drive line for control. The qubit energy level structure has high anharmonicity, with the ratio of the second to the first transition frequency larger than 3. The dependence of the first two transition frequencies on flux is in excellent agreement with numerical simulations. Close to the symmetry point, the qubit has long relaxation time, in the tens of microseconds range. We characterize qubit dephasing using spinecho and dynamical decoupling and we analyze the flux dependence of dephasing times to extract the flux noise power spectral density. In addition, we present measurement of multilevel coherence. Finally, we discuss the characterization of singlequbit gates using randomized benchmarking. 
Monday, March 4, 2019 10:12AM  10:24AM 
A29.00012: Engineering Sideband Interactions with the Very Small Logical Qubit (VSLQ) Device Part I Gabrielle Roberts, Yao Lu, Nelson Leung, Srivatsan Chakram, Eliot Kapit, David Schuster Like most other architectures, superconducting quantum bits are extremely sensitive to unwanted interactions with the environment. Determining effective and hardwareefficient quantum error correction protocols is thus an urgent scientific priority. I will describe a simple circuit and protocol that autonomously implements the error correction process [1]. The circuit consists of two transmons connected to dissipative baths in the form of lossy cavities and driven by superconducting quantum interference device couplings. Autonomous error correction is achieved using resonances to coherently correct errored qubit states while leaving unerrored states unaffected. The design is highly hardwareefficient and eliminates the need for fast external measurement and feedback. Here, we discuss the theoretical model, highlighting features of experimental relevance. We present simulations results demonstrating coherence improvements of an order of magnitude against realistic noise models. 
Monday, March 4, 2019 10:24AM  10:36AM 
A29.00013: Engineering Sideband Interactions with the Very Small Logical Qubit (VSLQ) Device Part II Yao Lu, Gabrielle Roberts, Nelson Leung, Srivatsan Chakram, Eliot Kapit, David Schuster We report our experimental progress towards implementing a VSLQ device for the autonomous quantum error correction. As a first step, we modify the original VSLQ superconducting circuit [1] and propose a new parametric modulation scheme for engineering the VSLQ Hamiltonian. The new scheme requires many fewer RF flux lines, which significantly reduces the device operational complexity and substantially mitigates crosstalks between the multiple flux loops. Next, in our experiment, we calibrate the static circuit parameters under DC flux biasing. We then further explore the dynamic behavior of the circuit under RF flux modulation, and demonstrate how to generate different types of sideband interactions from RF flux modulations with appropriately chosen frequencies and phases. These sideband interactions serve not only as critical components of the VSLQ protocol, but can also be used to realize instantaneous qubit interactions along arbitrary axes. 
Monday, March 4, 2019 10:36AM  10:48AM 
A29.00014: Efficient characterization of correlated SPAM errors Mingyu Sun, Michael Geller State preparation and measurement (SPAM) errors limit the performance of gatebased quantum computers, but are partly correctable after a calibration step that requires, for exact implementation on a register of n qubits, 2n additional characterization experiments. Here we introduce an approximate but efficient method for SPAM error characterization requiring 2n(n − 1) measurements. The technique assumes that multiqubit measurement errors are dominated by pair correlations, which are estimated with n(n − 1)/2 twoqubit experiments. We demonstrate this technique on the IBM and Rigetti online quantum computers, allowing comparison of their SPAM errors in both magnitude and degree of correlation. We find that the paircorrelation model is reasonably accurate on linear arrays of qubits. However qubits with more closely spaced twodimensional geometries exhibit significant higher order (e.g., 3qubit) SPAM error correlations. 
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