Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session B42: Gates in Superconducting QubitsFocus

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Sponsoring Units: DQI Chair: Antonio Corcoles, IBM Thomas J. Watson Research Center Room: BCEC 210A 
Monday, March 4, 2019 11:15AM  11:27AM 
B42.00001: Characterization of Single and Twoqubit Gates between Transmons and Capacitively Shunted Flux Qubits Jaseung Ku, Yebin Liu, Britton L Plourde, Xuexin Xu, Mohammad H. Ansari, Jared B Hertzberg, Markus Brink, Jerry M. Chow Capacitively shunted flux qubits (CSFQs) with high coherence hold promise for improved single and twoqubit gate operations due to their relatively large and positive anharmonicity. When paired with transmon qubits, CSFQs may lead to a novel regime for twoqubit gates based on the cross resonance interaction. We fabricated a twoqubit system consisting of a CSFQ and a fixedfrequency transmon coupled via a bus cavity and we characterized the fidelity of single and twoqubit gate operations. In this talk, we will present experimental results from these measurements. 
Monday, March 4, 2019 11:27AM  11:39AM 
B42.00002: Characterization of Single and Twoqubit Gates between Transmons and Capacitively Shunted Flux Qubits: Part 2, Theory Xuexin Xu, Mohammad H. Ansari, Jaseung Ku, Yebin Liu, Britton L Plourde, Jared B Hertzberg, Markus Brink, Jerry M. Chow We theoretically model an experiment on a superconducting circuit made of a capacitively shunted flux qubit (CSFQ) and a Transmon qubit both capacitively coupled to a bus resonator in dispersive regime. To model this circuit we take into account the contribution of higher excited states in qubits and blockdiagonalize the Hamiltonian perturbatively in the regime of small interaction couplings compared to frequency detuning. We apply external driving microwave pulses over all energy levels and consider the transitions they impose effectively within the computational subspace. More specifically we study single qubit gates and crossresonance gate, and analyze the fidelity of their operations. Our theoretical results are in agreement with experiment, showing a promising approach to controllably improve single and twoqubit gate operations in such circuits due to the relatively large and positive anharmonicity of CSFQ. 
Monday, March 4, 2019 11:39AM  11:51AM 
B42.00003: Demonstration of a high fidelity entangling gate in a superconducting architecture Sabrina Hong, Prasahnt Sivarajah, Anthony Polloreno, Nicolas Didier, Eyob Sete, Joshua Combes, Kyle Gulshen, Marcus Da Silva, Alexander Papageorge We demonstrate a high fidelity entangling gate in an architecture of alternating fixed and tunable qubits by directly modulating the flux of the tunable qubit. We achieve this by parking the DC flux of the tunable gate where the qubit is firstorder insensitive to 1/f DC flux noise in addition to performing the flux modulated gate at an "AC sweet spot" where the qubit is firstorder insensitive to noise under flux modulation, allowing us to leverage a higher ratio of gate time to coherence time. In addition to characterizing the gate using standard protocols, we qualify its performance over long periods of time, selfconsistently assessing the gate fidelity and drift therein. 
Monday, March 4, 2019 11:51AM  12:03PM 
B42.00004: Quantum Gates between Multimodal Quantum Circuits Sumeru Hazra, Kishor Salunkhe, Gaurav Bothara, Anirban Bhattacharjee, Suman Kundu, Meghan P. Patankar, Tanay Roy, Rajamani Vijayaraghavan Maximum interqubit connectivity and ability to do fast multiqubit operation are key ingredients towards building an efficient quantum processor. We have recently demonstrated trimon, a multimodal device[1,2] with alltoall longitudinal coupling as a threequbit quantum processor[3]. Always on strong longitudinal coupling and maximum interqubit interactions in such devices lead to simple implementation of fast high fidelity multiqubit gates. In order to scale up, we propose to use the trimon as a building block for a larger quantum processor using cross resonance drive[4] to turn on interaction between two such units. We will first discuss the coupling of a transmon and a longitudinally coupled twoqubit device named dimon, and explain how to construct a twoqubit CNOT gate. We will present preliminary experimental data and discuss extensions to gates between multiple dimon/trimon blocks. Finally, we will discuss the possibility of using dimon to improve connectivity and directionality of gates in transversely coupled multiqubit systems. 
Monday, March 4, 2019 12:03PM  12:15PM 
B42.00005: Fast single qubit gates in a capacitively shunted fluxonium Helin Zhang A capacitively shunted fluxonium circuit has among the longest lifetimes seen in cQED due to the exponential suppression of the charge transition matrix element from the extra capacitance. The suppressed matrix elements however make it challenging to perform fast fluxon gates via charge coupling, requiring the use of excited fluxonium levels and Raman transitions [1]. Here, we present and characterize new protocols for performing fast singlequbit gates near half flux quantum, using fast flux modulation. The low transition frequencies near half flux quantum allow for adiabatic gates, and direct synthesis of flux pulses. These approaches increase the ease of control of fluxonium circuit at its highest coherence point, and thereby its feasibility as a building block for future quantum information processors. 
Monday, March 4, 2019 12:15PM  12:27PM 
B42.00006: Sub100ns entangling gates between two strongly coupled transmon qubits Junling Long, HsiangSheng Ku, Russell Lake, Xian Wu, Mustafa Bal, Corey Rae McRae, David Pappas Superconducting qubits are very promising candidates for realizing a faulttolerant quantum computer. However, implementing twoqubit entangling gates with short gate operation time and high fidelity is still a challenge with superconducting qubits. In this talk, we demonstrate a new type of twoqubit entangling gate, the SWIPHT gate [1], on a coupledtransmon device. The twotransmon system, strongly coupled through a bus resonator, is tuned into the secondary resonance regime, where the effective σ^{1}_{z}σ^{2}_{z} interaction is enhanced due to the coupling to higher levels of the transmons. Unconditional single qubit gates twotransmon systems with strong σ^{1}_{z}σ^{2}_{z} interaction were developed. This allows a SWIPHT CNOT gate with roughly 90 ns gate time. The gate fidelity is extracted by quantum process tomography. 
Monday, March 4, 2019 12:27PM  12:39PM 
B42.00007: Blackbox optimization of quantum gates for two coupled transmons Zhaoqi Leng, Pranav Mundada, Andrew Houck As qubit coherence time sets a limit on the total time available for performing quantum gates, realizing highfidelity, fast quantum gates is critical for enabling longdepth quantum circuits in the nonfaulttolerant regime and represents a path way towards fault tolerant quantum computation. Here, we present simulated and experimental results on using blackbox optimization to tune up quantum gates for two transmon qubits coupled via a bus cavity. This optimization procedure does not require any knowledge of gradients derived from the system Hamiltonian, and thus it is ideal for automatic gate optimization on large quantum systems. 
Monday, March 4, 2019 12:39PM  12:51PM 
B42.00008: High fidelity fermionic simulation gates with superconducting gmon qubits Brooks Foxen, Ben Chiaro, Matthew McEwen, John M Martinis We present an experimental realization of a highfidelity 2qubit continuous fermionic simulation (fSim) gateset using superconducting gmon qubits. Each fSim gate is parameterized by two angles: the 01> to 10> swap angle θ, and the conditional phase angle φ. The full fSim(θ, φ) gateset covering θ: [0°, 90°] and φ: [180°, 180°] includes both the iSwap (fSim(90°, 0°)) and controlledZ (fSim(0°, 180°)) gates making it applicable to error correction, quantum supremacy, and preerror correction applications including quantum chemistry and quantum approximate optimization algorithms. 
Monday, March 4, 2019 12:51PM  1:03PM 
B42.00009: Experimental Realization of nonAdiabatic Shortcut to nonAbelian Geometric Gates Tongxing Yan, Baojie Liu, Kai Xu, Chao Song, Song Liu, Zhensheng Zhang, Hui Deng, Zhiguang Yan, Hao Rong, Keqiang Huang, ManHong Yung, Yuanzhen Chen, Dapeng Yu When a quantum system is driven slowly through a parametric cycle in a degenerate Hilbert space, the state would acquire a nonAbelian geometric phase, which is stable and forms the foundation for holonomic quantum computation (HQC). However, in the adiabatic limit, the environmental decoherence becomes a significant source of errors. Recently, various nonadiabatic holonomic quantum computation (NHQC) schemes have been proposed, but all at the price of increased sensitivity to control errors. Here we propose and experimentally demonstrate that HQC via shortcut to adiabaticity (STA) can be constructed with only three energy levels, using a superconducting qubit. With this scheme, all holonomic singlequbit operations can be realized nonadiabatically through a single cycle evolution. As a result, we are able to experimentally benchmark the stability of STA+HQC against NHQC. The flexibility and simplicity of our scheme makes it also implementable on other quantum systems. 
Monday, March 4, 2019 1:03PM  1:39PM 
B42.00010: Highfidelity parametric entangling gates at AC flux sweet spots Invited Speaker: Nicolas Didier Realizing highfidelity twoqubit gates is one of the main challenges in building quantum processors. A major limitation is often decoherence, in particular dephasing due to the ubiquitous presence of flux noise in tunable qubits. Under static magnetic flux biases, the dephasing time is greatly enhanced when the tunable qubit is parked at flux insensitive operating points, commonly referred to as "sweet spots". We show that under flux bias modulation around such DC flux sweet spots, even though the qubit continuously explores regions sensitive to flux noise, there exists a modulation amplitude where the qubit is insensitive to lowfrequency flux fluctuations like 1/f noise  the AC flux sweet spot. We show how this sweet spot is preserved in presence of instrumental white noise by lowpass filtering the flux line. We present how the AC flux sweet spot allows to reach stateoftheart fidelities in parametricallyactivated entangling gates. arXiv:1807.01310. 
Monday, March 4, 2019 1:39PM  1:51PM 
B42.00011: Coherent, LandauZener control of a superconducting composite qubit Daniel Campbell, Bharath Kannan, YunPil Shim, Roni Winik, Alexander Melville, Bethany M. Niedzielski, Jonilyn L Yoder, Charles Tahan, Terry Philip Orlando, Simon Gustavsson, William D Oliver We consider a computational subspace defined by two transmons coupled with a fixed capacitance. Following Ref. 1, we identify the resonantly coupled, hybridized states as a composite qubit (CQB). The CQB architecture has the desirable property that universal singleCQB gates may be implemented using solely coherent, LandauZener control methods based on fast, broadband pulses, without need for microwave control signals. The fidelity of these singleCQB gates is comparable to current stateoftheart implementations^{2}. We present these experimental demonstrations and discuss the susceptibility of the CQB to various noise channels. 
Monday, March 4, 2019 1:51PM  2:03PM 
B42.00012: Faulttolerant and Continuous Holonomic Gates for Topologically Protected Qubits. Andrey Klots, Lara Faoro, Robert F McDermott, Lev B Ioffe We present the design of the topologically protected qubit that allows fault tolerant discrete operations together with the continuous holonomic exp(iθσ_{z})gates. Topological protection of qubits is meaningless without ability to perform fault tolerant operations. These transformations should involve at least the generators of the discrete fault tolerant Clifford group and a rotation at an arbitrary angle such as π/3. We propose a superconducting circuit that satisfies these criteria. The proposed qubit allows protected π/2rotations in charge (Q) and flux (Φ)channels. Furthermore, by moving the state of the qubit along the closed loop in the {Q,Φ}space we can controllably and continuously gain a finite Berry phase difference between 0 and 1 states that is weakly sensitive to the errors in charge and flux biases. 
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