Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session A26: Superconducting Circuits: Qubit Control and Entanglement 
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Sponsoring Units: DQI Chair: Shannon Harvey, Harvard University Room: BCEC 160B 
Monday, March 4, 2019 8:00AM  8:12AM 
A26.00001: Measurement of GHZ and cluster state entanglement monotones in transmon qubits Amara Katabarwa, Michael Geller Experimental detection of entanglement in superconducting qubits has been mostly limited, for more than two qubits, to witnessbased and related approaches that can certify the presence of some entanglement, but not rigorously quantify how much. Here we measure the entanglement of three and fourqubit GHZ and linear cluster states prepared on the 16qubit IBM Rueschlikon (ibmqx5) chip, by estimating their entanglement monotones. We measure the decay of the monotones with time, and find in the GHZ case that they actually oscillate, which we interpret as a drift in the relative phase between the all zero and all one components, but not an oscillation in the actual entanglement. After experimentally correcting for this drift with virtual Z rotations we find that the GHZ states appear to be considerably more robust than cluster states, exhibiting higher fidelity and entanglement at later times. Our results contribute to the quantification and understanding of the strength and robustness of multiqubit entanglement in the noisy environment of a superconducting quantum computer. 
Monday, March 4, 2019 8:12AM  8:24AM 
A26.00002: Scalable generation of genuine multiparticle entanglement with superconducting qubits Ming Gong, MingCheng Chen, Yarui Zheng, Shiyu Wang, Chen Zha, Hui Deng, Zhiguang Yan, Hao Rong, Yulin Wu, Shaowei Li, Fusheng Chen, Youwei Zhao, Futian Liang, Jin Lin, Yu Xu, Cheng Guo, Lihua Sun, Anthony D Castellano, Haohua Wang, Chengzhi Peng, ChaoYang Lu, Xiaobo Zhu, JianWei Pan Solid state qubits by their nature are a scalable technology, but the number of entangled qubits still trails the size of stateoftheart systems considerably. Producing large entangled states requires many different measures of processor quality to be high simultaneously: coherence times and tunability must be high; crosstalk and state leakage must be minimized. In addition, gate operations need to be specifically calibrated to create entanglement. In this work, based only on singlequbit gates and controlledphase gates, we generated and verified the genuine multiparticle entanglement for more than 10 superconducting qubits. This gatebased approach to entanglement generation is directly scalable to larger systems. Our result is an important step towards achieving nearterm quantum supremacy. 
Monday, March 4, 2019 8:24AM  8:36AM 
A26.00003: Simple preparation of Bell and GHZ states using ultrastrongcoupling circuit QED Anton Frisk Kockum, Vincenzo Macrì, Franco Nori The ability to entangle quantum systems is crucial for many applications in quantum technology, including quantum communication and quantum computing. Here, we propose a new, simple, and versatile setup for deterministically creating Bell and GreenbergerHorneZeilinger (GHZ) states between photons of different frequencies in a twostep protocol. The setup consists of a quantum bit (qubit) coupled ultrastrongly to three photonic resonator modes. The only operations needed in our protocol are to put the qubit in a superposition state, and then tune its frequency in and out of resonance with sums of the resonatormode frequencies. By choosing which frequency we tune the qubit to, we select which entangled state we create. We show that our protocol can be implemented with high fidelity using feasible experimental parameters in stateoftheart circuit quantum electrodynamics. One possible application of our setup is as a node distributing entanglement in a quantum network. 
Monday, March 4, 2019 8:36AM  8:48AM 
A26.00004: Proposal to Generate and Characterize Quantum Entanglement using Coupled Metamaterial Resonators Francisco Rouxinol, Frederico B Brito, Matthew LaHaye, B.L.T. Plourde Entanglement is an underlying principle in quantum physics. Undoubtedly, the consequences of the correlations observed between measurements in spatially separated entangled systems has had a profound effect on our view of local realism of the world (EPR, Bell etc.). Using a lefthanded metamaterial coupled to a nanomechanical resonator, we propose a new procedure to couple multiple microwave resonant modes and investigate entanglement in these limits. These investigation allow for the possibility to develop new techniques for characterizing entanglement in multimode systems. The potential of using these circuits in QED architectures is discussed. 
Monday, March 4, 2019 8:48AM  9:00AM 
A26.00005: Propagating NonGaussian States Generated by Degenerate 3photon Downconversion Chung Wai Sandbo Chang, A. Vadiraj, Ibrahim Nsanzineza, Christopher Wilson Nonclassical light sources have generated broad interest across a number of fields for their potential to enable novel quantum protocols. Spontaneous parametric downconversion (SPDC) processes have been widely adopted as an essential source of nonclassical light, generating various resource states. However, SPDC beyond second order has been an experimental challenge for decades. Here, with a superconducting parametric cavity designed for cubic interactions, we present experimental results for spontaneous threephoton downconversion when the cavity is flux pumped at three times the mode frequency. We measure propagating output states that are clearly nonGaussian with a high degree of skewness in the quadrature amplitude distribution. Further, theory predicts that Wigner function negativity can persist for these states even after they escape from the cavity. With a linear detection scheme, we reconstruct the Wigner function of the propagating states from the directly measured moment matrix. We will present preliminary results on this reconstructed Wigner function. 
Monday, March 4, 2019 9:00AM  9:12AM 
A26.00006: Multipartite entanglement mediated by shared microwave resonators Marie Lu, Sydney Schreppler, Lukas F Buchmann, Felix Motzoi, Irfan Siddiqi Developing highly connected networks of qubits is invaluable for implementing various quantum codes and simulations. Increased connectivity allows for entangling qubits with reduced gate depth. For example, the MølmerSørensen gate generates high fidelity entanglement between multiple ions. We use shared coplanar waveguide (CPW) resonators to realize an analogous MølmerSørensen gate between superconducting qubits. This talk describes how the CPW enables multipartite entanglement of all qubits coupled to the resonator as well as arbitrary subsets of qubits. 
Monday, March 4, 2019 9:12AM  9:24AM 
A26.00007: Probing the TavisCummings level splitting with intermediatescale superconducting circuits Martin Weides, Ping Yang, Jan David Brehm, Juha Leppaekangas, Lingzhen Guo, Michael Marthaler, Isabella Boventer, Alexander Stehli, Tim Wolz, Alexey Ustinov We demonstrate the local control of up to eight twolevel systems interacting strongly with a microwave cavity. Following calibration, the frequency of each individual twolevel system (qubit) is tunable without influencing the others. Bringing the qubits one by one on resonance with the cavity, we observe the collective coupling strength of the qubit ensemble. The splitting scales up with the square root of the number of the qubits, being the hallmark of the TavisCummings model. The local control circuitry causes a bypass shunting the resonator, and a Fano interference in the microwave readout, whose contribution can be calibrated away to recover the pure cavity spectrum. The simulator's attainable size of dressed states is limited by reduced signal visibility, and if uncalibrated by offresonance shifts of subcomponents. Our work demonstrates control and readout of quantum coherent mesoscopic multiqubit system of intermediate scale under conditions of noise. 
Monday, March 4, 2019 9:24AM  9:36AM 
A26.00008: Improving the fidelity of entangling gates via in situ characterization of qubit control lines. Anatoly Kulikov, Markus Jerger, Arkady Fedorov One of the factors limiting the fidelity of entangling gates are control signal distortions arising from nontrivial transfer functions of microwave lines. Distortions can be canceled, in principle, if the complex transfer function of the control line is known, by applying its inverse to the signal before it is transmitted. We have previously developed a method for insitu measurement of the system function using a qubit as a network vector analyser [1]. I will present the refinement of the method and its experimental application to improve the fidelity of CPHASE gate between two superconducting transmon qutrits. I will also show how our technique can be combined with other recently reported insitu methods to calibrate lowfrequency response of control lines [2,3] to further enhance the fidelity of the twoqubit gates. 
Monday, March 4, 2019 9:36AM  9:48AM 
A26.00009: Fast Amplification and Rephasing of Entangled Cat States in a QubitOscillator System Tomoko Fuse, Zhihao Xiao, Sahel Ashhab, Fumiki Yoshihara, Kouichi Semba, Masahide Sasaki, Masahiro Takeoka, Jonathan P Dowling

Monday, March 4, 2019 9:48AM  10:00AM 
A26.00010: Groundstate cooling, Fockstate stabilization and photonresolved thermalization dynamics in a hot 170 MHz resonator. Mario Gely, Marios Kounalakis, Christian Dickel, Jacob Dalle, Rémy Vatré, Mark D Jenkins, Gary Steele In order to observe coherent quantum effects, systems are usually cooled to their thermal ground states. Despite millikelvin temperatures being a dominant energy scale in a 170 MHz circuit QED mode, we show that quantum control of the mode can still be realized. This is achieved by coupling it to a cold mode at 5.9 GHz through the nonlinearity of a Josephson junction shared between both modes. The resulting coupling leads to photon number splitting that allows reading out the state of the low frequency mode. Via fourwave mixing, we can sideband cool the lowfrequency mode to its ground state, as well as stabilize one and twophoton Fock states. In timedomain, we can then observe the photonresolved thermalization dynamics of these stabilized states. Our platform could be used to resonantly interface quantum circuits with MHz frequency systems (e.g. mechanical elements) or enable further exploration of thermodynamical processes at the quantum scale. 
Monday, March 4, 2019 10:00AM  10:12AM 
A26.00011: Probing the influence of manybody fluctuations on Cooper pair tunneling using circuit QED Sébastien Léger, Javier Puertas, Luca Planat, Remy Dassonneville, Vladimir Milchakov, Karthik Srikanth Bharadwaj, Jovian Delaforce, Farshad Foroughi, Olivier Buisson, Cecile Naud, Wiebke Guichard, Izak Snyman, Serge Florens, Nicolas Roch

Monday, March 4, 2019 10:12AM  10:24AM 
A26.00012: Driving not so forbidden state transitions in a frequencytunable transmon Alexander Opremcak, Ben Chiaro, Brooks Foxen, Matthew McEwen, Robert F McDermott, John M Martinis In a frequencytunable transmon, transitions between the 0> and 2> states are nominally forbidden by selection rules, yet an experiment (Sank et al., PRL 117) observed that it is possible to drive these transitions. In this talk we explain that observation. We show that the apparent selection rule violation is a direct consequence of parametric modulation of the Josephson energy of the compound junction of the device. From a theoretical analysis of the dc SQUID, we derive a drive term that explains the violation. We validate our theory using experimentally measured Rabi oscillations and Ramsey interferometry on the 0> > 2> transition. Surprisingly, the transition occurs for a transmon driven through a capacitor, indicating a modest degree of stray inductive coupling between the XY drive line and the compound junction of the transmon. These results identify a leakage channel and should inform efforts to integrate cryogenic control systems with arrays of frequencytunable transmons. 
Monday, March 4, 2019 10:24AM  10:36AM 
A26.00013: Perfect quantum state transfer in a superconducting qubit chain with parametrically tunable couplings Xuegang Li Faithfully transferring quantum state is essential for quantum information processing. Here, we demonstrate a fast (in 84 ns) and highfidelity (99.2%) transfer of arbitrary quantum states in a chain of four superconducting qubits with nearestneighbor coupling. This transfer relies on full control of the effective couplings between neighboring qubits, which is realized only by parametrically modulating the qubits without increasing circuit complexity. Once the couplings between qubits fulfill specific ratio, a perfect quantum state transfer can be achieved in a single step, therefore robust to noise and accumulation of experimental errors. This quantum state transfer can be extended to a larger qubit chain and thus adds a desirable tool for future quantum information processing. The demonstrated flexibility of the coupling tunability is suitable for quantum simulation of manybody physics which requires different configurations of qubit couplings. 
Monday, March 4, 2019 10:36AM  10:48AM 
A26.00014: Temperature Dependence of Fidelity for Parametrically Stimulated Gates Pranav Mundada, Gengyan Zhang, Andrew Houck Tunability and coherence are the desired properties of an ideal quantum computer. However, introducing tunability directly in the qubit often adversely affects its coherence. This tunabilitycoherence tradeoff can be optimized by using fixed frequency qubits coupled with a tunable coupler. This architecture has been widely used for demonstrating parametrically stimulated entangling gates. In previous theoretical studies of superconducting circuits, the coupler has always been assumed to be in the ground state and an examination of the temperature dependence of parametric gates has been lacking. Here we present the importance of thermalization of the tunable coupler, to the base temperature of a dilution refrigerator, for achieving a highfidelity parametric gate. This talk is based on the work presented in "Suppression of Qubit Crosstalk in a Tunable Coupling Superconducting Circuit" (arXiv:1810.04182 [quantph]) 
Monday, March 4, 2019 10:48AM  11:00AM 
A26.00015: Quantum entanglement dynamics due to dynamical Lamb effect Mirko Amico, Oleg Berman, Roman Kezerashvili We investigate the dynamics of a system of N qubits coupled to a common resonator with timedependent coupling. The instantaneous switching on and off of the qubit/cavity coupling gives rise to the dynamical Lamb effect. The dynamical Lamb effect is the parametric excitation of the qubits, and the creation of cavity photons, due to the sudden change in Lamb shift of the qubit. In the absence of dissipation, the Schrodinger equation which describes the dynamics of the N qubits is solved within a perturbative approach. When dissipation is taken into account, the Lindblad equation for the system of N qubits is solved numerically. Different measures of entanglement compatible with pure and mixed states are adopted. The concurrence and the negativity are obtained in the twoqubit case; the threeπ and the negativity are obtained in the threequbit case. It is demonstrated that the different measures show different level of details of the entanglement between the qubits. We find that the dynamical Lamb effect can be used to create GreenbergerHorneZeilinger states even in presence of dissipation. Furthermore, the dynamical Lamb effect can be used as a fast entangling gate between two qubits. 
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