Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session D28: Focus Session: Superconducting Qubits: Gates and Entanglement |
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Sponsoring Units: GQI Chair: Joel Strand, Northrop Grumman Room: 601 |
Monday, March 3, 2014 2:30PM - 3:06PM |
D28.00001: Driven superconducting quantum circuits Invited Speaker: Yasunobu Nakamura Driven nonlinear quantum systems show rich phenomena in various fields of physics. Among them, superconducting quantum circuits have very attractive features such as well-controlled quantum states with design flexibility, strong nonlinearity of Josephson junctions, strong coupling to electromagnetic driving fields, little internal dissipation, and tailored coupling to the electromagnetic environment. We have investigated properties and functionalities of driven superconducting quantum circuits. A transmon qubit coupled to a transmission line shows nearly perfect spatial mode matching between the incident and scattered microwave field in the 1D mode [1]. Dressed states under a driving field are studied there and also in a semi-infinite 1D mode terminated by a resonator containing a flux qubit [2]. An effective $\Lambda$-type three-level system is realized under an appropriate driving condition. It allows ``impedance-matched'' perfect absorption of incident probe photons and down conversion into another frequency mode [3]. Finally, the weak signal from the qubit is read out using a Josephson parametric amplifier/oscillator which is another nonlinear circuit driven by a strong pump field [4]. \\[4pt] [1] K. Koshino {\it et al.}, PRL {\bf 110}, 263601 (2013).\\[0pt] [2] K. Inomata {\it et al.}, PRB {\bf 86}, 140508(R) (2012).\\[0pt] [3] K. Koshino {\it et al.}, PRL {\bf 111}, 153601 (2013).\\[0pt] [4] Z. R. Lin {\it et al.}, APL {\bf 103}, 132602 (2013). [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D28.00002: Optimized pulse shapes for a resonator-induced phase gate Andrew Cross, Jay Gambetta, Stefano Poletto, Doug Mcclure, Oliver Dial, Matthias Steffen The resonator-induced phase gate is a multi-qubit controlled-phase gate for superconducting qubits. Through off-resonant driving of a bus cavity, coupled qubits acquire a state-dependent phase and are not excited outside of the qubit manifold. However, cavity loss leads to dephasing during the gate and any residual entanglement between the cavity and qubits after the gate leads to decoherence. In this talk we present strategies for shaping the drive pulse to minimize dephasing and reduce the pulse duration. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D28.00003: Cross-resonance interactions between superconducting qubits with variable detuning Matthew Ware, Blake Johnson, Jay Gambetta, Colm Ryan, Thomas Ohki, Jerry Chow, B.L.T. Plourde The cross-resonance effect is a promising route for generating two-qubit gates in an all-microwave architecture based on superconducting qubits. Because the strength of the cross-resonance effect, and hence the speed of a two-qubit gate, depends sensitively on the detuning between the qubits and the anharmonicity of each qubit, we are performing experiments with some fixed-frequency transmon qubits and others with some tunability. By using asymmetric transmon qubits, we are able to vary this detuning over a moderate range. This allows us to study the cross-resonance effect while varying the magnetic flux to generate different qubit-qubit detunings. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D28.00004: A circuit QED controlled-Z ``AMP'' gate (Adiabatic MultiPole gate) David C. McKay, Ravi Naik, Lev S. Bishop, David I. Schuster Circuit quantum electrodynamics --- superconducting Josephson junction ``transmon'' qubits coupled via microwave cavities --- is a promising route towards scalable quantum computing. Here we report on experiments coupling two transmon qubits through multiple strongly coupled planar superconducting cavities --- the multipole cavity QED architecture. This design enables large interactions (mediated by real cavity photons) when the transmons are resonant with the cavities, and low off rates when the qubits are tuned away from the cavity resonance. In this talk we will discuss our gate protocol --- the AMP gate --- and report on producing a high fidelity Bell state ($|gg\rangle+|ee\rangle$) measured from state and process tomography. We will discuss future plans for scaling this architecture beyond two qubits. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D28.00005: Achieving high contrast on/off ratios using the multipole circuit QED architecture Ravi Naik, David C. McKay, Lev S. Bishop, David I. Schuster An outstanding goal for scalable quantum information processing is to design gates (qubit-qubit interactions) that are fast, yet can be switched off with high contrast to permit high fidelity single qubit operations. We implement a possible candidate for such a gate using the multipole circuit QED architecture which consists of superconducting Josephson junction qubits coupled via multiple strongly interacting microwave cavities. In this architecture, the on/off ratio is expected to scale exponentially in the number of cavities (poles). Here we report on measurements of the off-resonant coupling rate for two flux-tuned transmon qubits coupled through three strongly coupled planar resonators (a 3-pole filter). We will also discuss progress towards a scheme to implement multipole QED for flux insensitive qubits in 3D microwave cavities where the longest coherence times for superconducting qubits have been demonstrated. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D28.00006: Remote entanglement of transmon qubits M. Hatridge, K. Sliwa, A. Narla, S. Shankar, Z. Leghtas, M. Mirrahimi, S.M. Girvin, R.J. Schoelkopf, M.H. Devoret An open challenge in quantum information processing with superconducting circuits is to entangle distant (non-nearest neighbor) qubits. This can be accomplished by entangling the qubits with flying microwave oscillators (traveling pulses), and then performing joint operations on a pair of these oscillators. Remarkably, such a process is embedded in the act of phase-preserving amplification, which transforms two input modes (termed signal and idler) into a two-mode squeezed output state. For an ideal system, this process generates heralded, perfectly entangled states between remote qubits with a fifty percent success rate. For an imperfect system, the loss of information from the flying states degrades the purity of the entanglement. We show data on such a protocol involving two transmon qubits imbedded in superconducting cavities connected to the signal and idler inputs of a Josephson Parametric Converter (JPC) operated as a nearly-quantum limited phase-preserving amplifier. Strategies for optimizing performance will also be discussed. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D28.00007: High fidelity gates and states in a 5 Xmon qubit Josephson quantum processor, part I: architecture J. Kelly, R. Barends, A. Megrant, A. Veitia, E. Jeffrey, D. Sank, T. White, J. Mutus, J. Bochmann, B. Campbell, Y. Chen, Z. Chen, B. Chiaro, A. Dunsworth, I. Hoi, C. Neill, P. O'Malley, C. Quintana, P. Roushan, A. Vainsencher, J. Wenner, A. Korotkov, A.N. Cleland, J.M. Martinis One of the greatest challenges in building a quantum architecture is to combine high fidelity logic gates with a multiqubit system. Here, we demonstrate high fidelity gates in a 5 Xmon qubit quantum processor using a multiqubit architecture which combines coherence, control and connectivity. The qubits are arranged in a linear chain with nearest neighbor coupling, have individual control and readout, and reach $T_1$ values up to 57 $\mu$s. We characterize single qubit gates with a fidelity above 99.9 \% for all qubits. Using the frequency tunability of the qubits, we employ a novel implementation of a fast, adiabatic two-qubit controlled-phase gate, measuring fidelities up to 99.45 \%. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D28.00008: High fidelity gates and states in a 5 Xmon qubit Josephson quantum processor, part II: multiqubit logic R. Barends, J. Kelly, A. Megrant, A. Veitia, E. Jeffrey, D. Sank, T. White, J. Mutus, J. Bochmann, B. Campbell, Y. Chen, Z. Chen, B. Chiaro, A. Dunsworth, I. Hoi, C. Neill, P. O'Malley, C. Quintana, P. Roushan, A. Vainsencher, J. Wenner, A. Korotkov, A.N. Cleland, J.M. Martinis One of the critical challenges in quantum computing is to employ simultaneous, high fidelity quantum logic gates across a system. Here, we show how a novel implementation of a fast, adiabatic controlled-phase gate achieves fidelities between 99.0 and 99.4 \% across all pairs in a 5 Xmon qubit quantum processor. We also show that nearest as well as next nearest neighbor qubits can be operated simultaneously without sacrificing fidelity. This, combined with low Z control crosstalk allows for direct control of single or multiqubit subspaces. To showcase the addressability of the qubits and modularity of the logic set, we use single and two-qubit gates to construct N=3, 4 and 5 Greenberger-Horne-Zeilinger states with fidelities of 96 \%, 86 \% and 82 \%, characterized by quantum state tomography. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D28.00009: High fidelity gates and states in a 5 Xmon qubit Josephson quantum processor, part III: controlled-Z theory John Martinis, R. Barends, J. Kelly, A. Megrant, A. Veitia, E. Jeffrey, D. Sank, T. White, J. Mutus, J. Bochmann, B. Campbell, Y. Chen, Z. Chen, B. Chiaro, A. Dunsworth, I. Hoi, C. Neill, P. O'Malley, C. Quintana, P. Roushan, A. Vainsencher, J. Wenner, A. Korotkov, A.N. Cleland I will explain how to construct a two-qubit controlled-Z gate that is adiabatic and fast (40 ns), yet requires only moderate coupling ($g/2\pi=$ 30 MHz). [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D28.00010: Model Free Gate Design and Calibration For Superconducting Qubits Daniel Egger, Frank Wilhelm Gates for superconducting qubits are realized by time dependent control pulses. The pulse shape for a specific gate depends on the parameters of the superconducting qubits, e.g. frequency and non-linearity. Based on ones knowledge of these parameters and using a specific model the pulse shape is determined either analytically or numerically using optimal control [arXiv:1306.6894, arXiv:1306.2279]. However the performance of the pulse is limited by the accuracy of the model. For a pulse with few parameters this is generally not a problem since it can be ``debugged'' manually. He we present an automated method for calibrating multiparameter pulses. We use the Nelder-Mead simplex method to close the control loop. This scheme uses the experiment as feedback and thus does not need a model. It requires few iterations and circumvents process tomogrophy, therefore making it a fast and versatile tool for gate design. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D28.00011: Frequency Modulating Entangling Gates Thomas Ohki, Colm Ryan, Blake Johnson, Kin Chung Fong, Matt Ware, Britton Plourde There are multiple approaches to generating entangling gates in a superconducting qubit architecture depending on the choice of tunable or fixed frequency qubits. The asymmetric transmon offers a compromise between the two extremes offering mild tunability and better coherence times. With the ability to modulate the qubits' frequency a new type of first-order sideband gate is made available [1,2]. These allow the qubits to exchange information with a cavity quantum bus without having to be dynamically tuned into resonance with the cavity and potentially acquiring unwanted phases from interactions with other qubits. We show progress towards using this interaction as a high-fidelity entangling gate. \\[4pt] [1] Beaudoin, F., da Silva, M. P., Dutton, Z., {\&} Blais, A. (2012). First-order sidebands in circuit QED using qubit frequency modulation. \textit{Physical Review A}, \textbf{86}, 022305. \\[0pt] [2] Strand et al. (2013). First-order sideband transitions with flux-driven asymmetric transmon qubits. \textit{Physical Review B}, \textbf{87}, 220505. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D28.00012: Monitoring the performance of an autonomous entanglement stabilization protocol in real time Y. Liu, S. Shankar, N. Ofek, M. Hatridge, A. Narla, K.M. Sliwa, R.J. Schoelkopf, M.H. Devoret Quantum feedback for error correction poses an open challenge for superconducting quantum information processing. Recently, we have demonstrated an autonomous feedback protocol to stabilize entanglement between two transmon qubits coupled to a cavity, and achieved a fidelity of 67\% in the steady state. The feedback protocol is designed such that the cavity output continuously provides information on the state of the qubits. Here, we report the integration of an external measurement-based feedback architecture with this experiment to monitor the cavity output in real-time. The cavity output is directed to a high-fidelity measurement chain based on a Josephson parametric converter and then processed in real-time using an FPGA (Field Programmable Gate Array). This real time monitoring capability combined with low-latency digital control allows conditional tomography of the qubits state, and thus enhances the fidelity of the entanglement. We can thus leverage the flexibility of measurement-based feedback with the rapid response of autonomous feedback to attain a level of performance that cannot be reached by either architecture alone. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D28.00013: Efficient three-qubit entangling (Toffoli) gates via excited states in qubit-cavity systems. Thomas Reinecke, Sophia Economou, Dmitry Solenov Efficient multi-qubit quantum operations are crucial for further development of quantum information processing using available physical designs. We report our results on efficient three-qubit entangling operations in qubit-cavity systems. The proposed gate design is based on non-commutativity of single-qubit pulse controls that can be achieved for systems in which auxiliary states above the qubit subspace are available. It does not rely on dynamical tuning of energy states, and, unlike traditional decomposition approaches, it provides efficiency comparable to that of a single control-NOT operation. We will focus on the transmon qubit systems, which have recently demonstrated coherence times suitable for multi-qubit computation. Other systems will also be discussed. [Preview Abstract] |
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