Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session U08: Superconducting Qubits: Gates, Couplers and Crosstalk II |
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Sponsoring Units: DQI Chair: Diego Ristè, BBN Technology - Massachusetts Room: 104 |
Thursday, March 5, 2020 2:30PM - 2:42PM |
U08.00001: Two-qubit coupler with exponential suppression of virtual interactions Catherine Leroux, Agustin Di Paolo, Alexandre Blais We present a superconducting two-qubit coupler where a tunable bus interacts with a driven mode such as to, on demand, exponentially suppress the qubit-qubit virtual interactions with respect to the amplitude of the external control field. The proposed mechanism can be combined with tuning the bus frequency, which is the standard approach to suppress dispersive interactions, thus making current tunable bus designs more efficient. |
Thursday, March 5, 2020 2:42PM - 2:54PM |
U08.00002: Suppression of Qubit Crosstalk between Transmons and Capacitively Shunted Flux Qubits : Part1, Experiment Jaseung Ku, Britton L Plourde, Jared Hertzberg, Markus Brink, David McKay, Jerry M. Chow, Xuexin Xu, Mohammad Ansari In multi-qubit cQED systems, the static ZZ crosstalk can be a limiting factor for high-fidelity two-qubit gates. Thus, mitigating such ZZ interactions becomes critically important for achieving high-fidelity entangling gates as the number of qubits increases. One potential way to suppress the static ZZ interaction involves combining different types of qubits. Capacitively shunted flux qubits (CSFQs), in contrast to transmon qubits, possess relatively large and positive anharmonicity. When they are paired with an appropriate detuning, the static ZZ term can be significantly suppressed for two-qubit gates based on the cross resonance interaction. We fabricated a two-qubit system consisting of a CSFQ and a fixed-frequency transmon coupled via a bus cavity. In this talk, we will present experimental results from these measurements, including the ZZ crosstalk and two-qubit gate fidelity versus detuning between the two qubits. |
Thursday, March 5, 2020 2:54PM - 3:06PM |
U08.00003: Suppression of Qubit Crosstalk between Transmons and Capacitively Shunted Flux Qubits: Part 2, Theory Xuexin Xu, Mohammad Ansari, Jaseung Ku, Britton L Plourde, Jared Hertzberg, Markus Brink, David McKay, Jerry M. Chow We compare two-qubit superconducting circuits composed of either different-species qubits or all-transmon qubits. In the different-species case, we use qubits with the opposite sign of anharmonicity, while for the all-transmon circuit, both qubits have a negative anharmonicity. We show that there are unique features in the different-species approach that makes it attractive for use in near-term quantum computers. Circuits with different-species qubits are capable of fast two-qubit gates and they allow for complete cancellation of ZZ crosstalk interactions. Our theoretical results are in agreement with an experiment on a two-qubit device consisting of a capacitively shunted flux qubit and a transmon and show a promising way to improve two-qubit gate operations. |
Thursday, March 5, 2020 3:06PM - 3:18PM |
U08.00004: High fidelity encoded gate operations for composite superconducting qubit Yun-Pil Shim, Daniel L Campbell, Bharath Kannan, Roni Winik, Alexander Melville, Bethany Niedzielski, Jonilyn Yoder, Terry Philip Orlando, Simon Gustavsson, William Oliver, Charles Tahan Our encoded qubit scheme for superconducting transmon qubits explores a non-traditional architecture where the computational states are encoded by the hybridized states formed by a pair of capacitively coupled degenerate transmons. Gate operations can be implemented on the encoded qubit using non-adiabatic Landau-Zener control at the small avoided crossing and requiring only baseband control, without individual microwave control for each transmon. Even with frequencies far below the effective temperature, high fidelity encoded gates can be achieved utilizing the sweet spot (optimal operating point) and dynamical sweet spot (optimal pulse shape). Further, these composite qubits show immunity to photon noise in readout resonators, which can be a limiting factor in many transmon-based quantum computing systems, often requiring special attenuators to mitigate. We also discuss the leakage errors due to relaxation to the physical ground state which is outside of the encoded qubit's computational subspace. |
Thursday, March 5, 2020 3:18PM - 3:30PM |
U08.00005: A multi-qubit gate for non-interacting qubits in circuit-QED using the Zeno effect Chen Mor, Boaz Koren, Asaf Diringer, Shay Hacohen-Gourgy The quantum Zeno effect is known for freezing dynamics due to observation. The observation can also confine the evolution to an unobserved subspace of the system. This confinement can lead to non-trivial evolution. We show that using a single qubit drive, and a strong continuous observation that blocks part of the evolution, we can create a multi-qubit entangling gate between non-interacting qubits. We use transmon qubits coupled to a 3D cavity. The qubits are far detuned from each other, such that they are effectively non-interacting. We present the experimental results and discuss the fidelity and limitations of this gate. |
Thursday, March 5, 2020 3:30PM - 3:42PM |
U08.00006: Cross-resonance Dynamics with Tunable Transmon Qubits Bradley Mitchell, Ravi Kaushik Naik, Akel Hashim, John Mark Kreikebaum, Irfan Siddiqi The cross-resonance gate is a widely-utilized entangling gate in modern superconducting quantum processors. Recent theoretical work [1] has illustrated that the dynamics of the cross-resonance interaction depend strongly on circuit and control parameters, such as frequency detuning and the effective coupling rate. We experimentally investigate the dynamics of the cross-resonance gate across a range of qubit detuning, intrinsic coupling strength, drive frequency, and compare our findings with the model. |
Thursday, March 5, 2020 3:42PM - 3:54PM |
U08.00007: Density Matrix Exponentiation on a Superconducting Quantum Processor (Part 1): Introduction and construction Morten Kjaergaard, Mollie Schwartz, Amy Greene, Gabriel Samach, Andreas Bengtsson, Michael O'Keeffe, Chris McNally, Youngkyu Sung, Milad Marvian, Philip Krantz, Jochen Braumueller, Roni Winik, David K Kim, Alexander Melville, Bethany Niedzielski, Jonilyn Yoder, Danna Rosenberg, Kevin Obenland, Terry Philip Orlando, Iman Marvian, Simon Gustavsson, Seth Lloyd, William Oliver In conventional quantum processors, the parameters of the unitary operators to be implemented as part of a quantum circuit are controlled using classical hardware. Here we experimentally demonstrate a two-qubit implementation of the Density Matrix Exponentiation (DME) algorithm, in which the unitary operators of a quantum circuit are programmed by other quantum states. By using multiple copies of a quantum state, the DME algorithm efficiently implements a unitary operator with that state acting as a Hamiltonian. In Part 1 of this talk, we will introduce the DME algorithm, its uses, and our experimental implementation on a small quantum processor using a 99.7% fidelity CPHASE gate between two superconducting transmons. |
Thursday, March 5, 2020 3:54PM - 4:06PM |
U08.00008: Density Matrix Exponentiation on a Superconducting Quantum Processor (Part 2): Demonstration and Characterization Mollie Schwartz, Morten Kjaergaard, Amy Greene, Gabriel Samach, Andreas Bengtsson, Michael O'Keeffe, Chris McNally, Youngkyu Sung, Milad Marvian, Philip Krantz, Jochen Braumueller, Roni Winik, David K Kim, Alexander Melville, Bethany Niedzielski, Jonilyn Yoder, Danna Rosenberg, Kevin Obenland, Terry Philip Orlando, Iman Marvian, Simon Gustavsson, Seth Lloyd, William Oliver Density matrix exponentiation (DME) represents a unique style of quantum operation, in which many copies of a quantum state ρ are used to perform a unitary operation U(ρ, θ) = eiρθ on a second quantum system σ. Traditionally, changing an operation on σ requires changing a sequence of classically-defined gates; in DME, a fixed gate sequence performs a range of operations simply by varying the quantum input. DME is a Trotter-style algorithm, in which the total angle of rotation θ is built up by performing N rotations of θ/N with algorithmic error θ2/N. In Part 2 of this talk, we benchmark an implementation of the DME algorithm in a two-qubit system. We characterize U(ρ, θ) via process tomography, and demonstrate that the unitary depends on the input quantum state. We also explore tradeoffs between algorithmic error at small N and noise/loss limits at large N. |
Thursday, March 5, 2020 4:06PM - 4:18PM |
U08.00009: Experimental considerations for zero noise extrapolation Abhinav Kandala, Kristan Temme, Seth Merkel, David McKay, Easwar M Magesan, Jay M Gambetta While decoherence and control errors limit the size of quantum computation in the absence of fault tolerance, a number of error mitigation techniques have been developed to access noise free estimates of expectation values. In particular, the zero-noise extrapolation technique was shown to extend the computational reach of a noisy superconducting processor. Here, quantum circuits are run at amplified noise rates, to extrapolate the results of these runs to the zero noise limit. Under the assumption of time invariant noise, such noise amplification may be achieved by stretching in time the gates employed in the circuit. We test these assumptions for superconducting processors with all-microwave drives and also consider the effect of errors in the gate rescaling on the extrapolation. |
Thursday, March 5, 2020 4:18PM - 4:30PM |
U08.00010: Realizing giant artificial atoms in superconducting waveguide QED A.M. Vadiraj, Andreas Ask, Ibrahim Nsanzineza, Chung Wai Sandbo Chang, Anton Frisk Kockum, C.M. Wilson In most studies of light-matter interaction, the atoms, either natural or artificial, are approximated as featureless dipoles, since the atomic dimension is much smaller than the wavelength of light. However, a new regime in waveguide QED, first proposed by Kockum et al, can realize a “giant” artificial atom by coupling to light at multiple points along a waveguide. As a result, the atom interacts with itself, resulting in a range of phenomena including non-Markovian dynamics and frequency-dependent coupling. The same proposal also discussed possibilities to extend this architecture to multiple giant atoms with interesting new physics. Motivated by this, we experimentally investigate circuits with one and two giant transmon qubits which are coupled to propagating microwaves at multiple points separated by wavelength-scale distances. For one qubit circuit, we demonstrate that we can enhance or suppress the relaxation rate of the 1-2 transition relative to the 0-1 transition by more than an order of magnitude. Using this capability, we show that we can engineer the giant transmon into an effective lambda system, including demonstrating EIT. We will also present preliminary measurements of a circuit with two giant qubits coupled in a braided geometry. |
Thursday, March 5, 2020 4:30PM - 4:42PM |
U08.00011: Progress Towards Fast Dissipation-Induced Entanglement In Circuit-QED Using Parametric Interactions Tristan Brown, Emery Doucet, Florentin Reiter, Raymond W Simmonds, Jose Aumentado, Diego Ristè, Leonardo M Ranzani, Archana Kamal
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Thursday, March 5, 2020 4:42PM - 4:54PM |
U08.00012: A novel bus design for a highly connected multi-qubit processor in 3D cQED architecture Sumeru Hazra, Anirban Bhattacharjee, Kishor V Salunkhe, Sanskriti Chitransh, Meghan P. Patankar, R Vijay Highly connected qubit networks offer efficient implementation of quantum algorithms by requiring fewer gates to implement a given circuit. While not considered to be highly scalable, the 3D cQED architecture offers a cleaner microwave environment and modular design which can be used effectively to build small to intermediate scale processors. We will present a novel bus design to couple multiple qubits in 3D cQED with high inter-qubit connectivity and sufficient spatial separation to minimize cross-talk effects. We will show preliminary experimental data to demonstrate qubit interactions using the cross-resonance gate but will also explain how it can be integrated with multiple gate schemes to build scalable multi-qubit processor in 3D architecture. |
Thursday, March 5, 2020 4:54PM - 5:06PM |
U08.00013: Waveguide-mediated Interactions Between Giant Superconducting Artificial Atoms Bharath Kannan, Max Ruckreigel, Daniel L Campbell, Anton Frisk Kockum, Jochen Braumueller, Roni Winik, Morten Kjaergaard, David K Kim, Alexander Melville, Bethany Niedzielski, Jonilyn Yoder, Terry Philip Orlando, Simon Gustavsson, William Oliver In quantum optics experiments, atoms are typically treated as small point-like objects compared to the wavelength of the modes they interact with. Superconducting circuits offer a platform to study the physics of systems where this approximation no longer holds. Here, we realize a “giant” artificial atom by coupling a transmon qubit to multiple positions along a superconducting waveguide with separation on the order of the qubit wavelength. We show that the coupling between the qubits and the waveguide can be strongly tuned with the qubit frequency. Furthermore, we demonstrate waveguide-mediated exchange interactions between two giant atoms that can be tuned to be “decoherence-free” with respect to the waveguide. Finally, we show how varying the number of and distance between coupling points can be used to engineer exotic decay and coupling spectra, with applications to quantum information and simulation. |
Thursday, March 5, 2020 5:06PM - 5:18PM |
U08.00014: Building and benchmarking diabatic entangling gates for frequency-tunable qubits, part I Andre Petukhov, Rami Barends, Chris Quintana, Yu Chen, Vadim Smelyanskiy One of the key needs in quantum computing are gates that are fast and precise, requiring minimization of the control errors and the suppression of leakage out of the computational states. We present a non-adiabatic protocol for iSWAP-like gates with minimal leakage and duration close to the speed limit. The key notion behind our approach is minimizing the gate error by synchronizing the Rabi clocks in the one- and two-excitation channels in a pair of transmon qubits to align minima of the leakage and residual swap population. This, in turn, can be accomplished by utilizing limited but sufficient tunability of the “fixed” interqubit coupling via a proper choice of the three key parameters such as interaction frequency, hold time and overshoot between the qubit frequencies near the avoided crossing. We show that our approach can be extended to other gate operations and discuss pulse-shape optimization suitable for particular gates and architectures. |
Thursday, March 5, 2020 5:18PM - 5:30PM |
U08.00015: Building and benchmarking diabatic entangling gates for frequency-tunable qubits, part II Rami Barends, Chris Quintana, Andre Petukhov, Yu Chen, Vadim Smelyanskiy One of the key needs in quantum computing are gates that are fast and precise, requiring precision control and the minimization of leakage out of the computational states. We present the experimental implementation of a synchronization protocol for constructing gates that have minimal leakage and a duration close to the speed limit. Using this method we achieve diabatic iSWAP-like and CPHASE gates with Pauli error rates down to 4.3(2)*10-3 in as fast as 18 ns with frequency-tunable superconducting qubits. In addition, we show how we use cross-entropy benchmarking (XEB) for quantifying the gates and error budgets. |
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