Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session B30: Superconducting Qubit Systems IFocus Live
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Sponsoring Units: DQI Chair: Oliver Dial, Massachusetts Institute of Technology MIT |
Monday, March 15, 2021 11:30AM - 11:42AM Live |
B30.00001: Protected Qubit Subspace within a Fluxonium Molecule Xanthe Croot, Xinyuan You, Anjali Premkumar, Jens Koch, Andrew Houck Qubits with inherent immunity to depolarization and pure dephasing processes are desirable for fault tolerant quantum processors, as both a complement and supplement to quantum error correction schemes. Such “protected” qubits in superconducting circuits have been proposed theoretically [1], and recently realized in the first experimental demonstration of the 0-π qubit [2]. The hunt for new species of protected qubits within the reach of current superconducting technologies is underway. Here we detail work towards a new flavour of protected qubit based on a fluxonium molecule circuit [3] using a non-traditional subspace which exhibits disjoint support and is to first order insensitive to flux noise. We present preliminary work towards realizing this protected qubit and detail its theoretical framework. |
Monday, March 15, 2021 11:42AM - 11:54AM Live |
B30.00002: Fast flux gates in inductively coupled fluxonium qubits Helin Zhang, Srivatsan Chakram, Tanay Roy, Chunyang Ding, Brian Baker, Daniel Weiss, Ziwen Huang, Jens Koch, David I Schuster The heavy-fluxonium qubit is a promising building block for superconducting quantum processors due to its long relaxation and dephasing times at the flux-frustration point. However, the suppressed charge matrix elements and small splitting between computational states have made it challenging to perform fast single and two-qubit gates with conventional methods. In order to achieve high-fidelity initialization and readout, we demonstrate protocols utilizing higher levels beyond the computational subspace. We realize fast qubit control using a universal set of single-cycle flux gates, which are comprised of directly synthesizable pulses, and reach fidelities exceeding 99.8%. Finally, we discuss a set of flux-controlled two-qubit gates for inductively coupled fluxonium qubits. We believe that the fast, flux-based control combined with the coherence properties of the heavy fluxonium make this circuit one of the most promising candidates for next-generation superconducting qubits. |
Monday, March 15, 2021 11:54AM - 12:30PM Live |
B30.00003: Combining CSFQ and transmon qubits to suppress unwanted ZZ interaction Invited Speaker: Jaseung Ku Building a fault-tolerant quantum computer requires not only highly coherent qubits but also tailored interactions between qubits to implement high-fidelity two-qubit entangling gates. One limiting factor to gate errors is crosstalk in the device corresponding to unwanted terms in the system Hamiltonian. Thus, mitigating crosstalk errors, whether classical or quantum mechanical, is critically important for achieving high-fidelity entangling gates in multi-qubit circuits. This is a particular concern in a superconducting qubit architecture with fixed-frequency transmons coupled to nearest neighbors via a static exchange term J. In this architecture, the two-qubit gate is enabled by activating the cross-resonance (CR) effect, whose strength is proportional to J. However, this J also produces an always-on ZZ coupling term. Such a ZZ interaction is an ever-present source of error since it leads to unwanted entanglement between pairs. This unwanted ZZ interactions can be suppressed by combining qubits with opposite anharmonicity. In this talk, we present the first such hybrid two-qubit device with a capacitively shunted flux qubit (CSFQ) and a transmon. Also, we show experimental measurements and theoretical modeling of two-qubit gate error for gates based on the cross resonance interaction, and demonstrate the elimination of the static ZZ interaction. |
Monday, March 15, 2021 12:30PM - 12:42PM Live |
B30.00004: A superconducting metamaterial quantum processor for studying quantum many-body physics: Part 1 Xueyue Zhang, Eun Jong Kim, Oskar Painter Superconducting quantum circuits have emerged as a promising platform to study quantum many-body physics. Compared to traditional AMO systems, superconducting qubits offer new possibilities to study higher-order many-body effects, at a high repetition rate, with full individual local qubit control, quantum non-demolition readout, and real-time feedback control. In this talk, we discuss the realization of a resource-efficient quantum processor based on a superconducting metamaterial waveguide. The metamaterial waveguide mediates tunable long-range exchange interaction between qubits, simultaneously acting as a channel to perform dispersive readout of qubits. In particular, the metamaterial itself is a Purcell filter with high contrast, enabling multiplexed single-shot readout of qubits with high fidelity. Our work marks an important step towards quantum simulation of many-body models beyond nearest-neighbor coupling. |
Monday, March 15, 2021 12:42PM - 12:54PM Live |
B30.00005: A superconducting metamaterial quantum processor for studying quantum many-body physics: Part 2 Eun Jong Kim, Xueyue Zhang, Oskar Painter Superconducting quantum circuits have recently drawn attention as a platform to study quantum many-body physics with high controllability and fidelity. In particular, the negative anharmonicity of superconducting transmon qubits naturally realizes attractive Bose-Hubbard model for photons, which was employed to study strongly-correlated quantum walks and dissipative stabilization of a many-body state in nearest-neighbor coupled one-dimensional arrays of superconducting qubits. In this talk, we report the progress towards the study of quantum many-body physics in a superconducting metamaterial quantum processor. With tunability of the on-site interaction and the range of hopping, we discuss an extended version of the Bose-Hubbard model realized in our system. We investigate the many-body phases emerging from the interplay of elementary parameters and discuss the characterization of higher-order quantum correlations. |
Monday, March 15, 2021 12:54PM - 1:06PM Live |
B30.00006: Suppressed crosstalk between two-junction superconducting qubits with mode-selective coupling. Aaron Finck, Santino Carnevale, Dave Klaus, Christopher Scerbo, John Blair, Thomas G McConkey, Cihan Kurter, April Carniol, George Keefe, Muir Kumph, Oliver E. Dial Fixed-frequency qubits can suffer from always-on interactions that inhibit independent control. While this problem can be alleviated with the use of flux-tunable buses, this introduces other challenges such as sensitivity to flux noise. Here, we describe a superconducting architecture using qubits that comprise of two capacitively-shunted Josephson junctions connected in series. Historically known as tunable coupling qubits, such two-junction qubits support two modes with distinct frequencies and different spatial symmetries. By selectively coupling only one type of modes and using the other as our qubit basis, we greatly suppress crosstalk between the data modes while permitting all-microwave two-qubit gates. |
Monday, March 15, 2021 1:06PM - 1:18PM Live |
B30.00007: ZZ freedom in two qubit gates Xuexin Xu, Mohammad Ansari Achieving high fidelity two qubit gates requires elimination of unwanted interactions among qubits. Weakly anharmonic superconducting qubits in the absence of external driving exhibit an always-on phase error mainly due to a sub-MHz repulsion between computational and non-computational energy levels, the so-called static ZZ interaction. Here we present that in general there are two theoretical ways for eliminating fundamental ZZ error: 1) static ZZ freedom by combining qubits with opposite sign anharmonicity 2) dynamic ZZ freedom in driven qubits with a microwave pulse, which can be universally realized by combining qubits with any anharmonicity signs. Scaling up the number of such qubits can mitigate high fidelity gate operation. |
Monday, March 15, 2021 1:18PM - 1:30PM Live |
B30.00008: Relaxation of a transmon qubit from unconfined states and resurgence of coherence Sourav Majumder, Tanmoy Bera, Ramya Suresh, Vibhor Singh A transmon qubit coupled to a circuit cavity is an extensively studied system, which has also been used to realize a wide variety of hybrid devices, such as with the cavity electromechanical systems. However, often the large power requirements for optomechanical systems render the c-QED setup incompatible. Motivated from this aspect, we investigate the relaxation of a transmon qubit after driving it to unconfined states with large power. We measure the resurgence time corresponding to the relaxation of qubit back to the ground state. After re-initializing transmon to its ground state, we create single photon states in the cavity by rapidly tuning the qubit frequency and swapping the single excitation to the cavity. |
Monday, March 15, 2021 1:30PM - 1:42PM Live |
B30.00009: Study of a two-state fluctuation in a ultra-strongly coupled qubit-resonator system Akiyoshi Tomonaga, Hiroto Mukai, Jaw-Shen Tsai We report experimental results of two-state-fluctuation (TSF) in a ultra-strongly coupled superconducting flux qubit-resonator system. Rabi model Hamiltonian well known as the representation of the qubit-resonator coupling system, but the spectrum will be reported in this talk has doubly split structure that cannot be described by the Rabi model. This is because the system has the two states of different parameters and the state fluctuate between them. Therefore, doubly split spectrum was observed in the vector network analyzer measurement. It is observed that the value of the splitting also fluctuates randomly with time by repeating the spectrum measurement and the fluctuation shows the Gaussian distribution. |
Monday, March 15, 2021 1:42PM - 1:54PM Live |
B30.00010: Full control of superconducting qubits with combined XYZ lines Riccardo Manenti, Eyob Sete, Angela Chen, Shobhan Kulshreshtha, Jen-Hao Yeh, Feyza Oruc, Keith Jackson, Mark Field, Andrew Bestwick, Stefano Poletto As the field of quantum computing proceeds to larger-scale devices, multiplexing will be crucial to scale quantum processors. While multiplexed readout is common practice for superconducting devices, relatively little work has been reported on the combination of flux and control lines. In this work, we present a method to integrate microwave lines and flux lines into a single “XYZ line” without significantly impacting the qubit relaxation time. This combined control line allows us to perform single-qubit gates in 20 ns as well as to deliver flux signals to the qubits. We benchmark the fidelity of single-qubit gates with randomized benchmarking achieving a fidelity above 99.5%. |
Monday, March 15, 2021 1:54PM - 2:06PM Live |
B30.00011: Fast feedback for active noise cancellation in superconducting qubits Antti Vepsalainen, Roni Winik, Amir Karamlou, Jochen Braumueller, Youngkyu Sung, Bharath Kannan, Morten Kjaergaard, David K Kim, Jonilyn Yoder, Bethany Niedzielski, Terry Philip Orlando, Simon Gustavsson, William Oliver Superconducting qubits are a promizing platform for realizing quantum computer. However, due to recent advances in qubit control, both single- and two-qubit gate fidelities are starting to be limited by the coherence times of the qubits. Here we show that by employing fast-feedback to cancel low-frequency noise disturbing the circuit, the fluctuations in the qubit frequency are stabilized, resulting in improved coherence time and gate fidelities. This enables high-fidelity operation of transmon qubits even when they are biased away from their flux sweetspot, helping to solve frequency crowding issues as well as reducing drifts in control parameters. |
Monday, March 15, 2021 2:06PM - 2:18PM Live |
B30.00012: Integrating low-loss magnetic couplers with superconducting 3d-cavities Ziyi Zhao, Eric Rosenthal, Leila Vale, Gene C Hilton, Konrad Lehnert Superconducting circuits are a leading platform for realizing intermediate scale quantum information processing. In particular, the modular approach distributes entanglement over a network of superconducting cavities, limiting the unwanted coupling between high-fidelity modules. As the scale of such networks increases, it is becoming essential to route and reroute signals with high fidelity. We present a design to magnetically couple cavities to the network via balanced superconducting switches, whose coupling strength can be varied to change the connectivity of a network. We exploit the differential nature of magnetic coupling to adapt to the balanced switch. We reduce seam loss by constructing the cavities and the wiring ground plane from the same piece of metal. Finally, we reduce packaging loss by eliminating circuit boards, wirebonds, and microwave connectors in the signal path. Simulation of this design predicts that it can provide sufficient coupling rate while introducing minimal loss, making it a suitable building block for the larger quantum network. |
Monday, March 15, 2021 2:18PM - 2:30PM Live |
B30.00013: Observation of qubit-photon bound states in a rectangular waveguide QED system Pradeepkumar Nandakumar, Jose Andres Rosario Hamann, Rohit Navarathna, Maximilian Zanner, Mikhail Pletyukhov, Arkady Fedorov Within the stopband, where the density of states is zero, no travelling modes are allowed through the waveguide. However, evanescent modes, exponentially localized around the atoms’ position can give rise to atom-photon bound states as theoretically predicted in Ref. [1] and experimentally observed in a photonic crystal configuration on a chip [2]. Here, we report observation of an atom-photon bound state with a transmon qubit inserted in a three-dimensional rectangular waveguide. Rectangular waveguides naturally exhibit low-frequency cut-off and the atom-photon bound states shows stronger localization compared to a photonic crystal realization [3]. We provide evidence of exponential localization of the atom-photon bound states through spectroscopic measurements, study their radiative emission and coherent interaction. |
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