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arXiv:1102.0307 · Physical Review B

Tunable coupling between three qubits as a building block for a superconducting quantum computer

Peter Groszkowski, Austin G. Fowler, Felix Motzoi, Frank K. Wilhelm

Published Oct 17, 2011• 25 citations
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Abstract

Large-scale quantum computers will consist of many interacting qubits. In this paper we expand the two flux qubit coupling scheme first devised by P. Plourde et al. [Phys. Rev. B 70, 140501 (2004)] and realized by T. Hime et al. [Science 314, 1427 (2006)] to a three-qubit, two-coupler scenario. We study L-shaped and line-shaped coupler geometries, and show how the interaction strength between qubits changes in terms of the couplers' dimensions. We explore two cases: the ``on-state'' where the interaction energy between two nearest-neighbor qubits is high, and the ``off-state'' where it is turned off. In both situations we study the undesirable crosstalk with the third qubit. Finally, we use the grape algorithm to find efficient pulse sequences for two-qubit gates subject to our calculated physical constraints on the coupling strength.

DOI
10.1103/physrevb.84.144516

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Large-scale quantum computers will consist of many interacting qubits. In this paper we expand the two flux qubit coupling scheme first devised by P. Plourde et al. [Phys. Rev. B 70, 140501 (2004)] and realized by T. Hime et al. [Science 314, 1427 (2006)] to a three-qubit, two-coupler scenario. We study L-shaped and line-shaped coupler geometries, and show how the interaction strength between qubits changes in terms of the couplers' dimensions. We explore two cases: the on-state'' where the interaction energy between two nearest-neighbor qubits is high, and the off-state'' where it is turned off. In both situations we study the undesirable crosstalk with the third qubit. Finally, we use the grape algorithm to find efficient pulse sequences for two-qubit gates subject to our calculated physical constraints on the coupling strength.
Nonpairwise multiqubit interactions present a useful resource for quantum information processors. Their implementation would facilitate more efficient quantum simulations of molecules and combinatorial optimization problems, and they could simplify error suppression and error correction schemes. Here, we present a superconducting circuit architecture in which a coupling module mediates two-local and three-local interactions between three flux qubits by design. The system Hamiltonian is estimated via multiqubit pulse sequences that implement Ramsey-type interferometry between all neighboring excitation manifolds in the system. The three-local interaction is coherently tunable over several MHz via the coupler flux biases and can be turned off, which is important for applications in quantum annealing, analog quantum simulation, and gate-model quantum computation.
We have realized controllable coupling between two three-junction flux qubits by inserting an additional coupler loop between them, containing three Josephson junctions. Two of these are shared with the qubit loops, providing strong qubit-coupler interaction. The third junction gives the coupler a nontrivial current-flux relation; its derivative (i.e., the susceptibility) determines the coupling strength J, which thus is tunable in situ via the coupler's flux bias. In the qubit regime, J was varied from approximately 45 (antiferromagnetic) to approximately -55 mK (ferromagnetic); in particular, J vanishes for an intermediate coupler bias. Measurements on a second sample illuminate the relation between two-qubit tunable coupling and three-qubit behavior.
A major challenge in the field of quantum computing is the construction of scalable qubit coupling architectures. Here, we demonstrate a novel tunable coupling circuit that allows superconducting qubits to be coupled over long distances. We show that the interqubit coupling strength can be arbitrarily tuned over nanosecond time scales within a sequence that mimics actual use in an algorithm. The coupler has a measured on/off ratio of 1000. The design is self-contained and physically separate from the qubits, allowing the coupler to be used as a module to connect a variety of elements such as qubits, resonators, amplifiers, and readout circuitry over distances much larger than nearest-neighbor. Such design flexibility is likely to be useful for a scalable quantum computer.
Superconducting qubits now offer coherence times exceeding 100 $\ensuremath{\mu}$s and single-qubit gate fidelities above 99.9%, but realizing a large-scale quantum computer is still impeded by the quality of two-qubit gates, with typical fidelities of 95--99%. Here a simple, broadly applicable scheme to improve two-qubit gates using a tunable coupler is proposed. It simplifies the coupling scheme, and reduces unwanted interactions typically present in schemes in which the interactions are always on. Numerical simulations show that this approach should allow two-qubit gate fidelities greater than 99.9%, when implemented using state-of-the-art superconducting qubits.
Tunable couplers in superconducting quantum computers have enabled fast and accurate two-qubit gates, with reported high fidelities over 99% in various architectures and gate-implementation schemes. However, there are few tunable couplers whose performance in multiqubit systems is clarified, except for the most widely used one: single-transmon coupler (STC). Achieving similar accuracy to isolated two-qubit systems remains challenging due to various undesirable couplings but is necessary for scalability. In this work, we numerically analyze a system of three fixed-frequency qubits coupled via two double-transmon couplers (DTCs) where nearest-neighbor qubits are highly detuned and also next-nearest-neighbor ones are nearly resonant. The DTC is a recently proposed tunable coupler, which consists of two fixed-frequency transmons coupled through a common loop with an additional Josephson junction. We find that the DTC cannot only reduce undesired residual couplings sufficiently, as well as in isolated two-qubit systems, but also enables implementations of 30-ns CZ gates and individual and simultaneous 10-ns π / 2 pulses with fidelities over 99.99%. For comparison, we also investigate the system where the DTCs are replaced by the STCs. The results show that the DTC outperforms the STC in terms of both residual coupling suppression and gate accuracy in the above systems. From these results, we expect that the DTC architecture is promising for realizing high-performance, scalable superconducting quantum computers. Published by the American Physical Society 2024
Controllable interaction between superconducting qubits is desirable for large-scale quantum computation and simulation. Here, based on a theoretical proposal by Yan et al. [Phys. Rev. Appl. 10, 054061 (2018)] we experimentally demonstrate a simply designed and flux-controlled tunable coupler with a continuous tunability by adjusting the coupler frequency, which can completely turn off adjacent superconducting qubit coupling. Utilizing the tunable interaction between two qubits via the coupler, we implement a different type of controlled-phase (cz) gate with ``dynamically decoupled regime,'' which allows the qubit-qubit coupling to be only ``on'' at the usual operating point while dynamically ``off'' during the tuning process of one qubit frequency into and out of the operating point. This scheme not only efficiently suppresses the leakage out of the computational subspace, but also allows for the acquired two-qubit phase being geometric at the operating point. We achieve an average cz gate fidelity of $98.3\ifmmode\pm\else\textpm\fi{}0.6\mathrm{%}$, which is dominantly limited by qubit decoherence. The demonstrated tunable coupler provides a desirable tool to suppress adjacent qubit coupling and is suitable for large-scale quantum computation and simulation.