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arXiv:1902.06569 · npj Quantum Information

Scalable quantum computer with superconducting circuits in the ultrastrong coupling regime

Roberto Stassi, Mauro Cirio, Franco Nori

Published Aug 6, 2020• 80 citations
arXivHTMLPDF

Abstract

Abstract So far, superconducting quantum computers have certain constraints on qubit connectivity, such as nearest-neighbor couplings. To overcome this limitation, we propose a scalable architecture to simultaneously connect several pairs of distant qubits via a dispersively coupled quantum bus. The building block of the bus is composed of orthogonal coplanar waveguide resonators connected through ancillary flux qubits working in the ultrastrong coupling regime. This regime activates virtual processes that boost the effective qubit–qubit interaction, which results in quantum gates on the nanosecond timescale. The interaction is switchable and preserves the coherence of the qubits.

DOI
10.1038/s41534-020-00294-x

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Abstract So far, superconducting quantum computers have certain constraints on qubit connectivity, such as nearest-neighbor couplings. To overcome this limitation, we propose a scalable architecture to simultaneously connect several pairs of distant qubits via a dispersively coupled quantum bus. The building block of the bus is composed of orthogonal coplanar waveguide resonators connected through ancillary flux qubits working in the ultrastrong coupling regime. This regime activates virtual processes that boost the effective qubit–qubit interaction, which results in quantum gates on the nanosecond timescale. The interaction is switchable and preserves the coherence of the qubits.
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.
Abstract Resonators in circuit quantum electrodynamics systems naturally carry multiple modes, which may have non-negligible influence on qubit parameters and device performance. While new theories and techniques are under investigation to deal with the multi-mode effects in circuit quantum electrodynamics systems, novel engineering designs are also proposed featuring multi-mode resonators to achieve enhanced functionalities of superconducting quantum processors. In this work, multi-mode bus coupling architecture is proposed, in which superconducting qubits are coupled to multiple bus resonators to gain larger coupling strength. Application of multi-mode bus coupler can help improve iSWAP gate fidelity and gate speed beyond the limit of single-mode scenario. The proposed multi-mode bus coupling architecture is compatible with a scalable variation of the traditional bus coupling architecture. It opens up new possibilities for realization of scalable superconducting quantum computation with circuit quantum electrodynamics systems.
Tunable couplings are a central requirement for scalable quantum processing with superconducting quantum circuits. Practical quantum processors must dynamically reconfigure their interaction landscape to accommodate different quantum operations, including idle, single-qubit gates, two-qubit gates, readout, and reset, while maintaining high coherence and suppressing spurious couplings. While widely used tunable interaction approaches rely on single-mode mediation, they are often constrained by nearest-neighbor connectivity, limited on-off ratios, and stringent requirements on qubit frequency allocation. This thesis develops and studies novel strategies for tailoring tunable interactions by moving beyond single-mode mediation, spanning multimode engineered interactions and modeless interaction schemes ("many to no modes"), while accounting for realistic constraints in superconducting hardware. Using a microwave metamaterial waveguide realized by coupled resonator arrays, I demonstrate tunable dissipative interactions that allow on-demand, fast, and high switching-ratio photon emission from a transmon used as a multi-level quantum emitter. This capability is leveraged to achieve deterministic generation of multidimensional photonic cluster states, as well as unconditional reset and leakage reduction of frequency-tunable superconducting qubits. Complementary to these multimode dissipative tunable interactions, this thesis also introduces architectures that realize tunable interactions mediated by many coherent modes or by no mediating modes. I discuss a long-range interaction scheme between superconducting dual-rail qubits mediated by spatially extended eigenmodes of a coupled-resonator array bus. Finally, I propose and analyze a novel modeless coupling architecture based on a SQUID coupler which provides intrinsic cross-Kerr interactions, enabling fast, hybridization-free CZ gates for far-detuned pairs, and discuss its implications for miniaturization of superconducting quantum processors.
Resonators in circuit quantum electrodynamics systems naturally carry multiple modes, which may have non-negligible influence on qubit parameters and device performance. While new theories and techniques are under investigation to deal with the multi-mode effects in circuit quantum electrodynamics systems, researchers have proposed novel engineering designs featuring multi-mode resonators to achieve enhanced functionalities of superconducting quantum processors. Here, we propose multi-mode bus coupling architecture, in which superconducting qubits are coupled to multiple bus resonators to gain larger coupling strength. Applications of multi-mode bus couplers can be helpful for improving iSWAP gate fidelity and gate speed beyond the limit of single-mode scenario. The proposed multi-mode bus coupling architecture is compatible with a scalable variation of the traditional bus coupling architecture. It opens up new possibilities for realization of scalable superconducting quantum computation with circuit quantum electrodynamics systems.
The existing scalable superconducting quantum processors have only nearest-neighbor coupling. This leads to a reduced circuit depth, requiring a large series of gates to perform an arbitrary unitary operation in such systems. Recently, multi-modal devices have been demonstrated as a promising candidate for small quantum processor units. Always on longitudinal coupling in such circuits leads to implementation of native high fidelity multi-qubit gates. We propose an architecture using such devices as building blocks for a highly connected larger quantum circuit. To demonstrate a quantum operation between such blocks, a standard transmon is coupled to the multi-modal circuit using a 3D bus cavity giving rise to small exchange interaction between the transmon and one of the modes. We study the cross-resonance interaction in such systems and characterize the entangling operation and the unitary imperfections and crosstalk as a function of device parameters. Finally, we tune up the cross-resonance drive to implement multi-qubit gates in this architecture.
We propose different designs of switchable coupling between a superconducting flux qubit and a microwave transmission line. They are based on two or more loops of Josephson junctions which are directly connected to a closed (cavity) or open transmission line. In both cases the circuit induces a coupling that can be modulated in strength, reaching the so-called ultrastrong coupling regime in which the coupling is comparable to the qubit and photon frequencies. Furthermore, we suggest a wide set of applications for the introduced architectures.