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arXiv:2004.08639 · Physical Review Applied

Switchable Next-Nearest-Neighbor Coupling for Controlled Two-Qubit Operations

Peng Zhao, Peng Xu, Dong Lan, Xinsheng Tan, Haifeng Yu, Yang Yu

Published Dec 4, 2020• 25 citations
arXivHTMLPDF

Abstract

Minimizing gate depth is important for extending the computational power of a noisy quantum processor, and native multiqubit gates could dramatically reduce the gate depth of generic circuits. To that end, although various approaches have been proposed, practical realization may be hindered by stringent control requirements, or by poor compatibility with existing schemes for one- or two-qubit operations. Here researchers employ a scalable architecture comprising two types of qubits to realize switchable coupling. This allows for a simple control strategy to realize native three-qubit gates, which may become a key element in $e.g.$ quantum chemistry simulations.

DOI
10.1103/physrevapplied.14.064016

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Minimizing gate depth is important for extending the computational power of a noisy quantum processor, and native multiqubit gates could dramatically reduce the gate depth of generic circuits. To that end, although various approaches have been proposed, practical realization may be hindered by stringent control requirements, or by poor compatibility with existing schemes for one- or two-qubit operations. Here researchers employ a scalable architecture comprising two types of qubits to realize switchable coupling. This allows for a simple control strategy to realize native three-qubit gates, which may become a key element in e.g.e.g.e.g. quantum chemistry simulations.
We introduce a scheme for realizing universal quantum computing in a linear nearest neighbor architecture with fixed couplings. We first show how to realize a controlled-NOT gate operation between two adjacent qubits without having to isolate the two qubits from qubits adjacent to them. The gate operation is implemented by applying two consecutive pulses of equal duration, but varying amplitudes, on the target qubit. Since only a single control parameter is required in implementing our scheme, it is very efficient. We next show how our scheme can be used to realize single qubit rotations and two-qubit controlled-unitary operations. As most proposals for solid state implementations of a quantum computer use a one-dimensional line of qubits, the schemes presented here will be extremely useful.
Native multiqubit gates could be essential for bridging the gap from current noisy devices to future utility-scale quantum computers, as they can substantially reduce circuit depth for near-term applications on noisy devices and may also lower the physical overhead of fault-tolerant quantum computation. Here we introduce a scalable protocol for implementing native multi-controlled gates on fluxonium qubits, supporting an arbitrary number of control qubits (N>1N > 1N>1) while remaining compatible with existing single- and two-qubit gate realizations. Our approach leverages engineered interactions in noncomputational state manifolds to enable qubit-state selective transitions, which is activated for the direct implementation of (C⊗N)Z(C^{\otimes N})Z(C⊗N)Z gates. We show that in square lattices with fluxonium qubits, CCZCCZCCZ, CCCZCCCZCCCZ, and CCCCZCCCCZCCCCZ gates with errors around 0.01 (0.001) are achievable, with gate lengths of 50 (100) ns50\,(100)\,\text{ns}50(100)ns, 100 (250) ns100\,(250)\,\text{ns}100(250)ns, and 150 (300) ns150\,(300)\,\text{ns}150(300)ns, respectively. Looking forward, integrating these native multi-controlled gates with primitive single- and two-qubit gate sets within a single quantum processor could significantly enhance flexibility in circuit synthesis and offer a promising alternative pathway toward utility-scale quantum computing.
We design a nearest-neighbor architectural layout that uses fixed positive and negative couplings between qubits, to overcome the effects of relative phases due to qubit precessions, both during idle times and gate operations. The scheme uses decoherence-free subspaces, and we show how to realize gate operations on these encoded qubits. The main advantage of our scheme is that most gate operations are realized by only varying a single control parameter, which greatly reduces the circuit complexity. Moreover, the scheme is robust against phase errors occurring as a result of finite rise and fall times due to non-ideal pulses.
Design and synthesis of quantum circuits have gained huge momentum during the last few years. With various physical constraint sone of the major issues is in making quantum circuits nearest neighbor complaint. The existing one and two dimensional architectures for the placement of qubits does not scale well for larger circuits. The situation thus demands for a faster approach to solve the problem. To make a quantum circuit nearest neighbor compliant, SWAP gates are inserted in the quantum gate circuit to bring the interacting qubits of a gate adjacent. We have proposed a faster method with a simple yet effective algorithm to minimize the number of SWAP gates for adjacent interactions. The time required for processing does not exceed one second even for larger benchmark circuits. The proposed method is found to reduce the number of SWAP gates on an average by 9.5% compared to the best results in previous works on 2D architectures, and by 45.7% compared to the best results on 1D architecture.
Fluxonium qubits demonstrate exceptional potential for quantum processing; yet, realizing scalable architectures using them remains challenging. We propose a fluxonium-based square-grid design with fast ∼63\sim63∼63~ns controlled-Z (CZ) gates, achieving coherent errors below 10−410^{-4}10−4, activated via microwave-driven transmon couplers. A central difficulty in such large-scale systems with all-microwave gates and, therefore, strong static couplings, is suppressing parasitic interactions that extend beyond nearest neighbors to include next-nearest elements. We address this issue by introducing several design strategies: the frequency allocation of both qubits and couplers, the localization of coupler wavefunctions, and a differential oscillator that suppresses residual long-range interactions. In addition, the architecture natively supports fast ∼70\sim70∼70~ns CZZ gates -- three-qubit operations composed of two CZ gates sharing a common qubit -- which reduce the incoherent error by ∼35%\sim 35\%∼35% compared to performing the corresponding CZs sequentially. Together, these advances establish an interaction-resilient platform for large-scale fluxonium processors and can be adapted to a variety of fluxonium layouts.
Implementing high-fidelity multiqubit gates is critical for reducing circuit depth in near-term quantum processors and fault-tolerant architectures. However, realizing multiqubit interactions and entanglement remains a critical challenge that limits the implementation of multiqubit gates. Here we propose an all-microwave scheme to realize single-step multiqubit gates incorporating n control and m target qubits, tailored for frequency-tunable superconducting transmon networks. By leveraging cross-resonance (CR) drives, this approach induces effective two-body ZX interactions that are significantly stronger than those achieved in conventional resonant regimes, circumventing the need for tunable couplers. The system's effective Hamiltonian is analytically derived using both quad frame rotation and rigorous block diagonalization. These combined theoretical methods facilitate the identification of optimal parameter regimes that simultaneously enhance desired target couplings and suppress parasitic higher-order terms, such as ZXX interactions. Through numerical simulations and gradient-based pulse-shape optimization, we demonstrate three-qubit gates achieving fidelities exceeding 99.9% within a 60 ns gate duration. Furthermore, we evaluate the fundamental scalability of this framework by extending the optimization to a five-qubit CXXXX architecture, confirming the physical viability of the multi-target driving scheme. Because these multiqubit operations naturally emulate stabilizer measurements, this architecture provides a hardware-efficient strategy for reducing the circuit depth of stabilizer-type operations by replacing sequential two-qubit gate decompositions with single-step multiqubit gates.