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npj Quantum Information

An efficient and compact switch for quantum circuits

Yulin Wu, Li‐Ping Yang, Ming Gong, Yarui Zheng, Huiqiu Deng, Zhiguang Yan, Yan-Jun Zhao, Keqiang Huang, Juno Clark, William J. Munro, Kae Nemoto, Dongning Zheng, C. P. Sun, Yu-xi Liu, Xiaobo Zhu, Li Lü

Published Oct 5, 2018• 48 citations
Publisher

Abstract

Abstract The engineering of quantum devices has reached the stage where we now have small-scale quantum processors containing multiple interacting qubits within them. Simple quantum circuits have been demonstrated and scaling up to larger numbers is underway. However, as the number of qubits in these processors increases, it becomes challenging to implement switchable or tunable coherent coupling among them. The typical approach has been to detune each qubit from others or the quantum bus it connected to, but as the number of qubits increases this becomes problematic to achieve in practice due to frequency crowding issues. Here, we demonstrate that by applying a fast longitudinal control field to the target qubit, we can turn off its couplings to other qubits or buses. This has important implications in superconducting circuits as it means we can keep the qubits at their optimal points, where the coherence properties are greatest, during coupling/decoupling process. Our approach suggests another way to control coupling among qubits and data buses that can be naturally scaled up to large quantum processors.

DOI
10.1038/s41534-018-0099-6

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Abstract The engineering of quantum devices has reached the stage where we now have small-scale quantum processors containing multiple interacting qubits within them. Simple quantum circuits have been demonstrated and scaling up to larger numbers is underway. However, as the number of qubits in these processors increases, it becomes challenging to implement switchable or tunable coherent coupling among them. The typical approach has been to detune each qubit from others or the quantum bus it connected to, but as the number of qubits increases this becomes problematic to achieve in practice due to frequency crowding issues. Here, we demonstrate that by applying a fast longitudinal control field to the target qubit, we can turn off its couplings to other qubits or buses. This has important implications in superconducting circuits as it means we can keep the qubits at their optimal points, where the coherence properties are greatest, during coupling/decoupling process. Our approach suggests another way to control coupling among qubits and data buses that can be naturally scaled up to large quantum processors.
The engineering of quantum devices has reached the stage where we now have small scale quantum processors containing multiple interacting qubits within them. Simple quantum circuits have been demonstrated and scaling up to larger numbers is underway. However as the number of qubits in these processors increases, it becomes challenging to implement switchable or tunable coherent coupling among them. The typical approach has been to detune each qubit from others or the quantum bus it connected to, but as the number of qubits increases this becomes problematic to achieve in practice due to frequency crowding issues. Here, we demonstrate that by applying a fast longitudinal control field to the target qubit, we can turn off its couplings to other qubits or buses (in principle on/off ratio higher than 100 dB). This has important implementations in superconducting circuits as it means we can keep the qubits at their optimal points, where the coherence properties are greatest, during coupling/decoupling processing. Our approach suggests a new way to control coupling among qubits and data buses that can be naturally scaled up to large quantum processors without the need for auxiliary circuits and yet be free of the frequency crowding problems.
As systems for quantum computing keep growing in size and number of qubits, challenges in scaling the control capabilities are becoming increasingly relevant. Efficient schemes to simultaneously mediate coherent interactions between multiple quantum systems and to reduce decoherence errors can minimize the control overhead in next-generation quantum processors. Here, we present a superconducting qubit architecture based on tunable parametric interactions to perform two-qubit gates, reset, leakage recovery, and to read out the qubits. In this architecture, parametrically driven multielement couplers selectively couple qubits to resonators and neighboring qubits, according to the frequency of the drive. We consider a system with two qubits and one readout resonator interacting via a single coupling circuit and experimentally demonstrate a controlled- Z gate with a fidelity of 98.30 ± 0.23 % , a reset operation that unconditionally prepares the qubit ground state with a fidelity of 99.80 ± 0.02 % and a leakage recovery operation with a 98.5 ± 0.3 % success probability. Furthermore, we implement a parametric readout with a single-shot assignment fidelity of 88.0 ± 0.4 % . These operations are all realized using a single tunable coupler and a statically decoupled resonator, demonstrating the experimental feasibility of the proposed architecture and its potential for reducing the system complexity in scalable quantum processors.
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.
In addition to striving to assemble more and more qubits in a single monolithic quantum device, taking a modular design strategy may mitigate numerous engineering challenges for achieving large-scalable quantum processors with superconducting qubits. Nevertheless, a major challenge in the modular quantum device is how to realize high-fidelity entanglement operations on qubits housed in different modules while preserving the desired isolation between modules. In this work, we propose a conceptual design of a modular quantum device, where nearby modules are spatially separated by centimeters. In principle, each module can contain tens of superconducting qubits and can be separately fabricated, characterized, packaged, and replaced. By introducing a bridge module between nearby qubit modules and taking the coupling scheme utilizing a tunable bus, tunable coupling of qubits that are housed in nearby qubit modules could be realized. Given physically reasonable assumptions, we expect that sub-100-ns two-qubit gates for qubits housed in nearby modules, which are spatially separated by more than two centimeters could be obtained. In this way, the inter-module gate operations are promising to be implemented with gate performance comparable with that of intra-module gate operations. Moreover, with the help of through-silicon vias technologies, this long-range coupling scheme may also allow one to implement inter-module couplers in a multi-chip stacked processor. Thus, the tunable longer-range coupling scheme and the proposed modular architecture may provide a promising foundation for solving challenges toward large-scale quantum information processing with superconducting qubits.
We study a coupling (decoupling) method between a superconducting qubit and a data bus that uses a controllable time-dependent electromagnetic field (TDEF). As in recent experiments, the data bus can be either an LCLCLC circuit or a cavity field. When the qubit and the data bus are initially fabricated, their detuning should be made far larger than their coupling constant, so these can be treated as two independent subsystems. However, if a TDEF is applied to the qubit, then a ``dressed qubit'' (i.e., qubit plus the electromagnetic field) can be formed. By choosing appropriate parameters for the TDEF, the dressed qubit can be coupled to the data bus and, thus, the qubit and the data bus can exchange information with the assistance of the TDEF. This mechanism allows the scalability of the circuit to many qubits. With the help of the TDEF, any two qubits can be selectively coupled to (and decoupled from) a common data bus. Therefore, quantum information can be transferred from one qubit to another.
Interqubit coupling and qubit connectivity in a processor are crucial for achieving high-fidelity multiqubit gates and efficient implementation of quantum algorithms. Typical superconducting processors employ relatively weak transverse interqubit coupling which is activated via frequency tuning or microwave drives. Here, we propose a class of multimode superconducting circuits which realize multiple transmon qubits with all-to-all longitudinal coupling. These ``artificial molecules'' directly implement a multidimensional Hilbert space that can be easily manipulated due to the always-on longitudinal coupling. We describe the basic technique to analyze such circuits, compute the relevant properties, and discuss how to optimize them to create efficient small-scale quantum processors with universal programmability.