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arXiv:1809.00668 · Physical Review Applied

Programmable Superconducting Processor with Native Three-Qubit Gates

Tanay Roy, Sumeru Hazra, Suman Kundu, Madhavi Chand, Meghan P. Patankar, R. Vijay

Published Jul 23, 2020• 49 citations
arXivHTMLPDF

Abstract

Higher-dimensional gates involving more than two qubits could play a major role in boosting the performance of quantum processors. While native two-qubit gates are ubiquitous on the superconducting-qubit platform, realizing high-fidelity three-qubit gates is challenging and typically requires multiple two-qubit gates, which leads to error accumulation. The authors utilize a multimodal ``trimon'' circuit to realize $n\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}v\phantom{\rule{0}{0ex}}e$ three-qubit gates that enable efficient implementation of important quantum algorithms, such as Grover's search. These results point to improved processor performance when the trimon is used as a building block for larger systems.

DOI
10.1103/physrevapplied.14.014072

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Higher-dimensional gates involving more than two qubits could play a major role in boosting the performance of quantum processors. While native two-qubit gates are ubiquitous on the superconducting-qubit platform, realizing high-fidelity three-qubit gates is challenging and typically requires multiple two-qubit gates, which leads to error accumulation. The authors utilize a multimodal ``trimon'' circuit to realize n\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}v\phantom{\rule{0}{0ex}}e three-qubit gates that enable efficient implementation of important quantum algorithms, such as Grover's search. These results point to improved processor performance when the trimon is used as a building block for larger systems.
Processing quantum information using quantum three-level systems or qutrits as the fundamental unit is an alternative to contemporary qubit-based architectures with the potential to provide significant computational advantages. We demonstrate a fully programmable two-qutrit quantum processor by utilizing the third energy eigenstates of two transmons. We develop a parametric coupler to achieve excellent connectivity in the nine-dimensional Hilbert space enabling efficient implementations of two-qutrit gates. We characterize our processor by realizing several algorithms like Deutsch-Jozsa, Bernstein-Vazirani, and Grover's search. Our efficient ancilla-free protocols allow us to show that two stages of Grover's amplification can improve the success rates of an unstructured search with quantum advantage. Our results pave the way for building fully programmable ternary quantum processors using transmons as building blocks for a universal quantum computer.
Processing quantum information using quantum three-level systems or qutrits as the fundamental unit is an alternative to contemporary qubit-based architectures with the potential to provide significant computational advantages. We demonstrate a fully programmable two-qutrit quantum processor by utilizing the third energy eigenstates of two transmons. We develop a parametric coupler to achieve excellent connectivity in the nine-dimensional Hilbert space enabling efficient implementations of two-qutrit gates. We characterize our processor by realizing several algorithms like Deutsch-Jozsa, Bernstein-Vazirani, and Grover's search. Our hardware-efficient protocols allow us to show that two stages of Grover's amplification can improve the success rates of an unstructured search with quantum advantage. Our results pave the way for building fully programmable ternary quantum processors using transmons as building blocks for a universal quantum computer.
Quantum algorithms on near-term quantum processors are typically executed using shallow quantum circuits composed of one- and two-qubit gates. However, as circuit depth and gate number increase, gate imperfections and qubit decoherence begin to dominate, limiting algorithmic complexity. An alternative approach is to explore gates involving more than two qubits. In previous work (X. Wu et al., Physical Review X 14, 041030 (2024)), we demonstrated a new superconducting qubit architecture with user-selectable two-qubit interactions via a reconfigurable router, used to connect pairs of qubits. Here, we leverage this novel architecture to realize programmable and efficient multi-qubit operations involving more than two qubits, resulting in faster preparation of multi-qubit entangled states with good fidelities. We also successfully apply model-free reinforcement learning to perform multi-qubit gates, including training a two-qubit controlled-Z gate as well as three-qubit controlled-SWAP and controlled-controlled-phase (Fredkin and Toffoli) gates. Higher nnnth-order gates may also be feasible, using our high-connectivity router design. This could provide a more efficient and higher-fidelity implementation of complex quantum algorithms and a more practical approach to quantum computation.
Three-qubit gates can be constructed using combinations of single-qubit and two-qubit gates, making their independent realization unnecessary. However, direct implementation of three-qubit gates reduces the depth of quantum circuits, streamlines quantum programming, and facilitates efficient circuit optimization, thereby enhancing overall performance in quantum computation. In this work, we propose and experimentally demonstrate a high-fidelity scheme for implementing a three-qubit controlled-controlled-z (ccz) gate in a flip-chip superconducting quantum processor with tunable couplers. This direct ccz gate is implemented by simultaneously leveraging two tunable couplers interspersed between three qubits to enable three-qubit interactions, achieving an average final state fidelity of 97.94% and a process fidelity of 93.54%. This high fidelity cannot be achieved through a simple combination of single- and two-qubit gate sequences from processors with similar performance levels. Our experiments also verify that multilayer direct implementation of the ccz gate exhibits lower leakage compared to decomposed gate approaches. As a showcase, we utilize the ccz gate as an oracle to implement the Grover search algorithm on three qubits, demonstrating high performance with the target probability amplitude significantly enhanced after two iterations. These results highlight the advantage of our approach, and facilitate the implementation of complex quantum circuits.
Quantum processors with native multi-qubit gates may offer very efficient implementations of near-term quantum algorithms on noisy hardware. Here, we introduce the multiunimon, a superconducting multimode circuit that encodes multiple qubits and enables native multi-qubit gates in a device consisting of a single Josephson junction embedded in a coplanar waveguide structure. Closely related to the unimon qubit, it inherits properties such as high anharmonicity, full protection against low-frequency charge noise, and partial protection against flux noise. By designing such a three-qubit device with Josephson-to-inductive energy ratio above unity and using a leakage-aware encoding scheme for the computational states, we simulate all twelve different controlled-controlled-NOT gates with a mean fidelity of 99.5% with simple sine-squared pulses of comparable length to single-qubit gates. The performance is limited by incoherent errors dominated by dielectric loss. With improvements in noise protection, design, and pulse shaping, the simulations suggest that fidelities approaching 99.99% are within reach. Our results demonstrate the potential of the multiunimon as a highly connected multi-qubit unit for larger superconducting quantum processors.
Superconducting circuits offer great flexibility in designing quantum hardware. The authors present a multimodal ``trimon'' circuit implementing three qubits, and exploit the strong all-to-all longitudinal coupling to demonstrate high-fidelity multiqubit gates. Their design is a marked departure from conventional ones that combine individual, decoupled qubits. Nonetheless, this scheme allows fairly straightforward scale-up using established multiqubit architectures. The trimon mimics natural molecules, as used in NMR techniques, and shows significant potential for applications in quantum information processing.