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arXiv:2404.18689 · Physical Review Applied

Diverse set of two-qubit gates for spin qubits in semiconductor quantum dots

Ming Ni, Rong-Long Ma, Zhenzhen Kong, Ning Chu, Sheng-Kai Zhu, Chu Wang, Ao-Ran Li, Wei-Zhu Liao, Gang Cao, Guilei Wang, Guang‐Can Guo, Xuedong Hu, Hai-Ou Li, Guo‐Ping Guo

Published Feb 26, 2025• 3 citations
arXivHTMLPDF

Abstract

On the path toward large-scale quantum information processing, the ability to implement diverse types of two-qubit gates is preferable for device structure designs and quantum circuit compilations. Here, taking advantage of the inherent Heisenberg interaction between spin qubits, we propose and verify a fast composite two-qubit gate scheme for spin qubits based on semiconductor quantum dots. Theoretical results indicate that the gate scheme facilitates the simultaneous implementation of various types of two-qubit gates, including five types of essential two-qubit gates, iswap-family gates, and fermionic simulation gates. Furthermore, we observe an excellent match between the measured and simulated results, representing the preliminary experimental demonstration of the composite gate scheme. We believe that, with this versatile composite two-qubit gate scheme, the available broad-spectrum two-qubit operations will allow us to efficiently utilize the hardware resources and accelerate the advancement of noisy intermediate-scale quantum computing.

DOI
10.1103/physrevapplied.23.024065

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On the path toward large-scale quantum information processing, the ability to implement diverse types of two-qubit gates is preferable for device structure designs and quantum circuit compilations. Here, taking advantage of the inherent Heisenberg interaction between spin qubits, we propose and verify a fast composite two-qubit gate scheme for spin qubits based on semiconductor quantum dots. Theoretical results indicate that the gate scheme facilitates the simultaneous implementation of various types of two-qubit gates, including five types of essential two-qubit gates, iswap-family gates, and fermionic simulation gates. Furthermore, we observe an excellent match between the measured and simulated results, representing the preliminary experimental demonstration of the composite gate scheme. We believe that, with this versatile composite two-qubit gate scheme, the available broad-spectrum two-qubit operations will allow us to efficiently utilize the hardware resources and accelerate the advancement of noisy intermediate-scale quantum computing.
In gate-defined semiconductor spin qubits, the highly tunable Heisenberg exchange interaction is leveraged to implement fermionic two-qubit gates such as CZ and SWAP. However, the broader family of fermionic simulation (fSim) gates remains unexplored, and has the potential to enhance the performance of near-term quantum simulation algorithms. Here, we demonstrate a method to implement the fSim gate set in spin qubits using a single pulse combining baseband and resonant exchange drives. This approach minimizes gate duration and drive amplitude, mitigating decoherence and crosstalk. We validate its effectiveness by realizing a resonant iSWAP gate between two hole spins in germanium, achieving a fidelity of 93.8(5)% extracted with interleaved randomized benchmarking. Quantum process tomography confirms accurate gate calibration and identifies qubit decoherence as the dominant error source. Our results establish a practical route toward a versatile and efficient two-qubit gate set for spin-based quantum processors.
Implementations for quantum computing require fast single- and multiqubit quantum gate operations. In the case of optically controlled quantum dot qubits, theoretical designs for long-range two- or multiqubit operations satisfying all the requirements in quantum computing are not yet available. We have developed a design for a fast, long-range two-qubit gate mediated by a photonic microcavity mode using excited states of the quantum-dot-cavity system that addresses these needs. This design does not require identical qubits, it is compatible with available optically induced single-qubit operations, and it advances opportunities for scalable architectures. We show that the gate fidelity can exceed 90% in experimentally accessible systems.
The benefit of exchange-only qubits compared to other spin-qubit types is the universal control using only voltage-controlled exchange interactions between neighboring spins. As a compromise, qubit operations have to be constructed from nonorthogonal rotation axes of the Bloch sphere and result in rather long pulsing sequences. This paper aims to develop a faster implementation of single-qubit gates using simultaneous exchange pulses and manifests their potential for the construction of two-qubit gates. We introduce pulse sequences in which single-qubit gates could be executed faster and show that subsequences on three spins in two-qubit gates could be implemented in fewer steps. Our findings can particularly speed up gate sequences for realistic idle times between sequential pulses and we show that this advantage increases with more interconnectivity of the quantum dots. We further demonstrate how a phase operation can introduce a relative phase between the computational and some of the leakage states, which can be advantageous for the construction of two-qubit gates. In addition to our theoretical analysis, we experimentally demonstrate and characterize a simultaneous exchange implementation of X rotations in a Si Ge quantum dot device and compare to the state of the art with sequential exchange pulses.
A crucial requirement for scalable quantum-information processing is the realization of multiple-qubit quantum gates. Universal multiple-qubit gates can be implemented by a set of universal single qubit gates and any one kind of two-qubit gate, such as a controlled-NOT (CNOT) gate. Semiconductor quantum dot qubits are a leading approach for the physical implementation of quantum computation, due to their potential for large-scale integration. Two-qubit gate operations have been so far only demonstrated in individual electron spin-based quantum dot systems. Due to the relatively short de-coherence time, charge qubits in quantum dots are generally considered to be inferior for going beyond single qubit level. Here, we demonstrate the benchmarking CNOT gate in two capacitively coupled charge qubits, each consisting of an electron confined in a GaAs/AlGaAs double quantum dot. Owing to the strong inter-qubit coupling strength, gate operations with a clock speed up to 5GHz has been realized. A processing tomography shows encouragingly that the universal two-qubit gate operations have comparable fidelities to that of spin-based two-qubit gates. Our results suggest that semiconductor charge qubits have a considerable potential for scalable quantum computing and may stimulate the use of long-range Coulomb interaction for coherent quantum control in other devices.
Recent achievements in the field of gate-defined semiconductor quantum dots reinforce the concept of a spin-based quantum computer consisting of nodes of locally connected qubits which communicate with each other via superconducting circuit resonator photons. In this paper, we theoretically demonstrate a versatile set of quantum gates between adjacent spin qubits defined in semiconductor quantum dots situated within the same node of such a spin-based quantum computer. The electric dipole acquired by the spin of an electron that moves across a double quantum dot potential in a magnetic field gradient has enabled strong coupling to resonator photons and low-power spin control. Here we show that this flopping-mode spin qubit also provides the tunability to program multiple two-qubit gates. Since the capacitive coupling between these qubits brings about additional dephasing, we calculate the estimated infidelity of different two-qubit gates in the most immediate possible experimental realizations.
We propose a universal approach based on Hamiltonian inverse engineering to realize a set of parameterized two-qubit gates. This method possesses unique advantages to simultaneous control of transitions among four energy levels, providing a simpler and effective way to construct composite two-qubit gates with fewer operations than traditional methods. Applied to silicon double quantum dots (DQDs), one can realize a one-step fSim gate and a B gate with only one pulse switch. Of note, the method can be further integrated with various optimization theories to enhance gate performance. Based on quantum optimal control theory, we develop a high-fidelity fSim gate scheme with experimentally feasible pulse shapes, featuring an average gate time of 50 ns and a theoretical fidelity of 99.95% in the presence of decoherence and approximation error. By incorporating geometric quantum gate principles, we propose a combined geometric and dynamic fSim gate scheme. Numerical simulations demonstrate that this hybrid scheme exhibits stronger robustness against systematic errors compared to the purely dynamic approach. Our method is generalizable to arbitrary two-qubit physical systems, offering a feasible pathway for rapidly and robustly constructing composite two-qubit gates.