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Chinese Physics Letters

Measurement of Spin Singlet-Triplet Qubit in Quantum Dots Using Superconducting Resonator*

Xingyu Zhu, Tao Tu, Ao-Lin Guo, Zong‐Quan Zhou, Guang‐Can Guo

Published Feb 1, 2020• 4 citations
Publisher

Abstract

The spin qubit in quantum dots is one of the leading platforms for quantum computation. A crucial requirement for scalable quantum information processing is the high efficient measurement. Here we analyze the measurement process of a quantum-dot spin qubit coupled to a superconducting transmission line resonator. Especially, the phase shift of the resonator is sensitive to the spin states and the gate operations. The response of the resonator can be used to measure the spin qubit efficiently, which can be extend to read out the multiple spin qubits in a scalable solid-state quantum processor.

DOI
10.1088/0256-307x/37/2/020302

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The spin qubit in quantum dots is one of the leading platforms for quantum computation. A crucial requirement for scalable quantum information processing is the high efficient measurement. Here we analyze the measurement process of a quantum-dot spin qubit coupled to a superconducting transmission line resonator. Especially, the phase shift of the resonator is sensitive to the spin states and the gate operations. The response of the resonator can be used to measure the spin qubit efficiently, which can be extend to read out the multiple spin qubits in a scalable solid-state quantum processor.
The hybrid system of electron spins and resonator photons is an attractive architecture for quantum computing owing to the long coherence times of spins and the promise of long-distance coupling between arbitrary pairs of qubits via photons. For the device to serve as a building block for a quantum processer, it is also necessary to readout the spin qubit state. Here we analyze in detail the measurement process of an electron spin singlet-triplet qubit in quantum dots using a coupled superconducting resonator. We show that the states of the spin singlet-triplet qubit lead to readily observable features in the spectrum of a microwave field through the resonator. These features provide useful information on the hybrid system. Moreover, we discuss the working points which can be implemented with high performance in the current state-of-the-art devices. These results can be used to construct the high fidelity measurement toolbox in the spin-circuit QED system.
Hybrid devices integrating quantum dots with Josephson junctions are gaining interest because they combine spin-based quantum computing with circuit quantum electrodynamics (circuit QED) methods. In particular, Andreev spin qubits have shown significant experimental progress including strong two-qubit coupling, and are predicted to exhibit all-to-all connectivity. Here we propose superconducting singlet-triplet (SST) qubits that rely on parallel-aligned double quantum dots in Josephson junctions. While Andreev spin qubits require spin-orbit interaction to unlock the spin degree-of-freedom, SST qubits do not require spin-orbit interaction, making the advantages of hybrid devices available to a wider range of materials. Similar to Andreev spin qubits, the qubit states couple to the superconducting phase across the junction, which allows for control and readout using circuit QED, and supports all-to-all connectivity. Only NNN flux lines are required to perform any single- and two-qubit gate among NNN qubits, and thus the overhead of control lines is small. Finally, linear protection from charge or flux noise makes these qubits interesting candidates for a future quantum processor.
Electron spins in semiconductor quantum dots are a promising platform for qubits. An essential step toward quantum error correction with spin qubits requires the ability to read out states and identify errors in the two-spin logical basis in a fast and high-fidelity way. Here we propose a scheme for spin singlet-triplet readout in a double quantum dot, with a superconducting resonator serving as the detector. Making use of a spin driving pulse sequence, this procedure can read out the spin states with a fidelity of 99.999.9%99.9 in a time 100 ns. Interestingly, this method makes use of the spin driving pulse to adjust the resonator signal field, leading to significant improvement of the spin readout. In addition, this method can be directly applied to multiplexed readout of qubits and repeated readout to identify logical errors. These results pave the way for the implementation of quantum error correction and scalable quantum processor with spin qubits.
A full-scale quantum computer requires physical qubits that can be controlled with high precision and accuracy. Unfortunately, few state-of-the-art qubits can perform all their elementary operations (preparation, measurement, single-qubit gates and two-qubit gates) with sufficient fidelity. In this thesis, we investigate alternative schemes for such operations in solid-state qubits. Specifically, two-qubit gates and measurements, which are often the noisiest of the four. \n \nWe first provide a preliminary introduction to quantum computing, and describe how quantum information can be encoded and manipulated in quantum systems. We include background information for the three different solid-state qubit architectures that feature in this thesis: spin qubits, superconducting qubits and Majorana qubits. \n \nFollowing this, we investigate a scheme for mediating a two-qubit interaction between spin qubits via a multielectron quantum dot. We study a multielectron dot in detail, and characterise its exchange interaction with a single spin. With the aid of a theoretical model, we show that the multielectron dot possesses an irregular triplet-preferring ground state, analogous to Hund's rule from atomic physics. Using these findings, we then demonstrate that the multielectron dot can be used to mediate a fast, long-range exchange interaction between two spin qubits. \n \nSubsequently, we examine two resonator-based measurement schemes for Majorana qubits. We first propose a readout technique based on a longitudinal qubit-resonator interaction. This leads to a measurement that is fast, high-fidelity and quantum non-demolition (QND). We then investigate a more conventional dispersive readout scheme. Not only does this yield a high quality measurement, but it can also offers a more protected readout mechanism in comparison to the dispersive readout of conventional superconducting qubits.
A design is proposed for scalable solid-state quantum computing, which is based on collectively enhanced magnetic coupling between nitrogen-vacancy center ensembles and superconducting transmission line resonators interconnected by current-biased Josephson junction superconducting phase qubit. In this hybrid system, we realize distant multi-qubit controlled phase gate operations and generate distant multi-qubit entangled W-like states, being indispensable resource to quantum computation. Our proposed architecture consists of solid-state spin ensembles and circuit QED, and could achieve quantum computing in a solid-state environment with high-fidelity and scalable way. The experimental feasibility is discussed, and the implementation efficiency is demonstrated numerically.