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1. Variability and Fidelity Limits of Silicon Quantum Gates Due to Random Interface Charge Traps

   https://doi.org/10.1109/LED.2020.2997009  

   Wu, Tong; Guo, Jing (January 2020, IEEE Electron Device Letters)

   null (Ed.)

   Silicon offers an attractive material platform for hardware realization of quantum computing. In this study, a microscopic stochastic simulation method is developed to model the effect of random interface charge traps in silicon metal-oxide-semiconductor (MOS) quantum gates. The statistical results show that by using a fast two-qubit gate in isotopically purified silicon, the two-qubit silicon-based quantum gates have the fidelity >98% with a probability of 75% for the state-of-the-art MOS interface quality. By using a composite gate pulse, the fidelity can be further improved to >99.5% with the 75% probability. The variations between the quantum gate devices, however, are largely due to the small number of traps per device. The results highlight the importance of variability consideration due to random charge traps and potential to improve fidelity in silicon-based quantum computing. 

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2. Noise Correlation in Silicon Spin Qubits: A Computational Study

   Cheng, Guoting; Guo, Jing (September 2024, IEEE International Conference on Quantum computing and Engineering)

   The platform of silicon-based spin qubits holds significant potential for the hardware implementation of quantum computing. Charge noise, however, notably hinders the performance and scalability of silicon-spin-based quantum computing technologies. Here we computationally investigated correlated charge noise in silicon spin quantum computing devices by developing and applying a Green’s transfer function approach. The approach allows for the systematic simulation and analysis of both the noise's auto-correlation and cross-correlation spectrums in a physics-based manner. We simulate the correlated noise's power spectral density (PSD) in silicon spin qubit devices. The results indicate strong cross-correlation and show phase-flipping features in neighboring silicon spin qubits, in agreement with a recent experiment. Given that each spin qubit device is small and influenced by a limited number of two-level fluctuators (TLFs), the arrangement of these TLFs plays a crucial role in the correlation of noise. The simulation study highlights the need to consider noise correlation and its related spectral features in developing robust quantum computing technologies based on silicon spin qubits. 

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3. Pairwise‐Parallel Entangling Gates on Orthogonal Modes in a Trapped‐Ion Chain

   https://doi.org/10.1002/qute.202300056  

   Zhu, Yingyue; Green, Alaina M.; Nguyen, Nhung H.; Huerta Alderete, C.; Mossman, Elijah; Linke, Norbert M. (September 2023, Advanced Quantum Technologies)

   Abstract Parallel operations are important for both near‐term quantum computers and larger‐scale fault‐tolerant machines because they reduce execution time and qubit idling. This study proposes and implements a pairwise‐parallel gate scheme on a trapped‐ion quantum computer. The gates are driven simultaneously on different sets of orthogonal motional modes of a trapped‐ion chain. This work demonstrates the utility of this scheme by creating a Greenberger‐Horne‐Zeilinger (GHZ) state in one step using parallel gates with one overlapping qubit. It also shows its advantage for circuits by implementing a digital quantum simulation of the dynamics of an interacting spin system, the transverse‐field Ising model. This method effectively extends the available gate depth by up to two times with no overhead when no overlapping qubit is involved, apart from additional initial cooling. This scheme can be easily applied to different trapped‐ion qubits and gate schemes, broadly enhancing the capabilities of trapped‐ion quantum computers. 

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4. Comparing Quantum Computing Platforms

   https://doi.org/10.1007/978-3-030-98012-2_32  

   Agarwal, Siddh; Jaoude, Gio Abou; Leider, Avery; Tappert, Charles C. (January 2022, Advances in Information and Communication: Proceedings of the 2022 Future of Information and Communication Conference (FICC), Volume 1)

   Arai, Kohei (Ed.)

   This research compares and contrasts two commonly available quantum computing platforms available today to academic researchers: the IBM Q-Experience and the University of Maryland's IonQ. Hands-on testing utilized the implementation of a simple two qubit circuit and tested the Pauli X, Y, and Z single-qubit gates as well as the CNOT 2+ qubit gate and compared the results, as well as the user experience. The user experience and the interface must be straightforward to help the user's understanding when planning quantum computing training for new knowledge workers in this exciting new field. Additionally, we demonstrate how a quantum computer's results, when the output is read in the classical computer, loses some of its information, since the quantum computer is operating in more dimensions than the classical computer can interpret. This is demonstrated with the ZX and XZ gates which appear to give the same result; however, using the mathematics of matrix notation, the phase difference between the two answers is revealed in their vectors, which are 180 degrees apart. 

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5. Quantum control of spin qubits using nanomagnets

   https://doi.org/10.1038/s42005-022-01041-8  

   Niknam, Mohamad; Chowdhury, Md. Fahim F.; Rajib, Md Mahadi; Misba, Walid Al; Schwartz, Robert N.; Wang, Kang L.; Atulasimha, Jayasimha; Bouchard, Louis-S. (November 2022, Communications Physics)

   Abstract Single-qubit gates are essential components of a universal quantum computer. Without selective addressing of individual qubits, scalable implementation of quantum algorithms is extremely challenging. When the qubits are discrete points or regions on a lattice, selectively addressing magnetic spin qubits at the nanoscale remains a challenge due to the difficulty of localizing and confining a classical divergence-free field to a small volume of space. Herein we propose a technique for addressing spin qubits using voltage-control of nanoscale magnetism, exemplified by the use of voltage control of magnetic anisotropy. We show that by tuning the frequency of the nanomagnet’s electric field drive to the Larmor frequency of the spins confined to a nanoscale volume, and by modulating the phase of the drive, single-qubit quantum gates with fidelities approaching those for fault-tolerant quantum computing can be implemented. Such single-qubit gate operations require only tens of femto-Joules per gate operation and have lossless, purely magnetic field control. Their physical realization is also straightforward using foundry manufacturing techniques. 

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