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1. 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)

   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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2. Quantum Computing in the NISQ era and beyond

   https://doi.org/10.22331/q-2018-08-06-79  

   Preskill, John ( August 2018 , Quantum)

   Noisy Intermediate-Scale Quantum (NISQ) technology will be available in the near future. Quantum computers with 50-100 qubits may be able to perform tasks which surpass the capabilities of today's classical digital computers, but noise in quantum gates will limit the size of quantum circuits that can be executed reliably. NISQ devices will be useful tools for exploring many-body quantum physics, and may have other useful applications, but the 100-qubit quantum computer will not change the world right away - we should regard it as a significant step toward the more powerful quantum technologies of the future. Quantum technologists should continue to strive for more accurate quantum gates and, eventually, fully fault-tolerant quantum computing. 

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3. Automatically Translating Quantum Programs from a Subset of Common Gates to an Adiabatic Representation

   https://doi.org/10.1007/978-3-030-21500-2_9  

   Regan M., Eastwood B. ( May 2019 , nternational Conference on Reversible Computation (RC), Springer LNCS)

   Adiabatic computing with two degrees of freedom of 2-local Hamiltonians has been theoretically shown to be equivalent to the gate model of universal quantum computing. But today’s quantum annealers, namely D-Wave’s 2000Q platform, only provide a 2-local Ising Hamiltonian abstraction with a single degree of freedom. This raises the question what subset of gate programs can be expressed as quadratic unconstrained binary problems (QUBOs) on the D-Wave. The problem is of interest because gate-based quantum platforms are currently limited to 20 qubits while D-Wave provides 2,000 qubits. However, when transforming entire gate circuits into QUBOs, additional qubits will be required. The objective of this work is to determine a subset of quantum gates suitable for transformation into single-degree 2-local Ising Hamiltonians under a common qubit base representation such that they comprise a compound circuit suitable for pure quantum computation, i.e., without having to switch between classical and quantum computing for different bases. To this end, this work contributes, for the first time, a fully automated method to translate quantum gate circuits comprised of a subset of common gates expressed as an IBM Qiskit program to single-degree 2-local Ising Hamiltonians, which are subsequently embedded in the D-Wave 2000Q chimera graph. These gate elements are placed in the chimera graph and augmented by constraints that enforce inter-gate logical relationships, resulting in an annealer embedding that completely characterizes the overall gate circuit. Annealer embeddings for several example quantum gate circuits are then evaluated on D-Wave 2000Q hardware. 

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4. Digital coherent control of a superconducting qubit

   Leonard Jr., E. ; Beck, M.A. ; Nelson, JJ ; Christensen, B.G. ; Thorbeck, T. ; Howington, C. ; Opremcak, A. ; Pechenezhskiy, I.V. ; Dodge, K. ; Dupuis, N.P. ; et al ( June 2018 , arXiv.org)

   High-fidelity gate operations are essential to the realization of a fault-tolerant quantum computer. In addition, the physical resources required to implement gates must scale efficiently with system size. A longstanding goal of the superconducting qubit community is the tight integration of a superconducting quantum circuit with a proximal classical cryogenic control system. Here we implement coherent control of a superconducting transmon qubit using a Single Flux Quantum (SFQ) pulse driver cofabricated on the qubit chip. The pulse driver delivers trains of quantized flux pulses to the qubit through a weak capacitive coupling; coherent rotations of the qubit state are realized when the pulse-to-pulse timing is matched to a multiple of the qubit oscillation period. We measure the fidelity of SFQ-based gates to be ~95% using interleaved randomized benchmarking. Gate fidelities are limited by quasiparticle generation in the dissipative SFQ driver. We characterize the dissipative and dispersive contributions of the quasiparticle admittance and discuss mitigation strategies to suppress quasiparticle poisoning. These results open the door to integration of large-scale superconducting qubit arrays with SFQ control elements for low-latency feedback and stabilization. 

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5. Practical implications of SFQ-based two-qubit gates

   https://doi.org/10.1109/QCE52317.2021.00061  

   Jokar, Mohammad Reza ; Rines, Richard ; Chong, Frederic T. ( October 2021 , 2021 IEEE International Conference on Quantum Computing and Engineering (QCE))

   Scalability of today’s superconducting quantum computers is limited due to the huge costs of generating/routing microwave control pulses per qubit from room temperature. One active research area in both industry and academia is to push the classical controllers to the dilution refrigerator in order to increase the scalability of quantum computers. Superconducting Single Flux Quantum (SFQ) is a classical logic technology with low power consumption and ultra-high speed, and thus is a promising candidate for in-fridge classical controllers with maximized scalability. Prior work has demonstrated high-fidelity SFQ-based single-qubit gates. However, little research has been done on SFQ-based multi-qubit gates, which are necessary to realize SFQ-based universal quantum computing.In this paper, we present the first thorough analysis of SFQ-based two-qubit gates. Our observations show that SFQ-based two-qubit gates tend to have high leakage to qubit non-computational subspace, which presents severe design challenges. We show that despite these challenges, we can realize gates with high fidelity by carefully designing optimal control methods and qubit architectures. We develop optimal control methods that suppress leakage, and also investigate various qubit architectures that reduce the leakage. After carefully engineering our SFQ-friendly quantum system, we show that it can achieve similar gate fidelity and gate time to microwave-based quantum systems. The promising results of this paper show that (1) SFQ-based universal quantum computation is both feasible and effective; and (2) SFQ is a promising approach in designing classical controller for quantum machines because it can increase the scalability while preserving gate fidelity and performance. 

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