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1. Differentiable Analog Quantum Computing for Optimization and Control

   Leng, Jiaqi; Peng, Yuxiang; Qiao, Yi-Ling; Lin, Ming; Wu, Xiaodi (January 2022, Advances in neural information processing systems)

   We formulate the first differentiable analog quantum computing framework with specific parameterization design at the analog signal (pulse) level to better exploit near-term quantum devices via variational methods. We further propose a scalable approach to estimate the gradients of quantum dynamics using a forward pass with Monte Carlo sampling, which leads to a quantum stochastic gradient descent algorithm for scalable gradient-based training in our framework. Applying our framework to quantum optimization and control, we observe a significant advantage of differentiable analog quantum computing against SOTAs based on parameterized digital quantum circuits by {\em orders of magnitude}. 

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2. Optimized Quantum Compilation for Near-Term Algorithms with OpenPulse

   https://doi.org/10.1109/MICRO50266.2020.00027  

   Gokhale, Pranav; Javadi-Abhari, Ali; Earnest, Nathan; Shi, Yunong; Chong, Frederic T. (October 2020, 53rd Annual IEEE/ACM International Symposium on Microarchitecture (MICRO))

   null (Ed.)

   Quantum computers are traditionally operated by programmers at the granularity of a gate-based instruction set. However, the actual device-level control of a quantum computer is performed via analog pulses. We introduce a compiler that exploits direct control at this microarchitectural level to achieve significant improvements for quantum programs. Unlike quantum optimal control, our approach is bootstrapped from existing gate calibrations and the resulting pulses are simple. Our techniques are applicable to any quantum computer and realizable on current devices. We validate our techniques with millions of experimental shots on IBM quantum computers, controlled via the OpenPulse control interface. For representative benchmarks, our pulse control techniques achieve both 1.6x lower error rates and 2x faster execution time, relative to standard gate-based compilation. These improvements are critical in the near-term era of quantum computing, which is bottlenecked by error rates and qubit lifetimes. 

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3. QisDAX: An Open Source Bridge from Qiskit to Trapped-Ion Quantum Devices

   https://doi.org/10.1109/QCE57702.2023.00097  

   Badrike, Kaustubh; Dalvi, Aniket S.; Mazurek, Filip; D’Onofrio, Marissa; Whitlow, Jacob; Chen, Tianyi; Phiri, Samuel; Riesebos, Leon; Brown, Kenneth R.; Mueller, Frank (September 2023, IEEE)

   Quantum computing has become widely available to researchers via cloud-hosted devices with different technologies using a multitude of software development frameworks. The vertical stack behind such solutions typically features quantum language abstraction and high-level translation frameworks that tend to be open source, down to pulse-level programming. However, the lower-level mapping to the control electronics, such as controls for laser and microwave pulse generators, remains closed source for contemporary commercial cloud-hosted quantum devices. One exception is the ARTIQ (Advanced Real-Time Infrastructure for Quantum physics) open-source library for trapped-ion control electronics. This stack has been complemented by the Duke ARTIQ Extensions (DAX) to provide modularity and better abstraction. It, however, remains disconnected from the wealth of features provided by popular quantum computing languages. This paper contributes QisDAX, a bridge between Qiskit and DAX that fills this gap. QisDAX provides interfaces for Python programs written using IBM's Qiskit and transpiles them to the DAX abstraction. This allows users to generically interface to the ARTIQ control systems accessing trapped-ion quantum devices. Consequently, the algorithms expressed in Qiskit become available to an open-source quantum software stack. This provides the first open-source, end-to-end, full-stack pipeline for remote submission of quantum programs for trapped-ion quantum systems in a non-commercial setting. 

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4. Ultrafast control of fractional orbital angular momentum of microlaser emissions

   https://doi.org/10.1038/s41377-020-00415-3  

   Zhang, Zhifeng; Zhao, Haoqi; Pires, Danilo Gomes; Qiao, Xingdu; Gao, Zihe; Jornet, Josep M.; Longhi, Stefano; Litchinitser, Natalia M.; Feng, Liang (October 2020, Light: Science & Applications)

   Abstract On-chip integrated laser sources of structured light carrying fractional orbital angular momentum (FOAM) are highly desirable for the forefront development of optical communication and quantum information–processing technologies. While integrated vortex beam generators have been previously demonstrated in different optical settings, ultrafast control and sweep of FOAM light with low-power control, suitable for high-speed optical communication and computing, remains challenging. Here we demonstrate fast control of the FOAM from a vortex semiconductor microlaser based on fast transient mixing of integer laser vorticities induced by a control pulse. A continuous FOAM sweep between charge 0 and charge +2 is demonstrated in a 100 ps time window, with the ultimate speed limit being established by the carrier recombination time in the gain medium. Our results provide a new route to generating vortex microlasers carrying FOAM that are switchable at GHz frequencies by an ultrafast control pulse. 

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5. 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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