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1. Calibration method for an extended depth-of-field microscopic structured light system

   https://doi.org/10.1364/OE.448019  

   Chen, Liming; Hu, Xiaowei; Zhang, Song (December 2021, Optics Express)

   This paper presents a calibration method for a microscopic structured light system with an extended depth of field (DOF). We first employed the focal sweep technique to achieve large enough depth measurement range, and then developed a computational framework to alleviate the impact of phase errors caused by the standard off-the-shelf calibration target (black circles with a white background). Specifically, we developed a polynomial interpolation algorithm to correct phase errors near the black circles to obtain more accurate phase maps for projector feature points determination. Experimental results indicate that the proposed method can achieve a measurement accuracy of approximately 1.0 $\mathrm{\mu <\#comment/>}$m for a measurement volume of approximately 2,500 $\mathrm{\mu <\#comment/>}$m (W) × 2,000 $\mathrm{\mu <\#comment/>}$m (H) × 500 $\mathrm{\mu <\#comment/>}$m (D). 

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2. Just-in-time Quantum Circuit Transpilation Reduces Noise

   Ellis Wilson, Sudhakar Singh (October 2020, International Conference on Quantum Computing and Engineering (QCE))

   Running quantum programs is fraught with challenges on on today’s noisy intermediate scale quantum (NISQ) devices. Many of these challenges originate from the error characteristics that stem from rapid decoherence and noise during measurement, qubit connections, crosstalk, the qubits themselves, and transformations of qubit state via gates. Not only are qubits not “created equal”, but their noise level also changes over time. IBM is said to calibrate their quantum systems once per day and reports noise levels (errors) at the time of such calibration. This information is subsequently used to map circuits to higher quality qubits and connections up to the next calibration point. This work provides evidence that there is room for improvement over this daily calibration cycle. It contributes a technique to measure noise levels (errors) related to qubits immediately before executing one or more sensitive circuits and shows that just-in-time noise measurements can benefit late physical qubit mappings. With this just-in-time recalibrated transpilation, the fidelity of results is improved over IBM’s default mappings, which only uses their daily calibrations. The framework assess two major sources of noise, namely readout errors (measurement errors) and two-qubit gate/connection errors. Experiments indicate that the accuracy of circuit results improves by 3-304% on average and up to 400% with on-the-fly circuit mappings based on error measurements just prior to application execution. 

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3. Quantum adiabatic theorem for unbounded Hamiltonians with a cutoff and its application to superconducting circuits

   https://doi.org/10.1098/rsta.2021.0407  

   Mozgunov, Evgeny; Lidar, Daniel A. (January 2023, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences)

   We present a new quantum adiabatic theorem that allows one to rigorously bound the adiabatic timescale for a variety of systems, including those described by originally unbounded Hamiltonians that are made finite-dimensional by a cutoff. Our bound is geared towards the qubit approximation of superconducting circuits and presents a sufficient condition for remaining within the $2^n$-dimensional qubit subspace of a circuit model of $n$qubits. The novelty of this adiabatic theorem is that, unlike previous rigorous results, it does not contain $2^n$as a factor in the adiabatic timescale, and it allows one to obtain an expression for the adiabatic timescale independent of the cutoff of the infinite-dimensional Hilbert space of the circuit Hamiltonian. As an application, we present an explicit dependence of this timescale on circuit parameters for a superconducting flux qubit and demonstrate that leakage out of the qubit subspace is inevitable as the tunnelling barrier is raised towards the end of a quantum anneal. We also discuss a method of obtaining a $2^n\times 2^n$effective Hamiltonian that best approximates the true dynamics induced by slowly changing circuit control parameters. This article is part of the theme issue ‘Quantum annealing and computation: challenges and perspectives’. 

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4. Energy-dependent barren plateau in bosonic variational quantum circuits

   https://doi.org/10.1088/2058-9565/ad80bf  

   Zhang, Bingzhi; Zhuang, Quntao (October 2024, Quantum Science and Technology)

   Abstract Bosonic variational quantum circuits (VQCs) are crucial for information processing in microwave cavities, trapped ions, and optical systems, widely applicable in quantum communication, sensing and error correction. The trainability of such VQCs is less understood, hindered by the lack of theoretical tools such ast-design due to the infinite dimension of the continuous-variable systems involved. We overcome this difficulty to reveal an energy-dependent barren plateau in such VQCs. The variance of the gradient decays as $1/E^{M\nu }$, exponential in the number of modesMbut polynomial in the (per-mode) circuit energyE. The exponentν = 1 for shallow circuits andν = 2 for deep circuits. We prove these results for state preparation of general Gaussian states and number states. We also provide numerical evidence demonstrating that the results extend to general state preparation tasks. As circuit energy is a controllable parameter, we provide a strategy to mitigate the barren plateau in bosonic continuous-variable VQCs. 

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5. Experimental demonstration of continuous quantum error correction

   https://doi.org/10.1038/s41467-022-29906-0  

   Livingston, William P.; Blok, Machiel S.; Flurin, Emmanuel; Dressel, Justin; Jordan, Andrew N.; Siddiqi, Irfan (December 2022, Nature Communications)

   Abstract The storage and processing of quantum information are susceptible to external noise, resulting in computational errors. A powerful method to suppress these effects is quantum error correction. Typically, quantum error correction is executed in discrete rounds, using entangling gates and projective measurement on ancillary qubits to complete each round of error correction. Here we use direct parity measurements to implement a continuous quantum bit-flip correction code in a resource-efficient manner, eliminating entangling gates, ancillary qubits, and their associated errors. An FPGA controller actively corrects errors as they are detected, achieving an average bit-flip detection efficiency of up to 91%. Furthermore, the protocol increases the relaxation time of the protected logical qubit by a factor of 2.7 over the relaxation times of the bare comprising qubits. Our results showcase resource-efficient stabilizer measurements in a multi-qubit architecture and demonstrate how continuous error correction codes can address challenges in realizing a fault-tolerant system. 

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