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1. Readout of a quantum processor with high dynamic range Josephson parametric amplifiers

   https://doi.org/10.1063/5.0127375  

   White, Theodore; Opremcak, Alex; Sterling, George; Korotkov, Alexander; Sank, Daniel; Acharya, Rajeev; Ansmann, Markus; Arute, Frank; Arya, Kunal; Bardin, Joseph C.; et al (January 2023, Applied Physics Letters)

   We demonstrate a high dynamic range Josephson parametric amplifier (JPA) in which the active nonlinear element is implemented using an array of rf-SQUIDs. The device is matched to the 50 Ω environment with a Klopfenstein-taper impedance transformer and achieves a bandwidth of 250–300 MHz with input saturation powers up to −95 dBm at 20 dB gain. A 54-qubit Sycamore processor was used to benchmark these devices, providing a calibration for readout power, an estimation of amplifier added noise, and a platform for comparison against standard impedance matched parametric amplifiers with a single dc-SQUID. We find that the high power rf-SQUID array design has no adverse effect on system noise, readout fidelity, or qubit dephasing, and we estimate an upper bound on amplifier added noise at 1.6 times the quantum limit. Finally, amplifiers with this design show no degradation in readout fidelity due to gain compression, which can occur in multi-tone multiplexed readout with traditional JPAs. 

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2. Broadband bandpass Purcell filter for circuit quantum electrodynamics

   https://doi.org/10.1063/5.0161893  

   Yan, Haoxiong; Wu, Xuntao; Lingenfelter, Andrew; Joshi, Yash J.; Andersson, Gustav; Conner, Christopher R.; Chou, Ming-Han; Grebel, Joel; Miller, Jacob M.; Povey, Rhys G.; et al (September 2023, Applied Physics Letters)

   In circuit quantum electrodynamics, qubits are typically measured using dispersively coupled readout resonators. Coupling between each readout resonator and its electrical environment, however, reduces the qubit lifetime via the Purcell effect. Inserting a Purcell filter counters this effect while maintaining high readout fidelity but reduces measurement bandwidth and, thus, limits multiplexing readout capacity. In this Letter, we develop and implement a multi-stage bandpass Purcell filter that yields better qubit protection while simultaneously increasing measurement bandwidth and multiplexed capacity. We report on the experimental performance of our transmission-line-based implementation of this approach, a flexible design that can easily be integrated with current scaled-up, long coherence time superconducting quantum processors. 

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3. Quantum Measurement Classification Using Statistical Learning

   https://doi.org/10.1145/3644823  

   Utt, Zachery; Volya, Daniel; Mishra, Prabhat (June 2024, ACM Transactions on Quantum Computing)

   Interpreting the results of a quantum computer can pose a significant challenge due to inherent noise in these mesoscopic quantum systems. Quantum measurement, a critical component of quantum computing, involves determining the probabilities linked with quantum states post-multiple circuit computations based on quantum readout values provided by hardware. While there are promising classification-based solutions, they can either misclassify or necessitate excessive measurements, thereby proving to be costly. This article puts forth an efficient method to discern the quantum state by analyzing the probability distributions of data post-measurement. Specifically, we employ cumulative distribution functions to juxtapose the measured distribution of a sample against the distributions of basis states. The efficacy of our approach is demonstrated through experimental results on a superconducting transmon qubit architecture, which shows a substantial decrease (88%) in single qubit readout error compared to state-of-the-art measurement techniques. Moreover, we report additional error reduction (12%) compared to state-of-the-art measurement techniques when our technique is applied to enhance existing multi-qubit classification techniques. We also demonstrate the applicability of our proposed method for higher dimensional quantum systems, including classification of single qutrits as well as multiple qutrits. 

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4. Parametrically controlled chiral interface for superconducting quantum devices

   https://doi.org/10.1103/PhysRevApplied.22.064023  

   Cao, Xi; Irfan, Abdullah; Mollenhauer, Michael; Singirikonda, Kaushik; Pfaff, Wolfgang (December 2024, Physical Review Applied)

   Nonreciprocal microwave routing plays a crucial role in measuring quantum circuits, and allows for realizing cascaded quantum systems for generating and stabilizing entanglement between noninteracting qubits. The most commonly used tools for implementing directionality are ferrite-based circulators. These devices are versatile, but suffer from excess loss, a large footprint, and fixed directionality. For utilizing nonreciprocity in scalable quantum circuits it is desirable to develop efficient integration of low-loss and controllable directional elements. Here, we report the design and experimental realization of a minimal controllable directional interface that can be directly coupled to superconducting qubits. In the device presented, nonreciprocity is realized through a combination of interference and phase-controlled parametric pumping. We have achieved a maximum directionality of around 30 dB, and the performance of the device is predicted quantitatively from independent calibration measurements. Using the excellent agreement of model and experiment, we predict that the circuit will be useable as a chiral qubit interface with inefficiencies at the $1\mathrm{\%}$level or below. Our work offers a promising route for realizing high-fidelity signal routing and entanglement generation in all-to-all connected microwave quantum networks, and provides a path for isolator-free qubit readout schemes. Published by the American Physical Society2024 

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5. Error-detected quantum operations with neutral atoms mediated by an optical cavity

   https://doi.org/10.1126/science.adr7075  

   Grinkemeyer, Brandon; Guardado-Sanchez, Elmer; Dimitrova, Ivana; Shchepanovich, Danilo; Mandopoulou, G Eirini; Borregaard, Johannes; Vuletić, Vladan; Lukin, Mikhail D (March 2025, Science)

   Neutral-atom quantum processors are a promising platform for large-scale quantum computing. Integrating them with optical cavities enables fast nondestructive qubit readout and access to fast remote entanglement generation for quantum networking. In this work, we introduce a platform for coupling single atoms in optical tweezers to a Fabry-Perot fiber cavity. Leveraging the strong atom-cavity coupling, we demonstrated fast qubit-state readout with ${99.960}_{-24}^{+14}\%$fidelity and two methods for cavity-mediated entanglement generation with integrated error detection. First, we used cavity-carving to generate a Bell state with 91(4)% fidelity and a 32(1)% success rate (the number in parentheses is the standard deviation). Second, we performed a cavity-mediated gate with a deterministic entanglement fidelity of 52.5(18)%, increased to 76(2)% with error detection. Our approach provides a route toward modular quantum computing and networking. 

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