Maintaining or even improving gate performance with growing numbers of parallel controlled qubits is a vital requirement for fault-tolerant quantum computing. For superconducting quantum processors, though isolated one- or two-qubit gates have been demonstrated with high fidelity, implementing these gates in parallel commonly shows worse performance. Generally, this degradation is attributed to various crosstalks between qubits, such as quantum crosstalk due to residual inter-qubit coupling. An understanding of the exact nature of these crosstalks is critical to figuring out respective mitigation schemes and improved qubit architecture designs with low crosstalk. Here we give a theoretical analysis of quantum crosstalk impact on simultaneous gate operations in a qubit architecture, where fixed-frequency transmon qubits are coupled via a tunable bus, and sub-100-ns controlled-Z (cz) gates can be realized by applying a baseband flux pulse on the bus. Our analysis shows that for microwave-driven single-qubit gates, the dressing from the qubit-qubit coupling can cause non-negligible cross-driving errors when qubits operate near frequency collision regions. During cz gate operations, although unwanted nearest-neighbor interactions are nominally turned off, sub-MHz parasitic next-nearest-neighbor interactions involving spectator qubits can still exist, causing considerable leakage or control error when one operates qubit systems around these parasitic resonance points. To ensure high-fidelity simultaneous operations, a request could be raised to figure out a better way to balance the gate error from target qubit systems themselves and the error from nonparticipating spectator qubits. Overall, our analysis suggests that towards useful quantum processors, the qubit architecture should be examined carefully in the context of high-fidelity simultaneous gate operations in a scalable qubit lattice.

We explore a topology-localization phase diagram by simulating a one-dimensional Su-Schrieffer-Heeger model with quasiperiodic disorder using a programmable superconducting simulator. We experimentally map out and identify various trivial and topological phases with extended, critical, and localized bulk states. We find that with increasing disorder strength, some extended states can be first replaced by localized states and then by critical states before the system finally becomes fully localized. The critical states exhibit typical features such as multifractality and self-similarity, which lead to surprisingly rich phases with different types of mobility edges and scaling behaviors on the phase boundaries. Our results shed light on the investigation of the topological and localization phenomena in condensed-matter physics. Published by the American Physical Society 2024

Variational quantum algorithms (VQAs) have shown strong evidence to gain provable computational advantages in diverse fields such as finance, machine learning, and chemistry. However, the heuristic ansatz exploited in modern VQAs is incapable of balancing the trade-off between expressivity and trainability, which may lead to degraded performance when executed on noisy intermediate-scale quantum (NISQ) machines. To address this issue, here, we demonstrate the first proof-of-principle experiment of applying an efficient automatic ansatz design technique, i.e., quantum architecture search (QAS), to enhance VQAs on an 8-qubit superconducting quantum processor. In particular, we apply QAS to tailor the hardware-efficient ansatz toward classification tasks. Compared with heuristic ansätze, the ansatz designed by QAS improves the test accuracy from $31\%$ to $98\%$. We further explain this superior performance by visualizing the loss landscape and analyzing effective parameters of all ansätze. Our work provides concrete guidance for developing variable ansätze to tackle various large-scale quantum learning problems with advantages.

The temperature dependence of proton's spin-lattice relaxation time T1 in water samples and Gd-based contrast agents with a measurement field of 72 μT was systematically studied by dc-SQUID-based ultralow field (ULF) nuclear magnetic resonance (NMR) spectrometer. The temperature of samples was controlled by a thermostatic bath and the standard error of T1 was around 0.01 s within five different groups, the results showed that T1 had a similar linear temperature dependence when it was compared to conventional NMR data and varied with the purity of the water samples. Then, the relaxivity r1 of Gd-based metal-organic frameworks (MOFs) and Gd-DTPA (where DTPA is diethylene triamine pentaacetate) were also studied. Both of them had a much higher r1 in ULF NMR than conventional NMR, which was around 45 mM ^-1 ·s ^-1 and 98 mM ^-1 ·s ^-1 , respectively; however, the T1 of Gd-based contrast agents did not follow the linear temperature dependence, and the relaxivity r1 would not be influenced by temperature, which implies Gd-MOFs and Gd-DTPA are also high performance T 1 contrast agents in ULF NMR and MRI.

MgB2, which has a high stransition temperature (Tc) of 40 K, has great potential for superconducting applications. Recently, significant improvements in the performance of MgB2 wires have enabled applications, such as superconducting magnets, magnetic shielding devices, flux transformers, and gradient meters. However, the problem of superconducting joints in some applications must be solved. In this study, low resistance joints between MgB2 wires were fabricated using the Mg diffusion method. The MgB2 wires were inserted into B bulks and compacted to different densities. Liquid Mg was reacted with the B bulk to form MgB2. The impact of the compaction pressure of the B bulk was studied. With the increase of compaction pressure, the Tc of the joint decreased while the critical current (Ic) of the joint increased. The highest Tc value of the joint was 39 K, and the highest Ic value of the joint at 0 T was 51 A at 20 K.

With the development of weak magnetic detection methods, the sensors, like superconducting quantum interference device (SQUID), become more and more sensitive. The magnetic shielding takes a more important part in weak signal detection systems than before. Traditional magnetic shielding methods are expensive, not flexible, and hard to prevent the low-frequency magnetic field clearly. For the superconducting magnetic shield, the operating temperature of a low critical temperature superconducting magnetic shield has high demand on refrigerating system. The high critical temperature superconducting magnetic shielding would introduce thermal magnetic noise. Due to the excellent superconductive property, the medium-temperature superconductor MgB2 has a huge potential to build the magnetic shielding system. In this article, we fabricate an MgB2 superconducting magnetic shielding cup with Mg Liquid Infiltration method. The highest TC of the sample is 39.4 K. The highest JC is 10 ⁶ A/cm ² at 4.2 K. The shielding effect of the cup has been tested at axial dc weak magnetic field and the shielding factor exceeds 2000.