| Abstract |
While the classification of zero-temperature topological quantum systems is near completion, classification of finite-temperature topological systems is still an actively researched area. I will first summarize the challenges from direct generalizations of zero-temperature topological indicators and then present a promising candidate for finite-temperature topological indicators known as the Uhlmann phase. Due to the incompatibility between the Uhlmann process and Hamiltonian dynamics, simulation and measurement of the Uhlmann phase can be challenging, but promising solutions arise from using quantum computers to simulate composite systems of a system plus its ancilla for generating and probing the Uhlmann phase. However, for a spin-1 (three-level) system with its ancilla and probe, the native circuits can reach 2000 gates and exceed the limit of noisy intermediate-scale quantum (NISQ) hardware. Through a series of optimizations using Qiskit and BQSKit to cut the gate count to about 200 and with a recent upgrade of the IBM hardware, we successfully extract the quantized Uhlmann phase of the spin-1 system at finite temperatures and show a topological regime sandwiched between low-and high-temperature trivial regimes.
Short bio: Chih-Chun Chien is an associated professor at the University of California, Merced (UCM). He got his PhD from University of Chicago, where he studied the BCS-BEC crossover of ultracold atomic Fermi superfluid. He was a Director’s fellow and then an Oppenheimer fellow at Los Alamos National Laboratory, where he applied quantum field theory to ultracold bosonic atoms. After joining UCM, he has expanded his research to study geometric and topological effects in many-body physics and utilized quantum computation and machine learning to investigate complex systems in and out of equilibrium.
2.IAMS Lecture
中研院原分所演講公告
| Title |
Massively Multiplexed Nanoscale Magnetometry with Diamond Quantum Sensors towards Multi-modal Sensing of Superconductivity |
| Speaker |
Mr. Kai-Hung Cheng
Princeton University |
| Time |
10:30 AM, June 29 (Monday), 2026 |
| Venue |
C. T. Chang Memorial Hall (NTU Campus), IAMS
本所張昭鼎紀念講堂（臺大校園內） |
| Contact |
Dr. Ying-Cheng Chen |
| Abstract |
Nitrogen vacancy (NV) centers in diamond enable both microscopic DC magnetometry and magnetic noise sensing at multiple frequency bands spanning the MHz and GHz range, making them powerful probes of material systems. A particularly complex family of materials are the cuprate superconductors, which exhibit high critical temperatures, large anisotropy, and abundance of defects, leading to rich superconducting vortex phase structure. In this presentation, I will first introduce the multiplexed NV magnetometry systems that we design and build for condensed matter sensing using both single, resolvable NV centers and dense ensemble of NV centers. The new platforms allow us to parallelize NV spin measurements as well as to sense non-local correlated magnetic field fluctuations. Next, I will describe how we apply multi-modal magnetometry with NV centers to study both the static field and magnetic noise of the cuprate superconductor, optimally doped Bi2Sr2CaCu2O8+x, across a wide temperature range. We image many individual superconducting vortices, which allows us to track their motion in real time and to quantitatively analyze their ordering. In the vortex solid phase, we measure the change of vortex crystallinity as function of temperature and observe thermally-activated vortex depinning, which is corroborated by measuring enhanced magnetic noise at higher temperatures. This result demonstrates that superconducting vortices produce magnetic field fluctuations, and highlights the ability of multi-modal magnetometry to illuminate complex dynamics in cuprate superconductors. Future directions include correlated noise spectroscopy in the vortex phase to further understand the nature of the vortex-related fluctuations. |
3.IAMS Lecture
中研院原分所演講公告
| Title |
Neural Computations Underlying Polarization Vision in Cephalopods |
| Speaker |
Dr. Tomoyuki Mano
Computational Neuroethology Unit , Okinawa Institute of Science and Technology |
| Time |
11:00 AM, August 26 (Wednesday), 2026 |
| Venue |
C. T. Chang Memorial Hall (NTU Campus), IAMS
本所張昭鼎紀念講堂（臺大校園內） |
| Contact |
Dr. Chia-Lung Hsieh |
| Abstract |
Underwater environments prominently feature a dimension of light invisible to our own eyes, polarization. Cephalopods (octopus, cuttlefish and squid) detect polarization with both high sensitivity and resolution, skillfully utilizing it for complex behaviors such as hunting and communication. However, the neural mechanisms for encoding patterns of light polarization remain poorly understood. Using the bigfin reef squid (Sepioteuthis lessoniana), we developed a new head-fixation method to perform two-photon calcium imaging in awake squid, providing the first in vivo recordings of the cephalopod visual system at cellular resolution. In the optic lobe cortex, the brain region receiving input from retinal photoreceptors, we identified distinct neuron classes defined by spatiotemporal tuning and light intensity vs polarization specificity, including (i) integrators of polarized photoreceptor inputs, (ii) neurons selective for a single polarization orientation, and (iii) neurons that subtract orthogonal polarization signals—together suggesting notable parallels with color coding in the vertebrate retina. Neuropixels recordings from downstream visual brain regions revealed evidence for hierarchical processing: receptive fields expanded with depth, and intensity and polarization signals were integrated with progressively greater complexity. Collectively, our study provides new insights into the neural computations underlying cephalopods’ specialized underwater vision and the convergent evolution of visual systems. |
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