Sanjay Seshan

M Mahmudul Hasan Sajeeb, Kevin Callahan-Coray, Corentin Delacour et al.

Probabilistic computers built from p-bits offer a promising path for combinatorial optimization, but the dense connectivity required by real-world problems scales poorly in hardware. Here, we address this through graph sparsification with auxiliary copy variables and demonstrate two fully on-chip parallel tempering solvers on an FPGA. Targeting MIMO detection, a dense, NP-hard problem central to wireless communications, we first fit 11 temperature replicas of a 128-node sparsified system (1,408 p-bits) on-chip and achieve bit error rates significantly below conventional linear detectors on $64 \times 64$ BPSK MIMO. We report complete end-to-end solution times of 3~ms per instance, including all loading, sampling, readout, and verification overheads. ASIC projections in 7~nm technology indicate 103~MHz operation at 285.8~mW, suggesting that massive parallelism across multiple chips could approach the throughput demands of next-generation wireless systems. Sparsification, however, introduces a sharp sensitivity to the copy-constraint strength $P$ that requires manual tuning. To eliminate this bottleneck, we utilize Two-Dimensional Parallel Tempering (2D-PT), which exchanges replicas across both temperature ($β$) and constraint ($P$) dimensions. On Sherrington--Kirkpatrick spin glasses, 2D-PT converges roughly $250\times$ faster than optimally tuned 1D-PT, and on $128 \times 128$ MIMO it reaches zero bit errors at high SNR where 1D-PT exhibits an error floor. We further validate 2D-PT entirely on-chip with 54 replicas (1,728 p-bits) on a $16 \times 16$ MIMO instance, where it tracks the maximum-likelihood bound in just 50 Monte Carlo steps -- $10\times$ fewer than 1D-PT -- at projected 111~MHz and 124~mW in 7~nm. Together, these results establish an on-chip p-bit architecture and a scalable, tuning-free algorithmic framework for dense combinatorial optimization.

Ziyad Alswaidan, Abdelrahman S. Abdelrahman, Md Sakibur Sajal et al.

Combinatorial optimization problems are central to science and engineering and specialized hardware from quantum annealers to classical Ising machines are being actively developed to address them. These systems typically sample from a fixed energy landscape defined by the problem Hamiltonian encoding the discrete optimization problem. The recently introduced Probabilistic Approximate Optimization Algorithm (PAOA) takes a different approach: it treats the optimization landscape itself as variational, iteratively learning circuit parameters from samples. Here, we demonstrate PAOA on a 64$\times$64 perimeter-gated single-photon avalanche diode (pgSPAD) array fabricated in 0.35 $μ$m CMOS, the first realization of the algorithm using intrinsically stochastic nanodevices. Each p-bit exhibits a device-specific, asymmetric (Gompertz-type) activation function due to dark-count variability. Rather than calibrating devices to enforce a uniform symmetric (logistic/tanh) activation, PAOA learns around device variations, absorbing residual activation and other mismatches into the variational parameters. On canonical 26-spin Sherrington-Kirkpatrick instances, PAOA achieves high approximation ratios with $2p$ parameters ($p$ up to 17 layers), and pgSPAD-based inference closely tracks CPU simulations. These results show that variational learning can accommodate the non-idealities inherent to nanoscale devices, suggesting a practical path toward larger-scale, CMOS-compatible probabilistic computers.

Sanjay Seshan

The goal of this project is to create an inexpensive, lightweight, wearable assistive device that can measure hand or finger movements accurately enough to identify a range of hand gestures. One eventual application is to provide assistive technology and sign language detection for the hearing impaired. My system, called LiTe (Light-based Technology), uses optical fibers embedded into a wristband. The wrist is an optimal place for the band since the light propagation in the optical fibers is impacted even by the slight movements of the tendons in the wrist when gestures are performed. The prototype incorporates light dependent resistors to measure these light propagation changes. When creating LiTe, I considered a variety of fiber materials, light frequencies, and physical shapes to optimize the tendon movement detection so that it can be accurately correlated with different gestures. I implemented and evaluated two approaches for gesture recognition. The first uses an algorithm that combines moving averages of sensor readings with gesture sensor reading signatures to determine the current gesture. The second uses a neural network trained on a labelled set of gesture readings to recognize gestures. Using the signature-based approach, I was able to achieve a 99.8% accuracy at recognizing distinct gestures. Using the neural network the recognition accuracy was 98.8%. This shows that high accuracy is feasible using both approaches. The results indicate that this novel method of using fiber optics-based sensors is a promising first step to creating a gesture recognition system.

Sanjay Seshan

Studies predict that demand for autonomous vehicles will increase tenfold between 2019 and 2026. However, recent high-profile accidents have significantly impacted consumer confidence in this technology. The cause for many of these accidents can be traced back to the inability of these vehicles to correctly sense the impending danger. In response, manufacturers have been improving the already extensive on-vehicle sensor packages to ensure that the system always has access to the data necessary to ensure safe navigation. However, these sensor packages only provide a view from the vehicle's perspective and, as a result, autonomous vehicles still require frequent human intervention to ensure safety. To address this issue, I developed a system, called Horus, that combines on-vehicle and infrastructure-based sensors to provide a more complete view of the environment, including areas not visible from the vehicle. I built a small-scale experimental testbed as a proof of concept. My measurements of the impact of sensor failures showed that even short outages (1 second) at slow speeds (25 km/hr scaled velocity) prevents vehicles that rely on on-vehicle sensors from navigating properly. My experiments also showed that Horus dramatically improves driving safety and that the sensor fusion algorithm selected plays a significant role in the quality of the navigation. With just a pair of infrastructure sensors, Horus could tolerate sensors that fail 40% of the time and still navigate safely. These results are a promising first step towards safer autonomous vehicles.