Matrix Chernoff concentration bounds for multipartite soft covering and expander walks

2025-04-05

AI Summary

Classical–quantum soft covering and multi-partite expander graph random walks lack Chernoff-type exponential concentration bounds. Method: We introduce the “bounded removal method,” extending McDiarmid’s inequality to non-independent expander walks. Contribution/Results: We establish, for the first time, a trace-distance concentration bound independent of Hilbert space dimension, tightly linked to smooth Rényi max-divergence. Leveraging matrix Chernoff bounds and expander analysis, we derive a dimension-independent one-shot private classical communication inner bound—applicable to quantum key distribution and quantum wiretap channels with multiple non-cooperating eavesdroppers. Notably, this result is also novel in purely classical settings.

Technology Category

• Machine Learning: Quantum Machine Learning
• Constraint Satisfaction and Optimization: Distributed CSP/Optimization
• Reasoning under Uncertainty: Other Foundations of Reasoning under Uncertainty

Application Category

• Security and Privacy: Large-scale security measurements
• Graph Algorithms and Modeling for the Web: Algorithms and analysis for incomplete, noisy, or partially observed Web-related graphs
• Responsible Web: Measurement, analysis, and circumvention of Web censorship

Abstract

We prove Chernoff style exponential concentration bounds for classical quantum soft covering generalising previous works which gave bounds only in expectation. Our first result is an exponential concentration bound for fully smooth multipartite classical quantum soft covering, extending Ahlswede-Winter's seminal result in several important directions. Next, we prove a new exponential concentration result for smooth unipartite classical quantum soft covering when the samples are taken via a random walk on an expander graph. The resulting expander matrix Chernoff bound complements the results of Garg, Lee, Song and Srivastava in important ways. We prove our new expander matrix Chernoff bound by generalising McDiarmid's method of bounded differences for functions of independent random variables to a new method of bounded excision for functions of expander walks. This new technical tool should be of independent interest. A notable feature of our new concentration bounds is that they have no explicit Hilbert space dimension factor. This is because our bounds are stated in terms of the trace distance of the sample averaged quantum state to the `ideal' quantum state. Our bounds are sensitive to certain smooth Renyi max divergences, giving a clear handle on the number of samples required to achieve a target trace distance. Using these novel features, we prove new one shot inner bounds for sending private classical information over different kinds of quantum wiretap channels with many non-interacting eavesdroppers that are independent of the Hilbert space dimensions of the eavesdroppers. Such powerful results were unknown earlier even in the fully classical setting.

Problem

• Exponential concentration bounds for multipartite quantum soft covering
• Concentration bounds for expander walks in quantum soft covering
• Private classical communication over quantum wiretap channels

Innovation

• Exponential concentration for multipartite soft covering
• Expander matrix Chernoff bound via bounded excision
• Dimension-free trace distance bounds for quantum states