The main focus of my research is to better understand the power and limits of information processing with quantum systems. I also aim to explore new applications of quantum information and new approaches to overcome theatrical challenges in realizing quantum technologies.

I study the capabilities of communication over quantum channels, the theory of quantum entanglement, and the quantum resource theories with applications in quantum information processing. I am also interested in the connections of quantum computation and information to optimization, Markov chains, thermodynamics, and learning theory.

My Ph.D. thesis **Semidefinite Optimization for Quantum Information** (pdf) aims to contribute to the development of quantum Shannon theory, entanglement theory, and zero-error information theory. It explores the fundamental properties of quantum entanglement and establishes efficiently computable approximations for elementary tasks in quantum information theory by using semidefinite optimization, matrix analysis, and information measures.

**Quantum Shannon Theory**

Quantum Shannon theory is the study of the ultimate performance of communication with quantum systems. One of my primary topics is to investigate the communication capabilities of quantum channels under both finite blocklength and asymptotic regime. The asymptotic regime focuses on the ultimate limits of communication, while the finite blocklength regime focuses on a more practical scenario involving only small and medium number of bits or qubits. Good examples of my results in this area are as follows:

- X. Wang, W. Xie, and R. Duan, “
*Semidefinite programming strong converse bounds for classical capacity*,” IEEE Transactions on Information Theory, 64 (1), 640–653, 2018 [link]. (**Contributed talk at QIP 2017**).

This work establishes SDP strong converse rates for channel coding and provides the best known upper bound for the classical capacity of amplitude damping channel. - X. Wang, K. Fang, and R. Duan, “Semidefinite programming converse bounds for quantum communication,” IEEE Transactions on Information Theory (in press, 2018) [link]. (
**Contributed talk at QIP 2018**).

This work determines efficiently computable upper bounds for quantum communication in both the non-asymptotic and asymptotic regime. - X. Wang, K. Fang, and M. Tomamichel, “
*On converse bounds for classical communication over quantum channels*,” arXiv:1709.05258 [link]. (**Contributed talk at QIP 2018**).

This work introduces the constant-bounded subchannels and its application to a new meta-converse on channel coding and establishes a finite resource analysis of quantum erasure channels.

**Quantum Entanglement and Resource Theory**

Quantum entanglement is a key ingredient in many quantum information processing tasks, including teleportation, superdense coding, and quantum cryptography. I am interested in exploring the fundamental structure and the resource theory of entanglement. For example, I demonstrate the irreversibility of asymptotic entanglement manipulation under quantum operations that completely preserve the positivity of partial transpose (PPT), resolving a major **open problem** in quantum information theory.

- X. Wang and R. Duan, “Irreversibility of Asymptotic Entanglement Manipulation Under Quantum Operations Completely Preserving Positivity of Partial Transpose,” Physical Review Letters, 119 (18), 180506, 2017 [link]. (
**Contributed talk at QIP 2017**).

I also work on the quantification and distillation of quantum entanglement. For example, I introduce an improved semidefinite programming upper bound on distillable entanglement and prove the nonadditivity of Rains’ bound.

- X. Wang and R. Duan, “Improved semidefinite programming upper bound on distillable entanglement,” Physical Review A (Rapid Communication), 94 (5), 050301, 2016. (Contributed talk at AQIS 2017).
- X. Wang and R. Duan, “Nonadditivity of Rains’ bound for distillable entanglement,” Physical Review A, 95 (6), 062322, 2017.

**Zero-error Information Theory**

While the ordinary Shannon theory studies the communication with asymptotically vanishing errors, Shannon also investigated the information theory when errors are required to be strictly zero, which is known as the zero-error information theory. In this area, the communication problem reduces to the study of the so-called confusability graph (non-commutative graph) of a classical channel (quantum channel). A good example of my research in this area is proving the separation between the quantum Lovász number and the entanglement-assisted zero-error capacity:

- X. Wang and R. Duan, “Separation Between Quantum Lovász Number and Entanglement-Assisted Zero-Error Classical Capacity,” IEEE Transactions on Information Theory, 64 (3), 1454–1460, 2018 [link]. (Contributed talk at AQIS 2017).

(My detailed research statement is available upon request.)