About

Todd Andrew Brun received his Ph.D. in Physics from Caltech in 1994. Since then, he has worked at the University of London, the Institute for Theoretical Physics in Santa Barbara, Carnegie Mellon University, and the Institute for Advanced Study in Princeton. He joined the faculty of USC in 2003, was promoted to Associate Professor in 2006, and to Full Professor in 2013. His primary appointment is in the Ming Hsieh Department of Electrical and Computer Engineering, with joint appointments in the Department of Computer Science and the Department of Physics and Astronomy. At USC, Prof. Brun has served as Associate Chair of Electrical Engineering, Vice Chair, and Chair of the Engineering Faculty Council, and has also served as a USC Senator from the Viterbi School of Engineering. Outside USC, he has held leadership roles in the American Physical Society's Division of Quantum Information, including Vice Chair, Chair Elect, Chair, and Past Chair. His research focuses on quantum computation and quantum information theory, with particular interest in quantum open systems, decoherence, environmental noise effects on quantum information processing, and quantum error correction. He has conducted recent work on fault-tolerant quantum computation, entanglement-assisted quantum error-correcting codes, quantum walks, continuous measurements, quantum control, orbital angular momentum of photons, weak measurements, quantum mechanics with closed timelike curves, and the arrow of time.

Research topics

• Computer science
• Physics
• Mathematics
• Quantum mechanics
• Statistical physics

Selected publications

• Adaptive Loss-tolerant Syndrome Measurements

  arXiv (Cornell University) · 2026-03-18

  preprintOpen accessSenior author

  In the presence of qubit losses, the building blocks of fault-tolerant error correction (FTEC) must be revisited. Existing loss-tolerant approaches are mainly architecture-specific, and little attention has been given to optimizing the syndrome measurement sequences under loss. Schemes designed for the standard Pauli error model are not directly applicable because the syndrome patterns differ when both Pauli errors and erasures can occur. Based on recent advances in loss detection units and loss-tolerant syndrome extraction gadgets, we extend the study of adaptive Shor-style measurement sequences to the mixed error model. We begin by discussing how to adaptively convert correctable erasures into located errors. The minimal overhead is quantified by the number of stabilizer measurements, which can be reduced to a subgroup dimension problem for erasures arising in any FTEC circuit for qubits and prime-dimensional qudits. As a byproduct, we provide the construction of the canonical generating set with respect to a given bipartite partition for a stabilizer group on qudits of composite dimension. We then generalize both the weak and strong FTEC conditions. Finally, we present adaptive syndrome-measurement protocols for the mixed error model, generalizing the adaptive protocols for the standard Pauli error model.

  PublisherDOI

• Protection of Exponential Operation using Stabilizer Codes in the Early Fault Tolerance Era

  Open MIND · 2026-02-13

  preprintSenior author

  Quantum error correction offers a promising path to suppress errors in quantum processors, but the resources required to protect logical operations from noise, especially non-Clifford operations, pose a substantial challenge to achieve practical quantum advantage in the early fault-tolerant quantum computing (EFTQC) era. In this work, we develop a systematic scheme to encode exponential maps of the form $\exp(-iθP)$ into stabilizer codes with simple circuit structures and low qubit overhead. We provide encoded circuits with small first-order logical error rate after postselection for the [[n, n-2, 2]] quantum error-detecting codes and the [[5, 1, 3]], [[7, 1, 3]], and [[15, 7, 3]] quantum error-correcting codes. Detailed analysis shows that under the level of physical noise of current devices, our encoding scheme is 4--7 times less noisy than the unencoded operation, while at most 3% of runs need to be discarded.

  DOI

• Steering paths mid-flight for fault-tolerance in measurement-based holonomic gates

  ArXiv.org · 2026-03-03

  articleOpen accessSenior author

  Continuous measurement-based holonomic quantum computation provides a route to universal logical computation in quantum error correcting codes. We introduce a fault-tolerant framework for implementing measurement-based holonomic gates that leverages continuous measurements with real-time feedback. We show that non-Markovian decoherence is intrinsically suppressed through the quantum Zeno effect, while Markovian errors are identified by the decoding of measurement records to reveal the rotated syndrome subspace populated during the evolution. This information enables steering holonomic paths mid-flight to ensure that the final evolution realizes the target logical gate. We further demonstrate that non-adiabatic effects give rise to measurement-induced errors, and we show that these can also be corrected by an analogous protocol. This approach relaxes the stringent adiabaticity requirement and enables faster implementation of holonomic gates.

  PublisherOA PDF

• Protection of Exponential Operation using Stabilizer Codes in the Early Fault Tolerance Era

  ArXiv.org · 2026-02-13

  articleOpen accessSenior author

  Quantum error correction offers a promising path to suppress errors in quantum processors, but the resources required to protect logical operations from noise, especially non-Clifford operations, pose a substantial challenge to achieve practical quantum advantage in the early fault-tolerant quantum computing (EFTQC) era. In this work, we develop a systematic scheme to encode exponential maps of the form $\exp(-iθP)$ into stabilizer codes with simple circuit structures and low qubit overhead. We provide encoded circuits with small first-order logical error rate after postselection for the [[n, n-2, 2]] quantum error-detecting codes and the [[5, 1, 3]], [[7, 1, 3]], and [[15, 7, 3]] quantum error-correcting codes. Detailed analysis shows that under the level of physical noise of current devices, our encoding scheme is 4--7 times less noisy than the unencoded operation, while at most 3% of runs need to be discarded.

  PublisherOA PDF

• Steering paths mid-flight for fault-tolerance in measurement-based holonomic gates

  arXiv (Cornell University) · 2026-03-03

  preprintOpen accessSenior author

  Continuous measurement-based holonomic quantum computation provides a route to universal logical computation in quantum error correcting codes. We introduce a fault-tolerant framework for implementing measurement-based holonomic gates that leverages continuous measurements with real-time feedback. We show that non-Markovian decoherence is intrinsically suppressed through the quantum Zeno effect, while Markovian errors are identified by the decoding of measurement records to reveal the rotated syndrome subspace populated during the evolution. This information enables steering holonomic paths mid-flight to ensure that the final evolution realizes the target logical gate. We further demonstrate that non-adiabatic effects give rise to measurement-induced errors, and we show that these can also be corrected by an analogous protocol. This approach relaxes the stringent adiabaticity requirement and enables faster implementation of holonomic gates.

  PublisherDOI

• Universal Weakly Fault-Tolerant Quantum Computation via Code Switching in the [[8,3,2]] Code

  arXiv (Cornell University) · 2026-03-16

  preprintOpen access

  Code-switching offers a route to universal, fault-tolerant quantum computation by circumventing the limitation implied by the Eastin-Knill theorem against a universal transversal gate set within a single quantum code. Here, we present a fault-tolerant code-switching protocol between two versions of the $[[8, 3, 2]]$ code. One version supports weakly fault-tolerant single-qubit Clifford gates, while the other supports a logical $\overline{\mathrm{CCZ}}$ gate via transversal $T/T^\dagger$ together with logical $\overline{\mathrm{CZ}}$, $\overline{\mathrm{CNOT}}$, and $\overline{\mathrm{SWAP}}$ gates. Because both codes have distance 2, the protocol operates in a postselected, error-detecting regime: single faults lead to detectable outcomes, and accepted runs exhibit quadratic suppression of logical error rates. This yields a universal scheme for postselected fault-tolerant computation. We validate the protocol numerically through simulations of state preparation, code switching, and a three-logical-qubit implementation of Grover's search.

  PublisherDOI

• Continuous quantum correction on Markovian and non-Markovian models

  Physical review. A/Physical review, A · 2026-01-07

  articleOpen accessSenior author

  We investigate continuous quantum error correction, comparing performance under a Markovian error model to two distinct non-Markovian models. The first non-Markovian model involves an interaction Hamiltonian between the system and an environmental qubit via an X-X coupling, with a "cooling" bath acting on the environment qubit. This model is known to exhibit abrupt transitions between Markovian and non-Markovian behavior. The second non-Markovian model uses the post-Markovian master equation (PMME), which represents the bath correlation through a memory kernel; we consider an exponentially decaying kernel and both underdamped and overdamped dynamics. We systematically compare these non-Markovian error models against the Markovian case and against each other, for a variety of different codes. We start with a single qubit, which can be solved analytically. We then consider the three-qubit repetition code and the five-qubit "perfect" code. In all cases, we find that the fidelity decays more rapidly in the Markovian case than in either non-Markovian model, suggesting that continuous quantum error correction has enhanced performance against non-Markovian noise. We attribute this difference to the presence of a quantum Zeno regime in both non-Markovian models.

  PublisherOA PDFDOI

• Universal Weakly Fault-Tolerant Quantum Computation via Code Switching in the [[8,3,2]] Code

  arXiv (Cornell University) · 2026-03-16

  articleOpen access

  Code-switching offers a route to universal, fault-tolerant quantum computation by circumventing the limitation implied by the Eastin-Knill theorem against a universal transversal gate set within a single quantum code. Here, we present a fault-tolerant code-switching protocol between two versions of the $[[8, 3, 2]]$ code. One version supports weakly fault-tolerant single-qubit Clifford gates, while the other supports a logical $\overline{\mathrm{CCZ}}$ gate via transversal $T/T^\dagger$ together with logical $\overline{\mathrm{CZ}}$, $\overline{\mathrm{CNOT}}$, and $\overline{\mathrm{SWAP}}$ gates. Because both codes have distance 2, the protocol operates in a postselected, error-detecting regime: single faults lead to detectable outcomes, and accepted runs exhibit quadratic suppression of logical error rates. This yields a universal scheme for postselected fault-tolerant computation. We validate the protocol numerically through simulations of state preparation, code switching, and a three-logical-qubit implementation of Grover's search.

  PublisherOA PDF

• Adaptive Loss-tolerant Syndrome Measurements

  ArXiv.org · 2026-03-18

  articleOpen accessSenior author

  In the presence of qubit losses, the building blocks of fault-tolerant error correction (FTEC) must be revisited. Existing loss-tolerant approaches are mainly architecture-specific, and little attention has been given to optimizing the syndrome measurement sequences under loss. Schemes designed for the standard Pauli error model are not directly applicable because the syndrome patterns differ when both Pauli errors and erasures can occur. Based on recent advances in loss detection units and loss-tolerant syndrome extraction gadgets, we extend the study of adaptive Shor-style measurement sequences to the mixed error model. We begin by discussing how to adaptively convert correctable erasures into located errors. The minimal overhead is quantified by the number of stabilizer measurements, which can be reduced to a subgroup dimension problem for erasures arising in any FTEC circuit for qubits and prime-dimensional qudits. As a byproduct, we provide the construction of the canonical generating set with respect to a given bipartite partition for a stabilizer group on qudits of composite dimension. We then generalize both the weak and strong FTEC conditions. Finally, we present adaptive syndrome-measurement protocols for the mixed error model, generalizing the adaptive protocols for the standard Pauli error model.

  PublisherOA PDF

• Continuous measurement-based holonomic quantum computation

  ArXiv.org · 2025-10-08

  preprintOpen accessSenior author

  We propose a scheme to generate holonomies using the Quantum Zeno effect, enabling logical unitary operations on quantum stabilizer codes purely through measurements. The quantum error-correcting code space is adiabatically rotated by measuring a succession of rotated stabilizer generators. When the rotation is sufficiently slow, the state remains confined to the instantaneous code space by the Zeno effect; otherwise, measurement-induced jumps can occur into a rotated orthogonal subspace. If the rotation completes a closed loop, the code state is transformed by a holonomy: a logical unitary transformation. We analytically derive the sequence of rotated stabilizer generators that produce a desired holonomy, and find the total time required to implement this procedure with a given success probability. If a measurement moves the state to the orthogonal subspace, we present a method to alter the path of the rotated observables to return the state either to the original code or the original error space with the desired holonomy; in the latter case, the holonomy is emulated. Finally, we establish conditions on the code and the measured observables that preserve the correctability of a given error set. When a code fails to meet the error-correcting conditions, our protocol remains applicable by augmenting the code with at most two ancilla qubits.

  PublisherOA PDFDOI

Recent grants

• CAREER: Realistic Models and Simulations of Systems for Quantum Information Processing

  NSF · $400k · 2005–2011

• FET: Small: Weak and Continuous Quantum Measurements with Feedback

  NSF · $490k · 2019–2023

• Quantum Walks and Weak Measurements

  NSF · $300k · 2008–2013

• QnTM: Weak Local Measurements, Entanglement Monotones, and Random Walks

  NSF · $150k · 2005–2008

• SHF: Small: Fault-Tolerant Quantum Computation in Multi-Qubit Block Codes

  NSF · $478k · 2014–2018

Frequent coauthors

• Ching–Yi Lai

  31 shared

• Daniel A. Lidar

  29 shared

• Mark M. Wilde

  Cornell University

  28 shared

• Yi-Cong Zheng

  Tencent (China)

  25 shared

• Shengshi Pang

  University of Science and Technology of China

  20 shared

• Min-Hsiu Hsieh

  15 shared

• Hari Krovi

  14 shared

• Ognyan Oreshkov

  13 shared

Education

• Ph.D., Computer Science

  University of Southern California

  1990

• M.S., Computer Science

  University of Southern California

  1986

• B.S., Computer Science

  University of Southern California

  1984

Awards & honors

• 2008 IEEE Senior Member
• 2005 NSF CAREER award
• 2001 IAS Chooljian Member for Natural Sciences