# Charles Bennett

> 1943– · Physicist
>
> **Recorded contribution:** Reversible computing; quantum teleportation; quantum cryptography; Maxwell's demon resolution

## How to use this dossier

Read for a causal chain, not a hero story: inherited problem → contribution → mechanism → downstream capability → limit. Then close the page and complete the reconstruction exercise from memory.

## 1. Historical orientation

Charles Bennett (born 1943), a physicist at IBM Research, showed how computation can be performed reversibly and became a founder of quantum information science. Building on Landauer's link between information erasure and heat, Bennett demonstrated that an irreversible computation can retain its intermediate history, copy the desired result, and run backward to clean its workspace. With Gilles Brassard he created the BB84 quantum key-distribution protocol; with collaborators he developed quantum teleportation and information measures. None of these results is solitary, and teleportation moves a quantum state using entanglement plus classical communication—not matter or faster-than-light information.

## 2. The problem inherited

Ordinary computations erase intermediate information and appear thermodynamically irreversible, while quantum states require communication mechanisms consistent with measurement and no-cloning constraints.

## 3. The central contribution

Bennett proved that logically reversible simulation can recover temporary information and helped establish quantum information as a resource governed by operational protocols.

## 4. Reconstruct the mechanism

1. Compute forward with reversible steps while recording enough history to distinguish every predecessor state.
2. Copy the final classical result into a clean output register without destroying the computed state.
3. Run the computation backward to restore history and workspace registers to their initial values.
4. Retain only input and copied output, avoiding logical erasure inside the idealized computation.

## 5. What changed downstream

- Reversible computation became foundational to quantum circuit design and low-energy limits.
- BB84 launched practical and theoretical quantum cryptography.
- Teleportation and entanglement studies made information an organizing concept in quantum physics.

## 6. Attribution, limits, and uncertainty

- Landauer, Brassard, Wiesner, Fredkin, Toffoli, and many quantum-information collaborators have essential priority.
- Reversible logic does not remove heat from noisy physical control, correction, or finite-speed devices.
- Quantum key distribution shifts trust to devices, authentication, and implementations and does not solve all security problems.

## 7. Reconstruction lab

Turn an irreversible AND operation into a reversible circuit with retained inputs and an ancilla. Copy the answer, uncompute garbage, and count remaining bits; then identify where a real implementation still dissipates energy.

## 8. Evidence trail

- [Logical Reversibility of Computation](https://doi.org/10.1147/rd.176.0525) — IBM Journal of Research and Development
- [Charles H. Bennett](https://research.ibm.com/people/charles-h-bennett) — IBM Research

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*Research checked 2026-08-09. Dates, roles, and claims about living people are historical snapshots. Linked sources remain the authority; this dossier is original instructional synthesis.*