# A quantum retrograde canon: Complete population transfer in $n^{2}$-state systems

## TL;DR

This paper introduces a method to simplify complex multi-state quantum control problems by reducing them to two-state systems, enabling easier analysis and implementation of complete population transfer and quantum gates.

## Contribution

The paper presents a novel analytical reduction technique for multi-state quantum systems to two-state systems, facilitating control and quantum computation applications.

## Key findings

- Reduces $n^{2}$-state systems to two-state systems for control analysis
- Enables complete population transfer in multi-state systems via two-state schemes
- Provides conditions for implementing universal quantum gates

## Abstract

We present a novel approach for analytically reducing a family of time-dependent multi-state quantum control problems to two-state systems. The presented method translates between $SU(2)XSU(2)$ controlled $n^{2}$-state systems and two-state systems, such that the former undergo complete population transfer (CPT) if and only if the latter reach specific states. For even n, the method translates any two-state CPT scheme to CPT schemes in $n^{2}$-state systems. In particular, facilitating CPT in a four-state system via real time-dependent nearest-neighbors couplings is reduced to facilitating CPT in a two-level system. Furthermore, we show that the method can be used for operator control, and provide conditions for producing several universal gates for quantum computation as an example. In addition, we indicate a basis for utilizing the method in optimal control problems.