# Markovian and non-Markovian quantum measurements

## TL;DR

This paper demonstrates that unamplified quantum measurements retain a memory of prior states, forming non-Markovian chains, while amplified measurements behave as Markov chains, revealing new insights into quantum measurement dynamics.

## Contribution

It shows that unamplified measurements preserve quantum memory and form non-Markovian chains, contrasting with amplified measurements which form Markov chains, advancing understanding of quantum measurement processes.

## Key findings

- Unamplified measurements encode the entire measurement history in ancilla density matrices.
- Non-Markovian quantum chains can be reconstructed from boundary information.
- Amplified measurements are equivalent to quantum Markov chains.

## Abstract

Consecutive measurements performed on the same quantum system can reveal fundamental insights into quantum theory's causal structure, and probe different aspects of the quantum measurement problem. According to the Copenhagen interpretation, measurements affect the quantum system in such a way that the quantum superposition collapses after the measurement, erasing any knowledge of the prior state. We show here that counter to this view, unamplified measurements (measurements where all variables comprising a pointer are controllable) have coherent ancilla density matrices that encode the memory of the entire set of quantum measurements, and that the quantum chain of a set of consecutive unamplified measurements is non-Markovian. In contrast, sequences of amplified measurements (measurements where at least one pointer variable has been lost) are equivalent to a quantum Markov chain. An analysis of arbitrary non-Markovian quantum chains of measurements reveals that all of the information necessary to reconstruct the chain is encoded on its boundary (the state preparation and the final measurement), reminiscent of the holographic principle.