# Quantifying Coherence with Untrusted Devices

## TL;DR

This paper investigates the possibility of witnessing and quantifying quantum coherence in single-party systems using untrusted devices, proposing a semi-DI scheme and analytical methods for coherence measures.

## Contribution

It introduces the first semi-DI scheme for quantifying single-party quantum coherence with untrusted devices and provides analytical tools for coherence measures.

## Key findings

- Proves a no-go theorem for fully DI coherence quantification.
- Proposes a semi-DI prepare-and-measure scheme for coherence.
- Develops analytical and numerical methods for coherence quantification.

## Abstract

Device-independent (DI) tests allow to witness and quantify the quantum feature of a system, such as entanglement, without trusting the implementation devices. Although DI test is a powerful tool in many quantum information tasks, it generally requires nonlocal settings. Fundamentally, the superposition property of quantum states, quantified by coherence measures, is a distinct feature to distinguish quantum mechanics from classical theories. In literature, witness and quantification of coherence with trusted devices have been well-studied. However, it remains open whether we can witness and quantify single party coherence with untrusted devices, as it is not clear whether the concept of DI tests exists without a nonlocal setting. In this work, we study DI witness and quantification of coherence with untrusted devices. First, we prove a no-go theorem for a fully DI scenario, as well as a semi DI scenario employing a joint measurement with trusted ancillary states. We then propose a general prepare-and-measure semi DI scheme for witnessing and quantifying the amount of coherence. We show how to quantify the relative entropy and the $l_1$ norm of single party coherence with analytical and numerical methods. As coherence is a fundamental resource for tasks such as quantum random number generation and quantum key distribution, we expect our result may shed light on designing new semi DI quantum cryptographic schemes.