Unspeakable Coherence Concentration
Benjamin Stratton, Chung-Yun Hsieh, Paul Skrzypczyk

TL;DR
This paper investigates the limits of increasing quantum coherence in multi-qubit systems using coherence non-increasing unitaries, revealing bound coherence and unbounded amplification possibilities, with implications for quantum advantage tasks.
Contribution
It provides a complete solution for qubit coherence enhancement, identifies bound coherence, and establishes fundamental bounds and no-go theorems for multi-qubit systems.
Findings
Optimal unitaries for qubit coherence increase identified
Existence of bound coherence demonstrated
Unbounded coherence amplification possible in certain states
Abstract
Unspeakable coherence is a key feature separating quantum and classical physics. Modelled as asymmetry with respect to a continuous transformation generated by a physically relevant observable, such as the Hamiltonian or angular moment, unspeakable coherence has been shown to be the relevant notion of coherence for achieving quantum advantage in the tasks of metrology, reference frame alignment and work extraction, among others. A question of both practical and foundational value is: Given some copies of a state with low coherence, can we prepare a more coherent state via coherence non-increasing operations? Here, we study this question in the minimal limiting case: Given two uncorrelated copies of a coherent state, can one, via globally coherence non-increasing unitaries, increase the coherence in a subsystem? We fully solve this problem for qubits, identifying the optimal unitaries…
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Taxonomy
TopicsQuantum Information and Cryptography · Quantum Mechanics and Applications · Quantum Computing Algorithms and Architecture
