### Abstract

A bosonic state is commonly considered nonclassical (or quantum) if its Glauber-Sudarshan P function is not a classical probability density, which implies that only coherent states and their statistical mixtures are classical. We quantify the nonclassicality of a single qubit, defined by the vacuum and single-photon states, by applying the following four well-known measures of nonclassicality: (1) the nonclassical depth, τ, related to the minimal amount of Gaussian noise which changes a nonpositive P function into a positive one; (2) the nonclassical distance D, defined as the Bures distance of a given state to the closest classical state, which is the vacuum for the single-qubit Hilbert space; together with (3) the negativity potential (NP), and (4) concurrence potential, which are the nonclassicality measures corresponding to the entanglement measures (i.e., the negativity and concurrence, respectively) for the state generated by mixing a single-qubit state with the vacuum on a balanced beam splitter. We show that complete statistical mixtures of the vacuum and single-photon states are the most nonclassical single-qubit states regarding the distance D for a fixed value of both the depth τ and NP in the whole range [0,1] of their values, as well as the NP for a given value of τ such that τ>0.3154. Conversely, pure states are the most nonclassical single-qubit states with respect to τ for a given D, NP versus D, and τ versus NP. We also show the "relativity" of these nonclassicality measures by comparing pairs of single-qubit states: if a state is less nonclassical than another state according to some measure then it might be more nonclassical according to another measure. Moreover, we find that the concurrence potential is equal to the nonclassical distance for single-qubit states. This implies an operational interpretation of the nonclassical distance as the potential for the entanglement of formation.

Original language | English |
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Article number | 042309 |

Journal | Physical Review A - Atomic, Molecular, and Optical Physics |

Volume | 91 |

Issue number | 4 |

DOIs | |

Publication status | Published - 2015 Apr 8 |

### All Science Journal Classification (ASJC) codes

- Atomic and Molecular Physics, and Optics

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## Cite this

*Physical Review A - Atomic, Molecular, and Optical Physics*,

*91*(4), [042309]. https://doi.org/10.1103/PhysRevA.91.042309