The physical model of non-interacting fermion quantum channel is proposed by fermionic linear optics.Based on covariance matrix property of even Gaussian state and majorization theory,a formula of fermion quantum channel minimum output entropy for even Gaussian input state is deduced.A output entropy correspondence between Gaussian state and even Gaussian state is established by adding one extra fermionic mode to n-qubit fermion system.A formula of tensor product state capacity for non-interacting fermion channel is deduced according this output entropy correspondence and under assumption that minimum output entropy is achieved on a Gaussian input state.For fermionic channel correct of proposed minimum output entropy formula is confirmed by the iterative minimization algorithm of output entropy.The numerical results is that for non-interacting fermion noisy quantum channel the proposed formula agrees with the numerical results with a precision about 10e-9.
The physical model of non-interacting fermion quantum channel is proposed by fermionic linear optics.Based on covariance matrix property of even Gaussian state and majorization theory,a formula of fermion quantum channel minimum output entropy for even Gaussian input state is deduced.A output entropy correspondence between Gaussian state and even Gaussian state is established by adding one extra fermionic mode to n-qubit fermion system.A formula of tensor product state capacity for non-interacting fermion channel is deduced according this output entropy correspondence and under assumption that minimum output entropy is achieved on a Gaussian input state.For fermionic channel correct of proposed minimum output entropy formula is confirmed by the iterative minimization algorithm of output entropy.The numerical results is that for non-interacting fermion noisy quantum channel the proposed formula agrees with the numerical results with a precision about 10e-9.
We study multi-qubit quantum channels that can be represented as a product of one-mode fermionic attenuation channels. An explicit formula for the classical capacity C1 and for the minimum output entropy Smin of these channels is proposed. We compute Smin analytically for any number of qubits under assumption that the minimum is achieved on a Gaussian input. Apart from that, a simple numerical method for evaluating Smin is developed. The method is applicable to any channels that are sufficiently noisy. For fermionic product channels the proposed formula for Smin agrees with the numerical results with a precision about 10−9.
We consider the minimal entropy of qubit states transmitted through two uses of a noisy quantum channel, which is modeled by the action of a completely positive trace-preserving (or stochastic) map. We provide strong support for the conjecture that this minimal entropy is additive, namely, that the minimum entropy can be achieved when product states are transmitted. Explicitly, we prove that for a tensor product of two unital stochastic maps on qubit states, using an entanglement that involves only states which emerge with minimal entropy cannot decrease the entropy below the minimum achievable using product states. We give a separate argument, based on the geometry of the image of the set of density matrices under stochastic maps, which suggests that the minimal entropy conjecture holds for nonunital as well as for unital maps. We also show that the maximal norm of the output states is multiplicative for most product maps on n-qubit states, including all those for which at least one map is unital. For the class of unital channels on C/sup 2/, we show that additivity of minimal entropy implies that the Holevo (see IEEE Trans. Inform. Theory, vol.44, p.269-73, 1998 and Russian Math. Surv., p.1295-1331, 1999) capacity of the channel is additive over two inputs, achievable with orthogonal states, and equal to the Shannon capacity. This implies that superadditivity of the capacity is possible only for nonunital channels.
We introduce a model of a non-Gaussian quantum channel that stems from the composition of two physically relevant processes occurring in open quantum systems, namely, amplitude damping and dephasing. For it we find input states approaching zero output entropy while respecting the input energy constraint. These states fully exploit the infinite dimensionality of the Hilbert space. Upon truncation of the latter, the minimum output entropy remains finite, and optimal input states for such a case are conjectured thanks to numerical evidence.
We find an upper bound for the minimum output entropy of a unital quantum channel, and obtain an exact formula for general qubit channels. Our techniques incorporate the Rényi entropies, particularly, with Rényi parameter α = 2. Moreover, since our upper bound is additive under tensor product, we get as a corollary an upper bound for the classical capacity of unital quantum channels. Interestingly, our upper bound for the classical capacity depends only on the operator norm of matrix representations of channels on the space of traceless Hermitian operators, and is tight in the sense that it gives the precise quantity of classical capacity of the Werner-Holevo channel. As an example, we study quantum channels with operator sum representation that is made of the discrete Weyl operators (generalized Pauli operators), and explain how our formula works in this case. Finally, we find new examples for which the minimum output Rényi 2-entropy is additive.
We investigate non-random quantum channels generated from the representation the- ory of orthogonal quantum groups to perform numerical computations for estimates of their minimum output entropies. In doing so, we address the violation of the additiv- ity conjecture for minimum output entropies, previously established through probabilistic proofs. This violation is relevant because given two quantum channels whose minimum output entropies sum to a value greater than the minimum output entropy of their tensor product channel it can be shown that more information can be sent with greater fidelity via the two channels in parallel than by using each separately. This is directly related to the relevance of quantum entanglement in quantum information theory. \nWe numerically compute minimum output entropies for single quantum channels and tensor products of quantum channels with respect to both Von Neumann and Renyi en- tropies. We then proceed to compute entropy estimates for nonplanar channels directly with Temperley-Lieb Category Theory. We conclude by discussing which of our choices for channels are optimal for entangled inputs.
We study collective state preparation and channel encoding in fermionic systems under parity superselection and composition governed by the canonical anticommutation relations. For a two-mode Gaussian state τ, we obtain the exact entanglement of formation, EFphys(τ⊗m)=⌈m/2⌉log2 for every m≥1. Two copies have the same formation value as one, violating additivity and strong superadditivity, while regularization halves the single-copy value. For an associated Gaussian channel, we determine the minimum output entropy and Holevo information at every blocklength and prove nonadditivity of both. Together, these results give exact answers to the fermionic counterparts of the four additivity questions associated with Shor and Hastings. One physical use carries no classical information, whereas two uses transmit one bit perfectly. The unassisted classical and zero-error classical capacities both equal one half bit per mode. A sharp finite-block bound gives a strong converse: the decoding success probability vanishes at every rate above capacity. These optimizations cover all physical pure-state decompositions and channel inputs, with Gaussian ensembles and classical codes attaining the optima. The formation and minimum output entropy formulas hold for every Rényi order α∈(0,∞]. We also determine the unassisted quantum and zero-error quantum capacities, both equal to one half logical qubit per mode, and prove a strong converse: entanglement fidelity vanishes at every rate above capacity. For a noisy Gaussian family, we determine the one- and two-copy formation values and prove strict nonadditivity at full rank. For τ, Bell violation under local parity-preserving measurements first appears at three copies.