Communications in Mathematical Physics

, Volume 280, Issue 1, pp 263–280 | Cite as

Gaussian Quantum Marginal Problem

  • Jens Eisert
  • Tomáš Tyc
  • Terry Rudolph
  • Barry C. Sanders
Article

Abstract

The quantum marginal problem asks what local spectra are consistent with a given spectrum of a joint state of a composite quantum system. This setting, also referred to as the question of the compatibility of local spectra, has several applications in quantum information theory. Here, we introduce the analogue of this statement for Gaussian states for any number of modes, and solve it in generality, for pure and mixed states, both concerning necessary and sufficient conditions. Formally, our result can be viewed as an analogue of the Sing-Thompson Theorem (respectively Horn’s Lemma), characterizing the relationship between main diagonal elements and singular values of a complex matrix: We find necessary and sufficient conditions for vectors (d1,..., dn) and (c1,..., cn) to be the symplectic eigenvalues and symplectic main diagonal elements of a strictly positive real matrix, respectively. More physically speaking, this result determines what local temperatures or entropies are consistent with a pure or mixed Gaussian state of several modes. We find that this result implies a solution to the problem of sharing of entanglement in pure Gaussian states and allows for estimating the global entropy of non-Gaussian states based on local measurements. Implications to the actual preparation of multi-mode continuous-variable entangled states are discussed. We compare the findings with the marginal problem for qubits, the solution of which for pure states has a strikingly similar and in fact simple form.

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Copyright information

© Springer-Verlag 2008

Authors and Affiliations

  • Jens Eisert
    • 1
    • 2
    • 3
  • Tomáš Tyc
    • 4
  • Terry Rudolph
    • 2
  • Barry C. Sanders
    • 5
  1. 1.Institute for Mathematical SciencesImperial College LondonLondonUK
  2. 2.Blackett LaboratoryImperial College LondonLondonUK
  3. 3.Physics DepartmentUniversity of PotsdamPotsdamGermany
  4. 4.Institute of Theoretical PhysicsMasaryk UniversityBrnoCzech Republic
  5. 5.Institute for Quantum Information ScienceUniversity of CalgaryAlbertaCanada

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