Abstract
The entanglement properties of random pure states are relevant to a variety of problems ranging from chaotic quantum dynamics to black hole physics. The averaged bipartite entanglement entropy of such states admits a volume law and upon increasing the subregion size follows the Page curve. In this paper, we generalize this setup to random mixed states by coupling the system to a bath and use the partial transpose to study their entanglement properties. We develop a diagrammatic method to incorporate partial transpose within random matrix theory and formulate a perturbation theory in $1/L$, the inverse of the Hilbert space dimension. We compute several quantities including the spectral density of partial transpose (or entanglement negativity spectrum), two-point correlator of eigenvalues, and the logarithmic negativity. As long as the bath is smaller than the system, we find that upon sweeping the subregion size, the logarithmic negativity shows an initial increase and a final decrease similar to the Page curve, while it admits a plateau in the intermediate regime where the logarithmic negativity only depends on the size of the system and of the bath but not on how the system is partitioned. This intermediate phase has no analog in random pure states, and is separated from the two other regimes by a critical point. We further show that when the bath is larger than the system by at least two extra qubits the logarithmic negativity is identically zero which implies that there is no distillable entanglement. Using the diagrammatic approach, we provide a simple derivation of the semi-circle law of the entanglement negativity spectrum in the latter two regimes. We show that despite the appearance of a semicircle distribution, reminiscent of Gaussian unitary ensemble (GUE), the higher order corrections to the negativity spectrum and two-point correlator deviate from those of GUE.
Highlights
Dynamics of strongly interacting quantum systems is an interdisciplinary research frontier across various fields of physics including quantum computation, condensed matter, and high-energy physics
We note that the last term is absent in the negative partial transpose (NPT)-positive partial transpose (PPT) transition point calculated from the semicircle law, this difference is negligible in the thermodynamic limit N → ∞
We investigated the entanglement properties of random mixed states through the window of the partial transpose and logarithmic negativity
Summary
Dynamics of strongly interacting quantum systems is an interdisciplinary research frontier across various fields of physics including quantum computation, condensed matter, and high-energy physics. Our main results in this paper are summarized in the phase diagram of Fig. 2, which is obtained analytically in the thermodynamic limit where the total number of qubits NA + NB are infinitely large while each subsystem contains a finite portion of the full system [33] This 2d phase diagram can be thought of as an extension of the 1d phase diagram of Page states, which corresponds to the horizontal line in the bottom with ρA pure, by adding a vertical axis to parameterize how mixed ρA is.
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