Ryu–Takayanagi conjecture
The Ryu–Takayanagi conjecture is a conjecture within holography that posits a quantitative relationship between the entanglement entropy of a conformal field theory and the geometry of an associated anti-de Sitter spacetime. The formula characterizes "holographic screens" in the bulk; that is, it specifies which regions of the bulk geometry are "responsible to particular information in the dual CFT". The conjecture is named after and , who jointly published the result in 2006. As a result, the authors were awarded the 2015 New Horizons in Physics Prize for "fundamental ideas about entropy in quantum field theory and  quantum gravity". The formula was generalized to a covariant form in 2007. Motivation. The thermodynamics of black holes suggests certain relationships between the entropy of black holes and their geometry. Specifically, the Bekenstein–Hawking area formula conjectures that the entropy of a black hole is proportional to its surface area: The Bekenstein–Hawking entropy formula_2 is a measure of the information lost to external observers due to the presence of the horizon. The horizon of the black hole acts as a "screen" distinguishing one region of the spacetime (in this case the exterior of the black hole) that is not  affected by another region (in this case the interior). The Bekenstein–Hawking area law states that the area of this surface is proportional to the entropy of the information lost behind it. The Bekenstein–Hawking entropy is a statement about the gravitational entropy of a system; however, there is another type of entropy that is important in quantum information theory, namely the entanglement (or von Neumann) entropy. This form of entropy provides a measure of how far from a pure state a given quantum state is, or, equivalently, how entangled it is. The entanglement entropy is a useful concept in  many areas, such as in condensed matter physics and quantum many-body systems. Given its use, and its suggestive similarity to the Bekenstein–Hawking entropy, it is desirable to have a holographic description of entanglement entropy in terms of gravity. Holographic preliminaries. The holographic principle states that gravitational theories in a given dimension are dual to a gauge theory in one lower dimension. The AdS/CFT correspondence is one example of such duality. Here, the field theory is defined on a fixed background and is equivalent to a quantum gravitational theory whose different states each correspond to a possible spacetime  geometry. The conformal field theory is often viewed as living on the boundary of the higher dimensional space whose gravitational theory it defines. The result of such a duality is a dictionary between the two equivalent descriptions. For example, in a CFT defined on formula_3 dimensional Minkowski space the vacuum state corresponds to pure AdS space, whereas the thermal state corresponds to a planar black hole. Important for the present discussion is that the thermal state of a CFT defined on the formula_3 dimensional sphere corresponds to the formula_5 dimensional Schwarzschild black hole in AdS space. The Bekenstein–Hawking area  law, while claiming that the area of the black hole horizon is proportional to the black hole's entropy, fails to provide a sufficient microscopic description of how this entropy arises. The holographic principle provides such a description by relating the black hole system to a quantum system which does admit such a microscopic description. In this case, the CFT has discrete eigenstates and the thermal state is the canonical ensemble of these states. The entropy of this ensemble can be calculated through normal means, and yields the same result as predicted by the area law. This turns out to  be a special case of the Ryu–Takayanagi conjecture. Conjecture. Consider a spatial slice formula_6 of an AdS space time on whose boundary we define the dual CFT. The Ryu–Takayanagi formula states: where formula_7 is the entanglement entropy of the CFT in some spatial sub-region formula_8 with its complement formula_9, and formula_10 is the Ryu–Takayanagi surface in the bulk. This surface must satisfy three properties: Because of property (3), this surface is typically called the "minimal surface" when the context is clear. Furthermore, property (1) ensures that the formula preserves certain features of entanglement entropy, such as  formula_16 and formula_17. The conjecture provides an explicit geometric interpretation of the entanglement entropy of the boundary CFT, namely as the area of a surface in the bulk. Example. In their original paper, Ryu and Takayanagi show this result explicitly for an example in formula_18 where an expression for the entanglement entropy is already known. For an formula_19 space of radius formula_20, the dual CFT has a central charge given by Furthermore, formula_21 has the metric formula_22 in formula_23 (essentially a stack of hyperbolic disks). Since this metric diverges at formula_24, formula_25 is restricted to formula_26. This act of imposing  a maximum formula_25 is analogous to the corresponding CFT having a UV cutoff. If formula_28 is the length of the CFT system, in this case the circumference of the cylinder calculated with the appropriate metric, and formula_29 is the lattice spacing, we have formula_30. In this case, the boundary CFT lives at coordinates formula_31. Consider a fixed formula_32 slice and take the subregion A of the boundary to be formula_33 where formula_34 is the length of formula_12. The minimal surface is easy to identify in this case, as it is just the geodesic through the bulk that connects formula_36 and  formula_37. Remembering the lattice cutoff, the length of the geodesic can be calculated as If it is assumed that formula_38, then using the Ryu–Takayanagi formula to compute the entanglement entropy. Plugging in the length of the minimal surface calculated in () and recalling the central charge charge (), the entanglement entropy is given by This agrees with the result calculated by usual means.