Tom,
The use of sphere packing is a sort of physical argument with respect to qubits. The Planck scale is a limit to the scale where one can identify a qubit. It is not possible to identify quantum information on a scale smaller than the Planck unit. Sphere packing is a configuration where each sphere with a qubit defines a quantum error correction code. The polytope is then a root configuration for the quantum error correction code. The fun group is the F_4 group, which has the root space of the 24-cell. The 24-cell has several representations, where the D_4 or SO(8)xS^3 is an interesting realization. The B_4 decomposition has a bearing on the BFSS M-matrix in M-theory.
If one works up the E_8, the most important thing which comes from this is the Freudenthal cubic system. Take a look at Phillip Gibbs' paper where he works out the cubic elliptic structure for the hyperdeterminant of the 3-qubit system. The 3-qubit system turns out to have a correspondence with BPS black hole types. I discuss that some below. The Freudenthal system is a general Jordan matrix algebraic result for the determinant and trace system of the heterotic group E_8 in a 3x3 matrix system. The hyperdeterminant for the 3-qubit system in the SLOCC group (see below on SLOCC group) defines a cubic form G_abcdψ^aψ^bψ^c = 0, which corresponds to a quadratic matrix M^{ab} = G^{abcd} ψ^bψ^c. This defines an elliptic curve from the hyperdeterminant of the cubic 2x2x2 form G_abcdψ^d
y^2 = det(M) = Det[G_abcdψ^d] Det means hyperdeterminant
This will be modular of course due to the A. Wiles proof of the Tanayama-Shimira conjecture. An explicit realization of this modularity comes from the equivalency with the AdS_n, and in particular with the near horizon condition AdS_{n+2} - -> AdS_2xS^n, which is conformal QM SL(2,R). This is the modular group, or its discrete subgroup SL(2,Z) defines the braid group.
The 4-qubit system describes extremal black holes, or there is a mathematical correspondence. The SLOCC group is a 2x2x2x2 matrix structure, which is entirely different again. This is given by a 4-qubit SLOCC group (see named section below). The M^{ab} matrix above is an SO(4) and the product is contained in SO(8) ~ SO(4,C). The moduli group is then SO(8)/SO(4)xSO(4) as a coset structure. The 4-qubit system is then given by a Cayley hyperdeterminant polynomai of degree 4.
So what does this have to do with my paper? The AdS_n space is the quotient
AdS_n = SO(n-1,2)/SO(n-1,1)
and the AdS_4 near a black hole horizon, assuming a black hole is contained in the spacetime, is AdS_2xS^2. There is the part about Taub-NUT spacetime in my paper, which has a correspondence with the AdS. The spacetime AdS_4xS^7 then has isometries SO(2,3)xSO(8). The S^7 is fixed by the G_2 group on SO(8). The G_2 group is the stabilizer group of F_4 (above) in the E_8. The spin(7) subgroup in SO(8) is then fixes an S^7, spin(7)/G_2 ~ S^7. Now on the 11-dimensional spacetime here we consider the lightcone or infinite momentum gauge to reduce the S^7 to S^6. This is then the Calabi-Yau manifold of compactification.
This CY manifold has an action on the AdS_4, now reduced to the AdS_2xS^2. The CY manifold is the space of Dp-brane wrapping. Duff et al (referenced below) work this for a 0,2,4,6 torus and even number branes. However, this clearly can be generalized. The result is that the electric and magnetic charges define a generator of a partition function with the appropriate BPS charges. The AdS_2 part corresponds to a Taub-NUT charge. This is equivalent to CFT_1, which turns out by Sen to be equivalent to the Hartle-Hawking states of the Wheeler DeWitt equation HΨ[g] = 0 The resulting partition function is an Eisenstein series realization. The BPS charges associated with the BPS - 3-qubit correspondence then construct a partition function. Further, the 3-form version of this is a partition function for the integers.
The F_4 or 24-cell turns out to have under it constant action with G_2 a form of CY manifold structure for h_{1,1}. This then connects back to the system of sphere packing in four dimensions.
This is then an eigenspectrum for the states of the universe. Due to the AdS_2xS^2 structure near a black hole horizon this is also related to the spectrum or counting of microstates on the event horizon of a black hole. A black hole has a set of quantal units of area on its event horizon. These are identified with quantum states, and are a partition of integers for counting the number of states on a black hole horizon. The area of a black hole is composed of little quanta of areas given by a sum of integers n_i >= 0,
A = 4π a(n_1 + n_2 + ... n_m)
where this total number N = n_1 + n_2 + ... n_m can be written according to the integer partition. Another way of thinking about this is that the string modes can exist in a distribution which is an integer parition. This is the holographic principle in action, where the event horizon or stretched horizon is composed of a "gas" of strings.
The density of states for a string is tr(w^N) , which for N = sum_nα_{-n}α_n the string number operator. Given there are 24 string operator the computation of this generating function is tr(w^N) = f(w)^{-24} for
F(w) = Π_{n=1}^∞(1 - w^n)
This is a form of the Dedekind η-function and the remaining calculation leads to a form of the Hardy-Ramanujan approximation for the integer partitions. Recent results by Ono, Brunier, Folsom, and Kent in the role of modular forms in number theory has result in an exact theory for integer partitions. What I am working towards is a string theoretic version of this theorem.
Cheers LC
The 3-qubit SLOCC group:
Based on a paper "Four-qubit entanglement from string theory" L. Borsten, D. Dahanayake, M. J. Duff, A. Marrani, W. Rubens.
In general we have Stochastic Local Operations and Classical Communication (SLOCC) in entanglement and the teleportation of states. Two states are SLOCC related by a teleporation if they can be inter-converted to each other in a reversible manner with some probability of success. This uses group theory, where the group G_{SLOCC} for this process is an N-partite system of q-bits with some group GL(2,C). The states further transform as a (2,2,...,2).
G_{SLOCC} = SL(2,C)_1(x)SL(2,C)_2(x) ... (x)SL(2,C)_N
where the composite state
|ψ_{12...N}> = SL(2,C)_1(x)SL(2,C)_2(x) ... (x)SL(2,C)_N|φ_{12...N}>
So this is an N-partite quantum information system where the entanglements are determined by the group element G_{SLOCC} and polynomials of this group. This is the moduli space for black holes composed of Q-bits and the U-duality group.
For a 2 Q-bit system this construction is apparent. You have a stat of the form sum_{ij}a_{ij}|i,j> for i and j running form 0 to 1. The elements a_{ij} transform as (2,2) of the G_{SLOCC}. The invariant element is the determinant of these matrices so det(a_{ij}) transformed under the G_{SLOCC} into
det(a_{ij}) - -> det(a'_{ij}) = det(U_{i'i}a_{ij}U'*_{j'j}) = det(a_{ij})
with the obvious result on the determinant of a product that the transformation elements have unit determinant. The entanglement entropy is given by this measure so S_{ij} = 4|det(a_{ij})|^2. For multipartite systems the same rule generally applies, but the matrix interpretation is different. For an N-partite system the entanglement entropy is given by a 2x2x...x2 (N times) set of elements. This then leads to the entangled states |00> + |11> and |01> + |10> (without normalization) for singlet and triplet entangled states.
For a 3 q-bit system things are more difficult. The amplitudes are elements a_{ijk} --- a 2x2x2 elements that is not a matrix. We have no diagonalization procedure here. We then have to focus on invariants. The determinant is replaced by a hyperdeterminant that transforms as a (2,2,2), and there are elements σ_i σ_j and σ_k, co-invariants, that transform as (3,1,1) (1,3,1) and (1,1,3) of the G){SLOCC}. So these four then construct entanglement measures S_{ijk}, S_{ij}, S_{ik} and S_{jk}.
The 4-qubit SLOCC
The four Q-bit system is G_{SLOCC} = SL(2,C)^4(four products of SL(2,C)) with 4x3 = 12 complex parameters and the state variable is ψ_{ABCD} and has 2^4 = 16 complex parameters. The entanglement of 4 Q-bits is then 16 - 12 = 4 coset space given by
(C^2)^4/(SL(2,C)^4
Now the entanglement space is not a discrete set as seen with the 3 Q-bit system, but is now given by a continuous set of parameters in this coset group. There are now an infinite number of possibilities. The 4 Q-bit has G_{SLOCC} flows or orbits of the state ψ_{ABCD} SO(4,R) ~ SL(2,C), and we can convert the SL(2,C)^4 into SO(4)^4 and use SO(4,C) = SO(4)^2. The orbits of the G_{SLOCC} are the an SO(8) conjugacy class. The set of nilpotent orbits is a classification of SO(8,C). With extremal black holes the condition is given by these nilpotent orbits on the moduli space. A nilpotent orbit is where there is a group G with algebra g, then for a \in G and b \in g then the adjoint action of a and b is
b - -> b' = aba^{-1}
A nilpotent orbit is given by b^n = bb ...b (n times) and this is stationary as it is clear than b'^n = b^n, and are a fixed point in the moduli space. This Lie group homomorphism SL(2,C)^2 ~ SO(4,C) converts ψ_{ABCD} to a (4,4) of SO(4,C)^2, so then under SL(2,C)^2 ψ_{ABCD} transforms as
ψ_{ABCD} - -> ψ'_{ABCD} = U_A(x)U_B(x)U_C(x)U_D ψ_{ABCD}
= [U_A(x)U_B(x)] ψ_{ABCD}[U_C(x)U_D ψ_{ABCD} ]^T
= M_{AB}ψ_{(AB)(CD)}N^T_{CD}
and is a convenient transformation Mψ N^T = ψ' for U in SL(2,C) and M and N in SO(4,C).