# Random subspaces of a tensor product (I)

This is the first post in a series about a problem inside RMT  QIT that I have been working on for some time now [cn2,bcn]. Since I find it to be very simple and interesting, I will present it in a series of blog notes that should be accessible to a large audience. I will also use this material to prepare the talks I will be giving this summer on this topic ;).

In what follows, all vector spaces shall be assumed to be complex and  are fixed constants. For a vector , the symbol  denotes its ordered version, i.e.  and  are the same up to permutation of coordinates and .

1. Singular values of vectors in a tensor product

Using the non-canonical isomorphism , one can see any vector

as a matrix

In this way, by using the singular value decomposition of the matrix  (keep in mind that we assume ), one can write

where , resp.  are orthonormal families in , resp. . The vector  is the singular value vector of  and we shall always assume that it is ordered . It satisfies the normalization condition

In particular, if  is a unit vector, then , where  is the probability simplex

and  is its ordered version.

In QIT, the decomposition of  above is called the Schmidt decomposition and the numbers  are called the Schmidt coefficients of the pure state .

2. The singular value set of a vector subspace

Consider now a subspace  of dimension  and define the set

called the singular value subset of the subspace .

Below are some examples of sets , in the case , where the simplex  is two-dimensional. In all the four cases,  and . In the last two pictures, one of the vectors spanning the subspace  has singular values .

3. Basic properties

Below is a list of very simple properties of the sets .

Proposition 1. The set  is a compact subset of the ordered probability simplex  having the following properties:

1. Local invariance: , for unitary matrices  and .
2. Monotonicity: if , then .
3. If , , then .
4. If , then .

Proof: The first three statements are trivial. The last one is contained in [cmw], Proposition 6 and follows from a standard result in algebraic geometry about the dimension of the intersection of projective varieties.

4. So, what is the problem ?

The question one would like to answer is the following:

How does a typical  look like ?

In order to address this, I will introduce random subspaces in the next post future. In the next post, I look at the special case of anti-symmetric tensors.

References

[bcn] S. Belinschi, B. Collins and I. Nechita, Laws of large numbers for eigenvectors and eigenvalues associated to random subspaces in a tensor product, to appear in Invent. Math.

[cn2] B. Collins and I. Nechita, Random quantum channels II: Entanglement of random subspaces, Rényi entropy estimates and additivity problems, Adv. in Math. 226 (2011), 1181--1201.

[cmw] T. Cubitt, A. Montanaro and A. Winter, On the dimension of subspaces with bounded Schmidt rank, J. Math. Phys. 49, 022107 (2008).