Tight informationally complete quantum measurements

We introduce a class of informationally complete positive-operator-valued measures which are, in analogy with a tight frame, "as close as possible" to orthonormal bases for the space of quantum states. These measures are distinguished by an exceptionally simple state-reconstruction formula which allows "painless" quantum state tomography. Complete sets of mutually unbiased bases and symmetric informationally complete positive-operator-valued measures are both members of this class, the latter being the unique minimal rank-one members. Recast as ensembles of pure quantum states, the rank-one members are in fact equivalent to weighted 2-designs in complex projective space. These measures are shown to be optimal for quantum cloning and linear quantum state tomography.Comment: 20 pages. Final versio

O polinomima u algebrama Maljceva

We establish several properties of higher commutators, which were introduced by A. Bulatov, in congruence permutable varieties. We use these commutators to prove that the clone of polynomial functions of a finite Mal’cev algebra whose congruence lattice is of height at most 2, can be described by a finite set of relations. For a finite nilpotent algebra of finite type that is a product of algebras of prime power order and generates congruence modular variety, we are able to show that the property of affine completeness is decidable. Moreover, polynomial equivalence problem has polynomial complexity in the length of the input polynomials.Ustanovljavamo osobine viˇsih komutatora, koje je uveo A. Bulatov, u kongruencijki permutabilnim varijetetima. Te komutatore koristimo da bi dokazali da se klon polinomijalnih funkcija konaˇcne Maljcevljeve algebre ˇcija je mreˇza kongruencija visine najviˇse dva moˇze opisati konaˇcnim skupom relacija. Za konaˇcne nilpotentne algebre konaˇcnog tipa koje su proizvod algebri koje imaju red stepena prostog broja i koje generiˇsu kongruencijki modularan varijetet pokazu-jemo da je osobina afine kompletnosti odluˇciva. Takod¯e, pokazujemo za istu klasu da problem polinomijalne ekvivalencije ima polinomnu sloˇzenost u zavisnosti od duˇzine unetih polinomijalnih terma

Multicoloured Random Graphs: Constructions and Symmetry

This is a research monograph on constructions of and group actions on countable homogeneous graphs, concentrating particularly on the simple random graph and its edge-coloured variants. We study various aspects of the graphs, but the emphasis is on understanding those groups that are supported by these graphs together with links with other structures such as lattices, topologies and filters, rings and algebras, metric spaces, sets and models, Moufang loops and monoids. The large amount of background material included serves as an introduction to the theories that are used to produce the new results. The large number of references should help in making this a resource for anyone interested in beginning research in this or allied fields.Comment: Index added in v2. This is the first of 3 documents; the other 2 will appear in physic