mop=twoquditop(op,k1,k2,varargin)

 % twoquditop   Operator acting on two qudits of a qudit register
%   twoquditop(op,k1,k2,n) defines an n-qudit quantum operator
%   which corresponds to operator op acting on the k1th and k2th qudits.
%   Qudit position is interpreted as with reorder.
%   The dimension of the qudit is deduced from the size of op.
%   If n is omitted, then its value is taken to be the value of global
%   variable N. If op is sparse, twoquditop() will also
%   produce a sparse matrix.

function mop=twoquditop(op,k1,k2,varargin)
if isempty(varargin),
    global N;
else
    if length(varargin)~=1,
        error('Wrong number of input arguments.')
    end %if
    N=varargin{1};
end %if

kk1=min(k1,k2);
kk2=max(k1,k2);

[sy,sx]=size(op);
d=sqrt(sx);

% If op is sparse then the result will also be sparse
if issparse(op),
   mop=kron(speye(d^(N-kk1-1)),op);
   mop=kron(mop,speye(d^(kk1-1)));
   mop=swapqudits(mop,kk2,kk1+1);
else   
   mop=kron(eye(d^(N-kk1-1)),op);
   mop=kron(mop,eye(d^(kk1-1)));
   mop=swapqudits(mop,kk2,kk1+1);
end %if