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r+4/bardbler/an}bracketri}ht/an}bracketri}ht/an}bracketle{t/an}bracketle{ter/bardbl (267)
Tr=d
d+1Qr+2d
d+1/bardbler/an}bracketri}ht/an}bracketri}ht/an}bracketle{t/an}bracketle{ter/bardbl (268)
(see Section 3). As we noted previously, it is possible to define everything in terms
of the adjoint representation matrices Jrand the rank-1 projectors /bardbler/an}bracketri}ht/an}bracketri}ht/an}bracketle{t/an}bracketle{ter/bardbl:
Qr=1
2Jr(Jr+I) (269)
¯Rr=J2
r (270)
Rr=J2
r+4/bardbler/an}bracketri}ht/an}bracketri}ht/an}bracketle{t/an}bracketle{ter/bardbl (271)
Tr=d
2(d+1)Jr(Jr+I)+2d
d+1/bardbler/an}bracketri}ht/an}bracketri}ht/an}bracketle{t/an}bracketle{ter/bardbl (272)
In that sense the structure constants of the Lie algebra, supple mented with the
vectors/bardbler/an}bracketri}ht/an}bracketri}ht, determine everything else.
In the next section we will show that there are some interesting geo metrical
relationships between the hyperplanes onto which Qr,QT
rand¯Rrproject. In this
section, as a preliminary to that investigation, we prove a number of identities
satisfied by the Q,Jand¯Rmatries. We start by computing their Hilbert-Schmidt
inner products:
Theorem 13. For allr,s
Tr/parenleftbig
QrQs/parenrightbig
=d3δrs+d2−d−1
(d+1)2(273)
Tr/parenleftbig
QrQT
s/parenrightbig
=d2(1−δrs)
(d+1)2(274)
Tr/parenleftbig
JrJs/parenrightbig
=2(d2δrs−1)
d+1(275)
Tr/parenleftbig¯Rr¯Rs/parenrightbig
=2(d−1)(d2δrs+2d+1)
(d+1)2(276)
Tr/parenleftbig
Jr¯Rs/parenrightbig
= 0 (277)
Proof.Let us first calculate some auxiliary quantities. It follows from the de finition
ofTr, andthe factthat the matrix P=1
dGdefined byEq.( 63) isarankdprojector,
that
Tr(TrTs) =d2/summationdisplay
u,v=1TruvTsvu34
=d2/summationdisplay
u,v=1K2
uvGruGusGsvGvr
=d
d+1d2/summationdisplay
u=1K2
ruK2
su+d4
d+1d2/summationdisplay
u,v=1PruPusPsvPvr
=d2(dδrs+d+2)
(d+1)3+d4
d+1/vextendsingle/vextendsinglePrs/vextendsingle/vextendsingle2
=d2(dδrs+d+2)
(d+1)3+d2
d+1K2
rs
=d2/parenleftbig
d(d+2)δrs+2d+3/parenrightbig
(d+1)3(278)
Also
Tr/parenleftbig
TrTT
s/parenrightbig
=d2/summationdisplay
u,v=1TruvTsuv
=d2/summationdisplay
u=1GruGsu
d2/summationdisplay
v=1GuvGuvGvrGvs
=2d
d+1d2/summationdisplay
u=1GruGsuGurGus
=2d2
(d+1)2/parenleftbig
1+K2
rs/parenrightbig
=2d2(dδrs+d+2)
(d+1)3(279)
where we made two applications of Eq. ( 23) (i.e.the fact that every SIC-POVM is
a 2-design). Finally, it is a straightforward consequence of the defi nitions ofTr,TT
r
and/bardbler/an}bracketri}ht/an}bracketri}htthat
/an}bracketle{t/an}bracketle{ter/bardblTs/bardbler/an}bracketri}ht/an}bracketri}ht=/an}bracketle{t/an}bracketle{ter/bardblTT
s/bardbler/an}bracketri}ht/an}bracketri}ht
=d+1
2dd2/summationdisplay
u,v=1TsuvK2
ruK2
rv