An elementary introduction to the geometry of quantum states with a picture book

inNdimensions

C=

{

x

∣

∣

∣x= (x 1 ,...,xN), |xj|≤

1

2

, j= 1,...,N

}

(B.1)

The radius of inertia ofCis

re^2 =E

(

x^2

)

=

∑

E

(

x^2 j

)

=N

∫ 1 / 2

− 1 / 2

x^2 dx=

N

12

(B.2)

Let us now considerr(θ), defined as the maximalrthat is inside the cube for a
given directionθ.
Choose a random directionθ= (θ 1 ,...,θN) by pickingθjto be normal iid with

θj∼N

[

0 ,

1

N

]

, xj=rθj (B.3)

WhenN is large there is a “phase transition” in the sense that

Prob

(

rθ∈C

)

≈

{

1 r < rC

0 r > rC

, rC=

1

2

√

N

logN

(B.4)

To see this we first observe that the probability thatx∼N(0,σ^2 ) takes values
outside the interval [−x 0 ,x 0 ] is given by the complementary error function

Prob(|x|> x 0 ) =

√

2

π

∫∞

x 0 /σ

e−x

(^2) / 2
dx= erfc(x 0 /σ)
Anticipating the result, Eq. (B.4), let us replacerby its re-scaled versionk:
r=krC (B.5)
For the case at hand
σ^2 = 1/N, x 0 =

1

2 r

=

1

2 krC
The probability forr|θj|=|xj|≤ 1 /2 happening for all coordinates simultaneously
is

Prob(rθ∈C) =

(

1 −erfc

(√

logN

k

))N

Since erfc(x)∈[0,1] forx∈[0,∞), the limitN→∞tends to

Prob(r θ∈C)→







0 ifNerfc

(√

logN

k

)

→∞

1 ifNerfc

(√

logN

k

)

→ 0

(B.6)

Since

Nerfc

(√

logN

k

)

→

{

∞ fork > 1

0 fork≤ 1

(B.7)

Eq. (B.4) follows.