SLIDE 2 XX0 ❍❡✐s❡♥❜❡r❣ ♠♦❞❡❧ ❘❛♥❞♦♠ ✇❛❧❦s ♦♥ ❛ ♣❧❛♥❡ ✇✐t❤ ❛ ✇❛❧❧ ❙❝❛❧❛r ♣r♦❞✉❝ts ❈♦rr❡❧❛t✐♦♥ ❢✉♥❝t✐♦♥s
❚❤❡ ❍❡✐s❡♥❜❡r❣ XX0 ♠♦❞❡❧ ♦♥ t❤❡ ❝❤❛✐♥ ✐s ❞❡✜♥❡❞ ❜② t❤❡ ❍❛♠✐❧t♦♥✐❛♥ H ≡ −1 2
k+1σ+ k + σ+ k+1σ− k ) .
❚❤❡ ❧♦❝❛❧ s♣✐♥ ♦♣❡r❛t♦rs σ±
k = 1 2(σx k ± iσy k) ❛♥❞ σz k ♦❜❡② t❤❡
❝♦♠♠✉t❛t✐♦♥ r✉❧❡s✿ [ σ+
k , σ− l ] = δkl σz l ✱ [ σz k, σ± l ] = ±2 δkl σ± l ✭δkl ✐s t❤❡
❑r♦♥❡❝❦❡r s②♠❜♦❧✮✳ ❚❤❡ s♣✐♥ ♦♣❡r❛t♦rs ❛❝t ✐♥ t❤❡ s♣❛❝❡ HM+1 s♣❛♥♥❡❞ ♦✈❡r t❤❡ st❛t❡s M
k=0 |sk✱ ✇❤❡r❡ |sk ✐♠♣❧✐❡s ❡✐t❤❡r s♣✐♥ ✏✉♣✑✱ |↑✱ ♦r
s♣✐♥ ✏❞♦✇♥✑✱ |↓✱ st❛t❡ ❛t ❦th s✐t❡✳ ❚❤❡ st❛t❡s |↑ ≡ 1
1
- ♣r♦✈✐❞❡ ❛ ♥❛t✉r❛❧ ❜❛s✐s ♦❢ t❤❡ ❧✐♥❡❛r s♣❛❝❡ C2✳ ❚❤❡ st❛t❡ |⇑ ✇✐t❤ ❛❧❧ s♣✐♥s
✏✉♣✑✿ |⇑ ≡ M
n=0 |↑n ✐s ❛♥♥✐❤✐❧❛t❡❞ ❜② t❤❡ ❍❛♠✐❧t♦♥✐❛♥ ✭✷✮✿
ˆ H |⇑ = 0
◆✳▼✳ ❇♦❣♦❧✐✉❜♦✈