1983 paper on entanglement entropy: "On the Entropy of the Vacuum   outside a Horizon"
  I introduce the concept of *entanglement entropy* (as it's now called) and point out that it follows an *area law* which renders it a suitable source of black hole entropy. I also suggest to conceive the latter as residing on the horizon at approximately one bit per "Planckian plaquette". 
Rafael D. Sorkin
gr-qc cond-mat.stat-mech hep-th quant-ph