It's many years since I visited, but the atomic bomb museum in Hiroshima has an exhibit that appeared to cover the evidence surrounding this pretty exhaustively

It's many years since I visited, but the atomic bomb museum in Hiroshima has an exhibit that appeared to cover the evidence surrounding this pretty exhaustively. My vague recollection is that most of it was evidence of perception on the US side that Japan was ready to surrender - as opposed to evidence about specific back-channel negotiations.