Gustaf can remember the precise date when he last used cash. 'It was October 7 last year,' he told me without hesitancy. 'I found an old note that I had forgotten about and used it to buy some sweets.' Like many others at his university in Gothenburg, Sweden, Gustaf relies on cards and smartphones to spend money. 'None of us use cash – you just don't need it these days,' he said. Card only: Sweden has become a cashless society so shoppers have switched to cards and smartphones But there is one problem – a big one. The 20-year-old computer...