How would it work?

Contactless

They can be used to pay for items by touching a reader at the till, rather than having to enter a pin number. The current contactless limit for a single transaction is £20 and it will increase to £30 from September 1.

The cards contain a chip that holds your account information and an antenna that picks up power from a signal sent out by the card reader.

Card companies will limit the number of contactless transactions that can be made in a row before you are asked to enter your pin though, and the limit could be about £50 or five transactions.

QR Code

QR code (abbreviated from Quick Response Code) is the trademark for a type of matrix barcode (or two-dimensional barcode) first designed for the automotive industry in Japan. A barcode is a machine-readable optical label that contains information about the item to which it is attached. A QR code uses four standardized encoding modes (numeric, alphanumeric, byte/binary, and kanji) to efficiently store data; extensions may also be used.

Apple Pay

Apple Pay requires the Near Field Communication antenna and Touch ID on iPhone 6 and later. To pay at checkout, just hold your iPhone near a contactless reader with your finger on Touch ID.

How would by app work?

Using the same technology as apple pay the device would use near field communication in order to communicate with the card machine. This making the process far faster. As this technology is already available the service and mock ups of the device would be far faster.