Passive RFID (radio frequency identification) tags are small, inexpensive, battery-less labels that are already used to track and identify a wide variety of items. If MIT’s experimental RFIQ system enters general use, they may also soon allow consumers to check if food products are contaminated.
Upon being temporarily powered up by radio waves emitted from a handheld reader device, passive RFID tags use a tiny integrated antenna to transmit a radio signal back to that device. That signal contains information on the item to which the tag is adhered, such as its stock number, batch number or production date.
When one of those tags is stuck to the outside of a container, the radio waves emitted by the tag travel back through the wall of that container, and are subtly distorted by the molecules and ions of its contents. As an example, the signals from identical RFID tags placed on identical containers filled with either water or air will be distorted in different ways, upon being received by a reader device.