A STAMP of molten wax was once the seal of secrecy for printed documents. But how do you seal digital documents? Turns out that quantum physics has the answer.
A seal doesn鈥檛 actually keep anyone from reading a message, but you will know if anyone has tampered with it. To create a digital seal, physicists Sudhir Singh of the University of California, Los Angeles, and R. Srikanth of the Raman Research Institute in Bangalore, India, say that each bit of information in the seal would need to be represented by a string of quantum bits or qubits, where a qubit can either be 0 or 1, or alternatively a 鈥渟uperposition鈥� of both states.
In their scheme, a sender encodes the actual value of the original bit in a third of the qubits. The remaining two-thirds of the qubits will be in a superposition, and randomly placed within the string of qubits. These qubits are the ones used to 鈥渟eal鈥� the information.
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When a receiver attempts to read the qubits, the two-thirds that are in a superposition will randomly become a 0 or a 1. By summing up the values of all the qubits, the receiver can determine whether the original bit was a 0 or a 1 鈥� a 1 would give a much larger sum for the qubits than a 0. As the number of qubits used to encode the original bit of information gets larger, the difference between the sums for a 1 or a 0 also gets larger. This ensures that the randomness of the superpositioned qubits does not obscure the information.
You will know that the seal on an electronic document has been broken because reading all the qubits leaves an imprint on the message 鈥� the qubits that were in a superposition are forced to become a 0 or 1. If you know which qubits were initially placed in a superposition of states, checking to see whether they are still in that state can reveal if someone has taken a peek at the data ().