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Quantum Coin Flipping

Because Alice and Bob are distrusting of each other, more effort must be spent on ensuring that neither Alice nor Bob can gain a significant advantage over the other to produce a desired outcome. An ability to influence a particular outcome is referred to as a bias, and there is a significant focus on developing protocols to reduce the bias of a dishonest player, otherwise known as cheating. Quantum communication protocols, including quantum coin flipping, have been shown to provide significant security advantages over classical communication, though they are difficult to realize in the practical world.

A coin flip protocol generally occurs like this:

1. Alice chooses a basis (either rectilinear or diagonal) and generates a string of photons to send to Bob in that basis.
2. Bob randomly chooses to measure each photon in a rectilinear or diagonal basis, noting which basis he used and the measured value.
3. Bob publicly guesses which basis Alice used to send her qubits.
4. Alice announces the basis she used and sends her original string to Bob.
5. Bob confirms by comparing Alice's string to his table. It should be perfectly correlated with the values Bob measured using Alice's basis and completely uncorrelated with the opposite.

Cheating occurs when one player attempts to manipulate a shortcoming of the coin flip protocol. The protocol discourages some forms of cheating; for example, Alice could cheat at step 4 by claiming that Bob incorrectly guessed her initial basis when he guessed correctly, but Alice would then need to generate a new string of qubits that perfectly correlates with what Bob measured in the opposite table. Her chance of generating a matching string of qubits will decrease exponentially with the number of qubits sent, and if Bob notes a mismatch, he will know she was lying. Alice could also generate a string of photons using a mixture of states, but Bob would easily see that her string will correlate partially (but not fully) with both sides of the table, and know she cheated in the process. There is also an inherent flaw that comes with current quantum devices. Errors and lost qubits will affect Bob's measurements, resulting in holes in Bob's measurement table. Significant losses in measurement will affect Bob's ability to verify Alice's qubit sequence in step 5.

One theoretically surefire way for Alice to cheat is to utilize the Einstein-Podolsky-Rosen (EPR) paradox. Two photons in an EPR pair are anticorrelated; that is, they will always be found to have opposite polarizations, provided that they are measured in the same basis. Alice could generate a string of EPR pairs, sending one photon per pair to Bob and storing the other herself. When Bob states his guess, she could measure her EPR pair photons in the opposite basis and obtain a perfect correlation to Bob's opposite table. Bob would never know she cheated. However, this requires capabilities that quantum technology currently does not possess, making it impossible to do in practice. In order to successful execute this, Alice would need to be able to store all the photons for a significant amount of time as well as measure them with near perfect efficiency. This is because any photon lost in storage or in measurement would result in a hole in her string that she would have to fill by guessing. The more guesses she has to make, the more she risks detection by Bob for cheating.