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Fast Quantum Byzantine Agreement: Difference between revisions
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==Outline== | ==Outline== | ||
Here we will sketch the outline of the protocol by Ben-Or [Quantum Byzantine Agreement#References|(3)] that solve Byzantine Agreement using quantum resources. A very nice summary of this protocol is also presented in [Quantum Byzantine Agreement#References|(1)]. The main idea of this protocol is for each player to classically send its proposed value/decision (a valid message) to every other player and then collaborate to determine what a majority of honest players proposed. In the case where adversaries make this difficult, a `good-enough' random coin is globally flipped (protocol for random coin flip using quantum resources explained below), which is then classically post-processed to reach agreement among the honest parties. More precisely, the protocol is outlined as follows. Each round consists of the following steps: | |||
* | |||
* Each player transmits its input to every other player. If one player receives more than 2/3 of the same values from the other players (including his own), then he changes his input also to this value (if that player already did not have the same choice). Otherwise, the same player executes a Quantum Oblivious Common Coin subroutine and sets his input to the outcome of this routine. | |||
* Then each player sequentially executes two classical subroutines to bias the agreement value towards <math>0</math> or <math>1</math> (outcomes of a coin flip). This guarantees that if the non-faulty players are in agreement, then they will terminate and successfully output the correct agreement value <math>d</math> (not an outcome of coin flip). | |||
</br> | |||
'''Quantum Oblivious Common Coin subroutine:''' The heart of this protocol comes from the quantum enhanced [[Oblivious Common Coin]]. At the end of this subroutine, each player outputs a random bit, such that with at least probability $p$ (called the fairness) $v_i = x$ for all players $i$ and all $x \in \{0,1\}$. Intuitively, this subroutines tosses a common coin, where all players get either heads or tails with probability at least $p$ each, but there may be executions (which occur with at most probability $1-2p$) where all players do not get the same output and no common coin is actually tossed. Since the players do not know whether the outcomes are all equal or not, this type of coin tossing is referred to as oblivious common coin tossing. In particular, using quantum resources, this task can be achieved in constant rounds (in the defined model). The implementation of this subroutine makes use of a weakened version of Verifiable Quantum Secret Sharing. | |||
==Notations Used== | ==Notations Used== | ||
**<math>|\psi\rangle_{in}:</math>The original input state | **<math>|\psi\rangle_{in}:</math>The original input state | ||