Shuffled Deck

Suppose we have a standard deck of cards arranged in some order. We shuffle the cards by cutting the deck into two halves: Cards in positions $1-26$ and $27-52$. Then, we alternate cards from each half of the deck. Namely, the cards in positions $1-26$ are now in positions $2,4,\dots, 52$. and the cards in positions $27-52$ are now in positions $1,3,\dots, 51$. What is the minimum number of shuffles needed before the deck returns to it's original state?