Is there a light but strong bike lock?

No!

Well, sort of no. If you want the strongest, most unbreakable bike lock it is going to be very heavy. On the other hand, if you want a very light lock then scumbag thieves will be able to cut through it with not much more than a pair of scissors.

In between these two extremes is a continuum of locks available at widely varying levels of security, weight and price. However, for quality lock brands like Kryptonite  and Abus - (never buy a cheap, poor quality lock) - strength is almost perfectly correlated with weight: The stronger the lock, the heavier it will be!

"Money no object, what's a very strong but very light lock?". I've had this question asked in the on more than one occasion, but sadly this combination doesn't exist.