Connect raspberry pi to arduino wirlessly

I am trying to connect my raspberry pi to my arduino wirelessly. I know there is a Wifi shield for the arduino but is there an alternative? My current setup use a usb to talk to each other with serial and I would like to make, if it is possible, no change to my code. Is there something that is like wireless usb to usb?

There is bluetooth.

Is there something that is like wireless usb to usb?

XBees. Whether they are appropriate, or not, depends on range, amount of data, reliability, and budget. You seem to have failed to quantify 4 of the 4 parameters.