You can use Dry Erase Cards as mini whiteboards for when you want to quickly get something down visually. I always keep one Dry Erase Card with me at all times so I can have access to it when I need to.
Recently at the pub me and my friends got into a discussion about quantum physics (as you do) and my Dry Erase Card was very useful as a mini dry wipe board to explain the different aspects, which, one drawn where a little easier to understand, such as the 'Double Slit Experiment'.
Dry Erase Cards are the perfect mini whiteboard because they fit into your wallet and lend themselves to any situation. With 30 in a pack they are inexpensive and reusable! In the past I have been at the pub and wanted to do something similar but had to settle for scrawling notes or pictures on receipt paper or even a napkin!