Behind the Scenes
Verify quantum link
What is really happening here?
Many pairs of entangled photons are created in our lab on campus. One photon from each pair is sent through fiber-optic cable to the library, where its polarization is measured with a polarizer set at the angle you chose. Back at the lab, the other photon from the pair is measured with a polarizer set at an angle slightly different from the one you chose. We then see how many of these pairs make it through the polarizers. This is repeated for the second angle you chose. By comparing the results for each set of angles we can check for quantum entanglement. Entanglement is verified when the number of pairs making it through exceeds what is possible classically. This happens because in quantum entanglement, the polarizations of the photons are undecided before measurement (the photons are in superpositions of polarizations), and yet at the time of measurement the polarizations are always related (for example, always the same).
What else can you use a quantum network for? Share your ideas with us!
Quantum fortune
What is really happening here?
When you press the button, you ask the entangled photons whether they’re a particular polarization or not. Because they’re entangled, their answers are related, even though their answers are unknowable ahead of time (they are in superposition). Every time they answer, a bit is created. All the bits together are used to create a random number corresponding to a message.
What else can you use a quantum network for? Share your ideas with us!
Send a secret message
What is really happening here?
When you and the other player answer each question, the direction you turn the wheel sets the angle of the polarizer your photons go through. Left is asking if they wiggle diagonally or anti-diagonally, and right is asking if they wiggle up-down or left-right. Every time the photons answer, a bit is created. The bit could be zero or one; the bits are always the same if you choose the same setting, because the photons are entangled that way. The more bits you and your friend create by choosing the same setting, the longer the secret message you can send, and the higher resolution the image you can share. The degree of security is shown by the resolution of the emoji.
This is the basic way secret information can be shared on a quantum network. The string of bits created through entanglement are used to encrypt the message you want to send. The longer the bit string, the longer the message can be. Since no one knows your measurement choices ahead of time, if someone tries to intercept the photons and measure them, they would have to guess what settings you will use, and will get it wrong sometimes. This will result in the bit being different than your friend’s bit. By comparing some of your bits you can figure out if someone is listening in or not, and keep making bits until you’re sure no one is eavesdropping.
What else can you use a quantum network for? Share your ideas with us!
Scientific article
Interested in more technical information?
If you're interested in the technical details, please see our recently published research article, "Public quantum network: the first node," in Applied Physics Letters: https://doi.org/10.1063/5.0241562.