In 1986, a "Captain Midnight" took control of HBO's insecure satellite system for four and a half minutes. During that time, Captain Midnight publicly slandered and defamed HBO. Not to say that he was wrong arguing that $12.95 a month is too much to pay for one television station, but this illustrates an important point: insecure satellite systems present a risk to the security and integrity of messages relayed on them. So how do we stop a more malignant breed of "Captain Midnights" from seriously compromising secure satellite systems? The answer lies in a form of applied quantum physics called quantum cryptography.
      Quantum cryptography, in simple terms, is a perfectly secure method of communication. Not only is quantum cryptography practically secure (no one has yet broken a quantum encrypted message), it is also theoretically secure (no one can break a quantum encrypted message). It is this theoretical security (something popular public key encryption algorithms do not have) that is the most exciting.  First, a brief word about encryption:
      In order for encryption to work, two elements are needed: an encryption algorithm and a way to distribute the key to that algorithm.  Now the former element is easy to get, smart math people have already done that.  The latter, however, presents more of a problem. How does one transmit a secret key to another party without it being able to be snooped by a third party?
      One way to do this is a secure quantum channel to exchange keys. To establish a quantum channel, the party wishing to send the message (call her Alice) needs to send a series of photons to the person she is trying to contact (call him Bob). Since a photon can have either spin up or spin down, Alice can select either of these states (spin up representing bit 1 and spin down representing bit 0) and since photons can also be polarized, Alice can either polarizes her photons vertically or at 45 degrees. So, to "quantumly" communicate with Bob, Alice randomly spins and polarizes a series of photons and sends that series of photons to Bob, recording them first.
     Bob, as the receiving party, does not know the polarizations. So, when he measures the spin on them, he can only guess with a device that is oriented either vertically or at 45 degrees. If he guesses the polarization correctly, then he will measure the spin correctly.  If he guesses incorrectly, then he will have only a 50% chance of measuring the spin correctly (this is because 45 deg is halfway to 90 deg - if the angle were different, the probabilities would be different as well.  This is where the quantum in "quantum cryptography" comes in).  Once Bob has received the entire string, he calls Alice and tells her what orientation he used to measure each photon and Alice tells him which ones he got right (for polarization only - if the polarization was wrong, the bit is tossed out, despite the fact that the spin *may* have been