Author: Wilf Nabil
Institution: Biology and International Affairs
Date: October 2005
It is probably the longest war in history, spanning millennia, and physicists are only baby steps away from resolving the conflict for eternity. What is this war, who is fighting it, and how is it about to end?
Want a hint? Visit the National Security Agency's webpage. The NSA is America's elite fighting force of code makers and code breakers, unparalleled in the world. You might be surprised to know that it is the largest employer of mathematicians in the world.
The war is fought in the field of secure, private communications. The NSA focuses on breaking foreign codes and encrypting America's so that eavesdroppers (i.e., spies) will remain baffled by classified messages. In technical speak, this is known as cryptography, the study of encoding messages, and cryptanalysis, the study of deciphering coded texts or messages.
One reason few know of this great war is because it is shrouded in secrecy, and advancements in the battlefield are concealed to delude the opponent. To some, cryptology is an art cloaked in a canvas of numbers and letters, arranged in unrecognizable patterns to the naked eye. Only viewers with access to the cipher are able to "X-ray" the image and peer into the message underneath the randomness.
How could physicists end the struggle for secure, private communication networks? It all boils down to quantum physics and its application to cryptography: quantum cryptography.
Quantum cryptography secures communications using certain phenomena of quantum physics. Unlike traditional cryptography, which employs various mathematical techniques to restrict eavesdroppers, quantum cryptography focuses on the physics of information. Instead of creating more complex mathematical ciphers (e.g. 256- versus 128-bit encryption), physicists try to encrypt messages using photons.
The sending and storing of information is always carried out by physical means, for example: photons in optical fibers or electrons in an electric current. Eavesdroppers must measure a physical object, in this case the carrier of the information. What the eavesdropper can measure, and how, depends exclusively on the laws of physics. Using quantum phenomena such as quantum superpositions or quantum entanglement, one can design a communication system that can always detect eavesdropping. When eavesdroppers try to measure the quantum carriers of information, or qubits, they disturb it. Thus, any attempt to decipher a message invariably destroys it, while only a receiver with the cipher can read it.
Some commercially-available products based on quantum cryptography have appeared. ID Quantique or MagiQ are companies that offer real-world solutions for corporations and governments to secure their fiber-based data networks.
Current research into quantum cryptography focuses on quantum entanglement, or what Einstein called "spooky action at a distance." It is the ability for two separated particles to instantly communicate with each other. Thus, whatever happens to one particle would immediately affect the other particle, wherever in the universe it may be.
In the real world, entangled quantum states are rarely stable enough to be useful. Difficulties exist in keeping them entangled long enough to meet the needs of highly-secure communication systems. Current applications of quantum cryptography have a limited reach of 100 kilometers between sender and reciever.
However, physicists at the Georgia Institute of Technology have just reached an important milestone in the development of these systems by entangling a photon and a single atom located in an atomic cloud. Researchers believe this is the first time an entanglement between a photon and a collective excitation of atoms has passed the rigorous test of quantum behavior known as the Bell inequality violation, an abstruse physics theorem. These findings are a significant step in developing secure long-distance quantum communications.
Still many challenges remain in developing these systems, one of which is how to get the particles to store information long enough and travel far enough to get to their intended destination. An entangled system of atoms and photons offers the best of both worlds. Photonic qubits are great carriers and can travel for long distances before being absorbed into the conduit, but they're not so great at storing the information for a long time. Atomic qubits, on the other hand, can store information for much longer. The trick is how to get them entangled in a simple way that requires the least amount of hardware.
Georgia Tech physicists Alex Kuzmich and Brian Kennedy think that taking a collective approach is the way to go. Instead of trying to isolate an atom to get it into the excited state necessary for it to become entangled with a photon, they decided to try to excite an atom in a cloud of atoms.
"Using a collective atomic qubit is much simpler than the single atom approach," says Kuzmich. "It requires less hardware because we don't have to isolate an atom. In fact, we don't even know, or need to know, which atom in the group is the qubit. We can show that the system is entangled because it violates Bell inequality."
"With single atoms, it is much more difficult to control the system because there is so much preparation that must be done," adds Kennedy. "For the collective excitation, the initial preparation of the atoms is minimal. You don't have to play too much with their internal state something that's usually a huge concern."
In addition to having the system pass the rigorous test of Bell inequality, researchers said they were able to increase the amount of time the atomic cloud can store information to several microseconds. That's fifty times longer than it takes to prepare and measure the atom-photon entanglement.
Another challenge of quantum communication networks is that since photons can only travel so far before they get absorbed into the conduit, the network has to be built in nodes with a repeater at each connection.
"A very important step down the road would be to put systems like this together and confirm they are behaving in a quantum mechanical way," notes Kennedy.
Ironically, the cherished aspect of quantum cryptography, the inability of eavesdroppers to measure qubits without disturbance, makes research into quantum communications difficult. Nevertheless, we are on the brink of another wave in the quantum physics revolution. The first was the atomic bomb. Thankfully, this time it will not be so devastating.