Scientists at the Department of Energy's Los Alamos National Laboratory and other research organizations around the world are harnessing the laws of quantum physics to develop what they hope will be impregnable data encryption systems.
Quantum cryptography systems would allow users to overcome the vulnerabilities of the public-key cryptosystems now widely deployed by businesses and government organizations to secure sensitive information from eavesdroppers.
Public-key cryptosystems use algorithms to encrypt and decrypt data. These cryptosystems send a set of specific parameters - called a key - together with the plain-text information to be scrambled as input into the encrypting algorithm. They generate public keys that senders can use to encrypt a secure message. The receiver then decrypts the message with a private key.
The security of the key relies on a randomly chosen long string of bits. Many public-key cryptosystems use large numbers with more than 100 digits. But such codes can eventually be broken by new algorithms or a powerful computational device that can factor the number into two smaller numbers.
"There is a danger that public-key encryption might be jeopardized," says Paul Kwiat, a researcher at the laboratory in Los Alamos, N.M.
To ensure the long-term security of data, physicists have turned to quantum mechanics to develop cryptography with uncrackable key distribution systems. Quantum cryptography creates and sends code made from a series of individual photons with different polarizations or other properties. The direction in which a photon's electric field vibrates represents the zeros and ones of computer language.
Richard Hughes, another researcher at Los Alamos, has already demonstrated that quantum cryptography can send secure messages through 48 kilometers of optical fibers and one mile of space.
Potential Drawbacks
But Quantum cryptography does have potential drawbacks. Sometimes, the faint pulses used to convey the light particle contain no photons. Or the pulses may contain more than one photon that might, five or 10 years from now, allow an eavesdropper to steal photons from the signal and secretly gather information about the encryption key.
Researchers are working on a new form of quantum cryptography that is based on a phenomenon called quantum entanglement. Entangled quantum cryptography uses a specially prepared crystal to split a single photon into a pair of "entangled" photons. The polarization of each photon then becomes an undetermined state representing a mixture of both zeros and ones.
Even when the entangled light particles are far apart, they influence one another's properties. Each photon could be detected either as a zero or a one, but once the polarization of one photon is detected, the second photon in the pair must assume a polarization that is identical to the first.
The laws of quantum mechanics dictate that a photon's polarization properties can be in a combination of states until it's measured. The photon then gains a definite polarization and can represent a particular value to build a key.
If Alice and Bob attempt to share an encryption key using quantum entanglement, each will receive one photon from a pair. Both then randomly decide to make one of two types of polarization measurements and can communicate which type of measurement they used.
In cases where the same type of measurement isn't used, the results are discarded. However, when Alice and Bob make identical measurements, the entangled photons will produce exactly the same results, which can be translated into bits. This string of digits then serves as the randomly generated information needed to create a secure key for encoding and decoding data.
Scientists believe that quantum entanglement could eventually offer higher transmission rates and greater security over longer distances. According to Kwiat, quantum entanglement might initially be employed by military or special-purpose applications suited to the point-to-point nature of exchanging such keys. He says it could also be used by banks encrypting information to branches or by a provider to encrypt all signals to a satellite. "It is difficult, however, to envision it on a large scale in networking," Kwiat adds.
Recent advances in quantum entanglement were announced in April by three groups: researchers from Los Alamos, a Swiss team and a joint German-Austrian team.
Long-Distance Success
Phil Schewe, a spokesman for the American Institute of Physics in College Park, Md., notes that the Swiss team used quantum entanglement to encrypt a message between two towns via fiber-optic lines. He says the German team encrypted and decrypted an image. Kwiat, who headed the Los Alamos research team, says his group experimented with better ways to detect eavesdroppers, who alter the photon's properties in ways that can be detected via shifts in error rates.
Schewe says the institute has received calls from several firms, including American Express Co. in New York, inquiring about the research. "Even if those photons have moved to opposite sides of the galaxy, quantum correlations do seem to travel faster than the speed of light," he says. "They cannot impart information faster than the speed of light, but they can still communicate with each other instantaneously."