Oct. 8, 1998 LOS ALAMOS, N.M., Oct. 7, 1998 -- Scientists at the Department of Energy's Los Alamos National Laboratory have achieved a significant advance in demonstrating the viability of an unbreakable encryption scheme for transmitting secure communications to and from satellites.
The encryption scheme, based on randomly generated characteristics of individual photons, could make major financial transactions or key military communications impenetrable to attempts to crack them and capable of revealing any attempts to eavesdrop on them.
"This remarkable advance in unbreakable cryptographic systems illustrates how our national laboratories can take fundamental scientific concepts -- sometimes involving physics too deep for most of us to understand -- and turn them into practical applications that support our national security objectives and other societal needs," Energy Secretary Bill Richardson said.
Los Alamos researchers have years of success transmitting their photon- based "quantum cryptographic keys" for encoding messages over optical fibers, a method useful for secure communications between fiber-linked facilities. Their latest achievement, described in the Oct. 12 issue of Physical Review Letters, was to successfully transmit a quantum key through the air over a distance of about one kilometer, demonstrating the feasibility of using the technique for secure satellite communications.
"This is an important result for our quantum key distribution system," said lead author William Buttler, "because it's the lowest few kilometers of the atmosphere that will cause an optical beam to deviate the most. We've shown we can operate this system in the lowest portion of the atmospheric boundary layer, where turbulence is at its worst."
If the optical signal emerges mostly intact after passing through the boundary layer, the rest of the travel to a satellite orbiting 300 kilometers or so overhead will have negligible effect on the signal.
The PRL paper describes nighttime transmission and detection of the individual photons used to build the quantum cryptographic key for encoding and decoding messages. The researchers are now conducting their demonstration in daylight, an immensely more difficult challenge. The initial results of the daytime demonstration have been positive.
Encryption works by converting a message into a new form with a secret key. The key could be as simple as making A=1, B=2, C=3, etc. and writing the message in numbers. If the sender and recipient share the key, then decoding the message is straightforward.
Existing encryption schemes used to protect financial transactions, national security information and other significant communications suffer two weaknesses: the numerical-based keys used to encrypt messages are potentially vulnerable as computers become more powerful, and the key can possibly be intercepted when recipient and sender share it.
Quantum cryptographic keys, by contrast, are generated as needed between the sender and receiver, creating a random string of numbers known only to the two people. The key is created through their shared communication, and any attempts to intercept the communication or eavesdrop on it can be detected because of the quantum-based nature of the information.
Once the sender and receiver share a unique key, they can code, transmit and decode messages securely.
The Los Alamos demonstration consists of a laser that can emit extremely short pulses; an attenuator that damps each pulse to a single photon, on average; and a system that randomly assigns one of two polarization states to the photon. The two polarization states represent "1" and "0" in a binary number sequence
Polarization describes a preferred direction of oscillation for the electromagnetic wave of a photon. Devices known as polarizers will transmit only specific polarization states.
The receiver includes a telescope and optics that randomly direct the photons collected along one of two paths, each configured to look for a specific polarization state.
The quantum key is generated when the sender, conventionally dubbed "Alice," generates a series of individual photons, shot out at a rate of a million per second, and randomly changes the polarization to create a sequence of zeroes and ones. She has told the recipient, dubbed "Bob," which polarization state represents a one and which a zero; this information can also be shared with the world at large without threatening the encryption scheme.
Bob captures as many of the incoming photons as possible, given the difficulty of plucking specific individual photons out of a sea of background photons. The Los Alamos team has shown that with precision timing and properly chosen filters a sufficiently high number of photons can be detected to make the quantum encryption scheme work.
Bob's receiver randomly switches between his chosen polarization values for zero and one. He doesn't, however, try to measure Alice's original polarization states; instead, he looks for related polarization states. This ensures that when Bob is looking for a zero, he will never see a photon if Alice transmitted a one. Yet, a fraction of the time that he is looking for a zero and Alice transmits a zero, he will record a photon and know their two values were in agreement.
In less than a second, Alice can transmit a sequence of many thousands of photons. Bob will detect and agree with the value of some random fraction of these, about a quarter of the original photon stream on average. Bob then indicates the positions in the sequence where his value agreed with Alice's. This positional information, in which only Bob and Alice know the values for each point in agreement, allows them to form their secret quantum key.
If anyone intercepts the stream of photons the act will reveal itself by raising the error rate above a threshold value or eliminating the photon stream altogether.
Los Alamos' quantum key transmission operates at a wavelength that passes through the atmosphere with high efficiency and where high- quality detectors are commercially available. Generating the single-photon quantum key is a routine procedure for the system the Los Alamos team has developed.
The Los Alamos researchers say the quantum key distribution system could provide secure uplink and downlink satellite communications, and connect cities anywhere in the world via satellite. Authors on the PRL paper were Buttler, Richard Hughes, Paul Kwiat, Steve Lamoreaux, George Morgan, Glen Peterson, Chuck, Gabe, and Beth Nordholt.
Los Alamos National Laboratory is operated by the University of California for the U.S. Department of Energy.
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