Quantum password

Basic Concepts

Quantum cryptography with the traditional password systems, it relies on key aspects of physics rather than mathematics as a safe mode. In essence, the quantum cryptography system is unbreakable cryptographic applications and their inherent single photon quantum properties based on the development, because they can not measure the quantum state of the system without interfering system. Theoretically other particles can also be used, only photons with all of the required quality, their behavior is relatively well understood, but it is also the most promising high-bandwidth communication media information carrier fiber optic cable.


theoretical model

In theory, quantum cryptography work (this view is from the traditional model of Bennett and Brassard in 1984 in the following modes of development , there are other modes):

Suppose two people who want to exchange information securely, named Alice and Bob. Alice to Bob by sending initialization information to a key, this key may be the encryption data mode information. It is a random sequence of bits, some type of transmission mode, two different initial values ​​that represent a specific binary bit (0 or 1).

we forget that this key is transmitted in one direction flow of photons, each single photon particle represents a data bit (0 or 1). In addition to straight running, all photons are also vibrates in some manner. These vibrations along any axis in the space of 360 degrees, for simplicity (at least simplified problem in the quantum cryptography), we put these vibrations specific state divided into four groups, i.e., upper, lower, left, right, upper left , lower right and upper right, lower left, vibration angle on poles along the photon. Filter, which allows atomic vibrations in a certain state of no change, so that other atoms by the changed state of shock (it can also completely blocked by photons, but here we will ignore this attribute). Alice has a polarizer allows photons by four states in fact, she may choose to filter (upper, lower, left, right) or diagonal (upper left, lower right, upper right, lower left) along a straight line.

Alice in her conversion between straight and diagonal vibration mode to filter random single photon transmission. When so, it means a single bit, 1 or 0 in one of two modes of vibration.

When the received photons, Bob must be a straight line or a diagonal of a polarizer to measure each photon bit. He may select the correct polarization angle, it may be wrong. Since Alice is very arbitrary choice of the polarizer, the photon would react when you select the wrong polarizer it?


Heisenberg uncertainty principle states that we can not determine each individual photon will happen, because we measure its behavior changed its properties (if we want to measure two properties of a system, a measure of our rights while excluding another quantized). However, we can estimate what occurred in this group. When Bob measuring the upper left / upper right and lower right / lower left (diagonal) with a linear side light photons, a photon state will change these when passing through the polarizer, half into a vertical vibration mode, into the other half way around. But we can not determine a single photon which will be transformed into state (of course, in real applications, some photons are blocked off, but little to do with this theory).

may also be possible to correct errors when Bob measuring photons, visible, Alice and Bob have created insecure communication channel, others who may be listening. Then Alice tells Bob which she photon bit transmitted by polarizer, not how she polarized photons. She might say photon number 8597 (theoretically) using linear mode transmission, but she would not say whether or when to spend sent down or left and right. Bob This is to determine whether he used the correct polarizer accepted each photon. Then Alice and Bob to abandon all of the photons he used the wrong polarizer measurements. They have, the original sequence of transmission is half the length of 0 and 1. But this forms the basis of one-time pad (OTP) theory, that once they are properly implemented, it is considered to be completely random and secure password system.

theoretical assumptions

We assume that there is a listener, Eve, trying to wiretap information, he has a polarizer with Bob same, you need to select a straight or diagonal photons filtration. However, he faces the same problems as Bob, half of the possibility he would choose the wrong polarizer. Bob's advantage is that he can confirm the type of polarizer used to Alice. And Eve is no way, half of the possibility that she chose the wrong detector, misinterpret the photon information to form the final key, rendering it useless.

Further, in the quantum cryptography there is another inherent level of security is intrusion detection. Alice and Bob would know if Eve is listening to them. Eve on the fact that the photon line will be very easy to find, for the following reasons:

Let's assume that Alice by way of transfer number upper right / lower left of photons 349 to Bob, but this time, with the linear Eve polarizer only accurate determination of vertical or horizontal type photons. If Bob with a linear polarizer, then it does not matter, because he would abandon the photons from the last key value in. But if Bob with a diagonal polarizer, a problem arises, he may be correct measurements, according to Heisenberg uncertainty theory, it may erroneous measurements. Eve with the wrong polarizer changes the state of the photon, Bob even with the correct polarizer can make mistakes.

Once you have found Eve's bad behavior, they must take the above measures, obtain a unique key sequence consisting of 0's and 1's, unless it has been stolen, will produce contradiction. Then they will take further action to check the validity of keys. If the last key in an insecure channel compare binary digits it is very foolish, is not necessary.

We assume last key value 4000 comprises binary digits, Alice and Bo.b need to do is to select a random subset from among these figures, 200 bar, according to the two states (Digital 2,34,65,911 serial number, etc.), and a digital state (0 or 1), are compared, and if all matches that Eve can not listening. If she listens, then the probability is not to be found one trillionth, that is impossible not to be found. Alice and Bob will find someone to listen no longer use this key, they will re-start the key exchange over the secure channel can not be reached Eve, of course, the above-mentioned activities can be carried out relatively insecure channel. If Alice and Bob infer their key values ​​are safe because they were tested with 200, 200 which will be discarded from the last key value, the 4000 changes to the 3800.

Thus, a quantum cryptography key exchange connection relatively easy and convenient manner in a common key cryptography.

practical significance

In practice, quantum cryptography has been demonstrated in IBM's labs, but only suitable for relatively short distances. Successful fiber optic cable transmission of photons over a longer distance, with extremely pure optical characteristic distance of 60 km. Just BERs (error rate) with the Heisenberg uncertainty principle impurities and micro fiber closely linked so that the system does not work stably. Although studies have been successfully transmitted through the air, but the transmission distance is very short even under ideal weather conditions. Quantum cryptography applications need to further develop new technologies to enhance the transmission distance.

Application examples

1, between the United States, the White House and the Pentagon in Washington have dedicated lines for practical applications, it is also attached near the main military sites, defense systems and research laboratory. Beginning in 2003, the company id Quantique in Geneva and New York-based MagiQ Technologies, Inc., introduced a quantum key transmission distance beyond 30 cm Bennett experiment commercial products. NEC after the demo record transmission distance of 150 km, will launch the first products to the market in 2010. IBM, Fujitsu and Toshiba and other companies are also actively engaged in research and development. Key products to the market through the optical fiber transmission tens of kilometers.

2, in the country, Asky quantum Tech CO., LTD (Q-day quantum) Construction of Wuhu quantum-government network, so that our country has made great progress in this area.


addition to the initial use of photon polarization characteristics encoded, but also the emergence of a new coding method - the phase encoding photons. Compared to the polarization encoding, phase encoding benefits are not so harsh polarization requirements.

This technology can make the operation, generally need to go through this procedure: transmitting quantum information on the ground - quantum signals transmitted through the atmosphere - receiving satellite signals and forwarded to walk around the world to accept target . One of the challenges facing this technology is that air molecules will stand quantum atmosphere of a catapult to all sides, it is difficult to make them absorb the designated satellite.

In addition, this technology will have to face "Encryption at low temperatures and can not guarantee the encryption speed" challenge. Confidentiality and theft as spear and shield, like go hand in hand with the struggle between them has been going on for thousands of years appeared, quantum cryptography, in theory, end the feud, wanted it to be real Terminator.

extended reading

quantum cryptography

to our stable and rapid transmission of large amounts of data through an optical fiber. But in fact we can have another option, the beam is passed directly to the data without using optical fiber. However, confidential information is very important, how can securely transmit data, has become a science, known as "quantum cryptography (Quantum Cryptography)."

theory of quantum cryptography is quantum mechanics, theory is different from the conventional mathematical cryptography. If quantum cryptography transmitting data, this data will not be retrieved or inserted into any other section of malicious data, data flow may be safely coding and decoding. The encoding and decoding tool is a random sequence (bit-strings), you can also call him a key (Key). Currently, the core of quantum cryptographic research is how to use quantum technology safe and reliable distribution channel in the quantum key.

different from the conventional cryptography, using principles of physics quantum cryptography protection information. "The method of quantum information carrier, transmitted through a quantum channel, the establishment of a shared key between a legitimate user," usually, known as quantum key distribution (QKD), its safety by the "Heisenberg Uncertainty Principle "and" single quantum can not be copied Theorem "guarantee.

Heisenberg uncertainty principle

are the basic principles of quantum mechanics, explains at the same time observer can not accurately measure the "position" and "momentum" of the analyte. "Single quantum theorems can not be copied" the Heisenberg uncertainty principle is a corollary, it refers to copy a single quantum in the case do not know the quantum state is not possible, because you want to copy only a single quantum would first make measurements, and measurement necessarily change the quantum state.

quantum key

In terms of making quantum cryptography key, this key can not be copied, and therefore it is absolutely safe. If unfortunately hackers captured, because the measurement process will change the quantum state, hackers Theft of data would be meaningless. The team

from Germany and the United Kingdom in the latest issue of the journal Nature, said the scientists by key (Key), a distance of 23.4 kilometers in two places, with a wavelength of 850nm laser, send encrypted data to each other in the air. Since no two optical fibers, data transfer is carried out in general air, and therefore in order to reduce the interference environment, at a lean air scientists selected (altitude 2244 ~ 2950m) and evening (to avoid light pollution), experiment. Such a distance (23.4 km) have broken the world record by American scientists established 10 kilometers.

Scientists now have a quantum key can be transmitted in the optical fiber. However, with the progress of the times, more and more frequent exchange of information humans, scientists hope to establish a quantum key transmission 1600 kilometers away in the future if such a mature way data transmission, you can be on the surface, fast and secure transmission of information. This technique can also be used as a means of communication with the surface low-orbit satellites, thereby establishing a global information confidential transmission system.

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