In quantum mechanics, some observables such as position and momentum cannot be determined simultaneously. Also, in Ekert invented an entanglement-based QKD protocol. QKD provides unconditional security based on quantum mechanics and it is secure against even quantum computers. More concretely, Alice, by sending Bob quantum signals e. If Eve is detected, Alice and Bob can simply abort the protocol without losing any confidential information and try it again in future. The first QKD experiment was done in over the transmission of 30cm of open air.
Tremendous experimental progress has been made in QKD over the last 25 years. QKD has now been successfully performed over hundreds of kilometers of telecom fibers and open air. Commercial QKD systems are currently being deployed. Some countries such as the US and China are currently installing quantum communication networks with trusted relays between major cities e. In theory, quantum communication is perfectly secure. Loopholes could appear in both the source and the detectors.
Practical sources and detectors rarely conform to the idealized theoretical model used in security proof. For instance, in a gated detector, its detection efficiency is switched by a biased voltage and is thus a function of time.
Ideally, there are two detectors,one for the bit, 0, and one for the bit, 1 and it is important to ensure that the detection efficiencies of the two detectors are perfectly matched. In practice, we have shown that a tiny mismatch in the timing of the biased voltage could introduce a big mismatch in the detection efficiencies.
This is the principle behind time-shift attack. This is just one example. Various loopholes have been found in QKD systems. As a counter-measure to quantum hacking, recently there has been a lot of theoretical and experimental interest in measurement-device-independent quantum key distribution MDI-QKD , an idea proposed by me, Marcos Curty and Bing Qi in As mentioned above, there are loopholes in both detectors and sources.
The most fatal loopholes are often at the detectors. In the longer term, all photonics quantum repeaters, if realized, will be a great way to extend the idea of MDI-QKD to longer distances. A Bell test is a test of the idea of local reality.
Two systems are said to obey local reality if we could assign a local physical reality i. Unfortunately, a remarkable feature of quantum mechanics is that it violates local reality. This puzzled even Einstein. In this sense, quantum mechanics is non-local. In particular, experimentally quantum mechanics violates Bell inequalities.
A different view, however, was presented by Hoi-Kwong Lo from the quantum computing and cryptography firm Magiq Technologies. He is worried that there are many possible technology developments in the near future and many loopholes in the system which are not obvious to anybody today, but may well be discovered tomorrow.
Theoretical safety against eavesdroppers is not enough, and mathematical tricks can not fully compensate for the possible danger of cloning.
All that matters is the physical safety of the information, which is obviously threatened by the ability to clone photons, which was thought to be impossible earlier. Clearly, quantum cryptography is still a long way from being common in information transfer, because the real world is still a long way from theoretical perfection.
However, it's important to keep in mind that the basic foundation for this great technique has been laid, and so now it only needs to be refined. For now, computers capable to transmitting information using quantum cryptography are very large, custom-made and, thus, expensive.
A couple of banks have already taken advantage of this security method, but few other organizations would be able to afford it in the foreseeable future. With regards to entangled photons, which seem to be absolutely safe, there is also a serious practical problem not only with the cost, but also with keeping them entangled long enough to meet the needs of the real world. While the system is perfect in theory, it is going to be very hard to implement it in practice.
Another problem is that for distances beyond 50 kilometers or so, the noise becomes so great that error rates skyrocket. This not only leaves the channel very vulnerable for eavesdroppers, but also makes it virtually impossible to send information. However, it is potentially possible for quantum keys to be exchanged through the air in the future. Tiny telescopes would then have to be aligned to detect the signal. Some calculations even suggest that photons could be detected by a satellite, which would allow communication between continents!
For now, non-quantum cryptography is still very safe, because it relies on algorithms that can't be cracked in less than the lifetime of the universe by all the currently existing computers. So in theory, there is not much need for quantum cryptography yet; however, we never know when technology will take a leap forward and quantum techniques will become necessary to protect our information.
When quantum computers will come into play, the computational speeds will increase dramatically, so the mathematical complexity of algorithms will become less of a challenge.
It is still debatable whether or not it will be possible to simply increase the numbers use in the algorithms and thus increase the complexity enough to outrun even quantum computer. Yet there is no debate about the fact that quantum cryptography is a true breakthrough in the field.
It is still being refined and developed further. However, already it is clear that even with its current imperfections, it is many steps above everything that was developed before it. All we need is some years, or maybe decades or even centuries, to refine the technique and make it practical in the real world. Modern Cryptography: Theory and Applications. Quantum Cryptography Introduction Quantum cryptography is an attempt to allow two users to communicate using more secure methods than those guaranteed by traditional cryptography.
History from s to Quantum cryptography was first proposed by Stephen Weisner in his work "Conjugate Coding" in the early s. Outline of the process In quantum cryptography, data is converted to bits of 0s and 1s, and then transferred using polarized photons.
Sending Alice determines the polarization horizontal, vertical, left-circular or right-circular of each burst of photons which she's going to send to Bob. Since a lot of this information will later be discarded, this can probably be done randomly. The goal is not to transfer a specific key, but to agree on a key that is common to both parties. A light source from a light-emitting diode LED or from a laser is filtered to produce the desired polarized photons. Ideally, each pulse consists of a single photon.
However, in real life it actually has to be a beam of light with very low intensity. If the intensity is too low, a pulse may be undetectable for the receiver, yet if it's too high, then the polarization of the photons can be detected discreetly i. Both cases are undesireable to say the least, so the intensity has to be regulated very carefully.
Bob tells Alice which sequence of bases he used, without worrying about other people hearing this information. Alice publicly responds with which bases were chosen correctly.
Alice and Bob discard all observations except for those with the correctly-chosen bases. The remaining observations are converted on to binary code left-circular or horizontal is 0, and right-circular or vertical is 1. Correcting errors - step 1 Alice and Bob agree on random permutations of bits in the resulting string, to randomize the positions of errors. Two errors next to each other are very hard to detect, yet it is likely that an error with the instruments or because of random noise would alter a sequence of bits one after the other.
Randomization helps account for that. The strings are partitioned into blocks of size k, with k ideally chosen to make the probability of multiple errors per block very small. Note that if Alice's string contains and Bob's contains , the parity is the same, 1, even though there are two mistakes in the block. Making k small is one of the steps taken to minimize the chance of this happening, since the chances of having two errors in a block of size 50 is much less than in a block of , especially after the errors are randomized.
Alice and Bob compute and exchange parities for each block. This information can be made public, but, to ensure security, the last bit of each block is then discarded, making the information useless for Eve.
Any block with different reported is broken down further and a binary search is used to locate and correct the error. Alternatively, if the length of the key is already sufficiently large, those blocks could even be discarded. Steps are then repeated with increasing block size, k, in an attempt to discover multiple errors that could've gone undetected within original blocks.
Correcting errors - step 2 Finally, to determine if additional errors remain, Alice and Bob do another randomized check. They publicly agree on a random assortment of half the bit positions in their string, and compare parities, followed discarding the last digit, as always. Then a binary search is used to find and eliminate error, as described above. Alternate methods of exchanging keys Actually, the information can be exchanged in a number of ways, not just the one described above.
She uses B1 to select the basis and B2 to select the polarization of the photon. Typically, information is encoded on single photons, as shown in the figure.
If he measures in a base that is different from the one Alice used to prepare, then his answer will be random and discarded, but if they chose the same one, then they will have perfectly correlated results; Alice sends H and Bob detects H, and these are kept. This last step requires Alice and Bob to communicate about which base was used but reveals no information about the result, which now becomes the secret key.
This is just one way to do but there are now many variations. The beauty that quantum physics brings to this solution is that if a spy or a hacker tries to intercept the key generation, they will introduce errors and reveal themselves. Importantly, this happens before any information is encoded or communicated! Laboratory demonstrations and some field tests of QKD in the s paved the way for the first commercial systems in the early s.
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