Category: Entanglement

We had mentioned earlier that quantum information is very different than classical information. We saw that while classical information is independent of its physical representation, quantum information cannot be read without disturbing the physical process encoding it. In addition, we mentioned that the Quantum No-Cloning Theorem prevents us from copying quantum information without errors. However,…

Coding messages was traditionally of interest only to diplomats, the military, and spy agencies. However, cryptography is now an essential part of everyone’s life, given that the Internet is used to communicate all of our bank transactions, industrial designs, and commercial dealings. Most systems today use an encrypting method known as public-key cryptography, in which Bob…

The fact that the state of Bob’s photon changes instantaneously when Alice makes her Bell-state measurement is often confused by nonphysicists as a technology that could be used for communicating faster than light. The error is in thinking that an entangled photon encodes the information, when in fact the information is encoded in the correlation between the…

It would seem that it is impossible to communicate or manipulate an unknown quantum state without destroying its fragile nature and causing its collapse. Furthermore, we have already seen that any measurement, however mild, causes Heisenberg’s Uncertainty Principle to kick in and cause a random change in the system being measured. In 1993, scientists Charles…

Let’s try to shed some light onto the matter of quantum information by defining “information” as something that is encoded in the state of a physical system. As we have discussed, information may be encoded in the voltage on a line, a specific polarization of light, the side of a coin facing up, or vibrations…

It isn’t easy at all to understand how a bit that is both a digital one and a digital zero at the same time could be used to convey or process data. In fact, the perfectly random outcome of measuring such a bit is now used to generate random streams of numbers. The random-number function…

So far, we have explored the fundamentals of quantum physics, perhaps paying a bit too much attention to its mind-boggling philosophical implications. In the real world of academic and industrial physics however, quantum mechanics is applied very successfully to the solution of physics problems and in the development of electronic devices without the need for…

There are a number of experimental problems or “loopholes” that may affect the validity of results in Bell test experiments conducted with the type of apparatus we just described. The main challenge relates to the low detection efficiency of optical systems, and the way in which it affects the “fair sampling” of coincidences that could…

In 1982, French physicist Alain Aspect was able to run the first experimental test of Bell’s Inequality.50 Aspect used a very complex entangled source in which calcium atoms (in an atomic jet) are pumped by a two separate lasers.51 Using the CHSH form of Bell’s Inequality, Aspect was looking for the value of a certain coincidence statistical…

Our single-photon experiments (Figure 33, Figure 90, Figure 121, Figure 125, and Figure 132) have all used a highly attenuated laser beam to produce single photons. However, a precise analysis of this method shows that we could be assured that single photons fly through the apparatus only if the source photons are completely independent of each other. But photons…