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,… Continue reading QUANTUM COMPUTING AND TECHNOLOGIES FOR THE FUTURE

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… Continue reading QUANTUM CRYPTOGRAPHY

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… Continue reading FASTER-THAN-LIGHT COMMUNICATION?

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… Continue reading QUANTUM TELEPORTATION

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… Continue reading QUANTUM INFORMATION

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… Continue reading A QUANTUM RANDOM-NUMBER GENERATOR

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… Continue reading THE AGE OF QUANTUM INFORMATION

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… Continue reading CLOSING THE LOOPHOLES

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… Continue reading TESTING BELL’S INEQUALITY

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… Continue reading HIGH-PURITY SINGLE-PHOTON SOURCE