Quantum information processing is the result of using the physical reality that quantum theory tells us about for the purposes of performing tasks that were previously thought impossible or infeasible. Devices that perform quantum information processing are the " quantum computers".
And as far as Quantum Communication is concerned, basically there are two communication protocols, which, as is known so far, can be implemented using quantum mechanics: Super-Dense coding and Quantum Teleportation.
The basic idea behind Quantum Teleportation is the following: Let's say Alice has particle 1 in a certain quantum state, a qubit A ( I am refraining from throwing in mathematical representations as that is not easy to do here) which has two orthogonal states { 0> and 1> } with complex amplitudes. She wishes to deliver this state to Bob but cannot do so directly ( let's say). Now, according to the postulates of QM ( quantum mechanics) any measurement performed by Alice on her particle will destroy the quantum state at hand without giving Bob all the necessary info required in order to reconstruct the quantum state. Then how do they communicate? The way around is to use an ancilliary pair of "ENTANGLED" particles, say, X and Y ( the EPR pair, Einstein, Podolsky & Rosen, Physics Review, 1935, 47, 777-780), where Alice has particle X and Bob particle Y. The EPR pair is also knows as the Bell state. Such a state, for sake of intuitive experimentation, would have to be created ahead of time, when the qubits are in a lab together and can be made to interact in a way which will give rise to the entanglement between them. After the state is created, Alice and Bob each take one of the two qubits away with them. Alternatively, a third party could create the EPR pair and give one particle to Alice and the other to Bob. If they are careful not to let them interact with the environment, or any other quantum system, Alice and Bob's joint state will remain entangled. This entanglement becomes a resource which Alice and Bob can use to achieve protocols such as super-dense coding and teleportation.
For quantum teleportation, the scenario is that Alice wishes to communicate the state of a qubit to Bob. Suppose Alice only has a classical channel linking her to Bob. To send the state of a qubit exactly, it would seem that Alice would either have to send the physical qubit itself, or she would have to communicate the two complex amplitudes with infinite precision. However, if Alice and Bob possess an entangled state this intuition is wrong, and a quantum state can be sent exactly over a classical channel. Teleportation is a protocol which allows Alice to communicate the state of a qubit exactly to Bob, sending only two bits of classical information to him. Technical implementation of many qubit systems has been, so far, a challenge especially the necessity to have a shared EPR pair for every qubit ( or electron, photon, nucleon) whose state is to be teleported. Single qubit states have been successfully teleported in more than one laboratory using optical and NMR techniques ( see references 1 and 2).
THAT leads us to Quantum non-locality of EPR pairs which is so impressive and difficult to accept that it does appear to be on the verge of the mystical as it seems to demonstrate existence of superluminal effects, in other words, exchange of signaling at a speed faster than light. Efforts to clarify non-locality using the transactional interpretation of quantum mechanics ( EPR experiments), and the possibility of superluminal effects (e.g., faster-than-light communication) from nonlocality and non-linear quantum mechanics is still under investigation ( Prof. J. Cramer's group at University of Washington for e.g.).
Excited atoms often produce two photons in a process called a "cascade" involving two successive quantum jumps. Because of angular momentum conservation, if the atom begins and ends with no net angular momentum, the two photons must have correlated polarizations. When such photons travel in opposite directions, angular momentum conservation requires that if one of the photons is measured to have some definite polarization state, the other photon is required by quantum mechanics to have exactly the same polarization state, no matter what measurement is made.Such correlated photon pairs are in an "entangled" quantum states. Experimental tests of Bell's theorem, i.e, the "EPR experiments", usually use entangled photons from such an atomic cascade.
It just so happens that quantum mechanics and Bell's Theorem make qualitatively different predictions about EPR measurements. In other words, the intrinsic nonlocality of quantum mechanics has been demonstrated by the experimental tests of Bell's theorem. It has been experimentally demonstrated that nature arranges the correlations between the polarization of the two photons by some faster-than-light mechanism that violates Einstein's intuitions about the intrinsic locality of all natural processes. What Einstein called "spooky actions at a distance" are an important part of the way nature works at the quantum level. Einstein's faster-than-light spooks cannot be ignored.
Question: Can quantum nonlocality be used for faster-than-light or backward-in-time communication? Perhaps, for example, a message could be telegraphed from one measurement site of the EPR experiment to the other through a judicious choice of which measurement was performed. The simple answer to this question is "No!" Briefly, the quantum operators characterizing the separated measurements always commute, no matter which measurement is chosen, so non-local information transfer is impossible.
HOWEVER, this prohibition against superluminal communication, as stated above, is a part of standard quantum mechanics. This is broken if quantum mechanics is allowed to be slightly "non-linear", a technical term meaning that when quantum waves are superimposed they may generate a small cross-term not present in the standard formalism.
The onset of non-linear behavior is seen in other areas of physics, e.g. laser light in certain media, and, it is suggested by Nobel Laureate Steven Weinberg that this might also be present but unnoticed in quantum mechanics. Weinberg's non-linear QM subtly alters certain properties of the standard theory, producing new physical effects that can be detected through precise measurements.
So, the answer to the query of "Theoretical Possibility" of teleportation (and Beam-me-up Scotty type transport) as it stands today would be an emphatical "YES". Practically, it has not been proven to be so YET. But that does not mean quantum non-locality or entanglement cannot be faster than light in vacuum. It just means that we have, so far, been unable to prove it to be so.
Briefly, at a quantum level it can happen that two objects form a single entity, even at arbitrarily large separation from each other. A classical (macroscopic) physical object broken into pieces can be described and measured as separate components. An n-particle quantum system cannot always be described in terms of the states of its component pieces. For instance, the state I00> + I11> cannot be decomposed into separate states of each of the two qubits . Any attempt to view this quantum entity as a combination of two indepnedent objects fails, UNLESS the posibility of signal propagation at superluminal speeds is allowed.
An entangled system consisting of two subsystems can't be described as the product of quantum state of two subsystems. In this sense the entangled system is considered as inseparable and non-local. The creation of entanglement between two long-lived and distant systems is important and challenging role in quantum information research. There are various schemes of the employing entanglement for quantum computation, teleportation, optical memory, cryptography, and error correction.
The EPR paradox introduced the concept of entanglement in quantum optics. Modern quantum mechanics contains the novel and counterintuitive features as witnessed in the famous dialogue between Niles Bohr and Albert Einstein. Einstein argued that quantum mechanics is incomplete. In 1935, A. Einstein, B. Podolsky and N. Rosen proposed the EPR paradoxical Gedankenexperiment. Consider a quantum system consisting of two particles such that the individual momentum or position isn't well defined. But the sum of the position that is their center mass and the difference of the momentum that is their individual momentum in the center mass system are well defined. Assuming the two particles are separated in arbitrary distances. The measurement of either momentum or position of particle 1can immediately implies the precise momentum or position of particle 2. The measurement of particle 1 doesn't have any influence on particle 2 (locality condition). Thus the property of particle 2 is independent on of the measurement performed on particle 1.
In the famous reply from Bohr, he argued that two particles in EPR case are always part of quantum system and thus measurement on one particle changes the possible predictions that can be made for the system thus for the other particle.
Since then physicists pay more attention to the entangled state especially after invention of laser. Conventionally entanglement is realized in the microscopic system of a few particles like the trapped ions or atoms. There exist several sources of entangled quantum systems. Entangled ions have been prepared in electromagnetic Paul traps. Controlled entanglement between nuclear spins within a single molecule can be achieved by the technique of nuclear magnetic resonance. In quantum optics there are two classes of the entanglement: (a). Entanglement between single photon and (b). Between the quadrature components of light beam.
Parametric down-conversion processes have been utilized for creation a pair of entangled photons. Two different kinds of polarization entanglement between the two photons-parallel and orthogonal can be made by two type of the phase matching scheme in the down-conversion. Varying the phase relation to make the emission of the two photons in different frequency (momentum) and different direction (mode) can also achieve the momentum entanglement. A so-called time entanglement can also be observed if we can vary the phase of the two light path. The quadature component entanglement proposed by L. Vaidman, further elaborated on by Braubstein and Kimble and experimentally realized at Caltech.
A solid step towards a Quantum Internet has been taken by Prof. Seth Lloyd of MIT and Prof. Prem Kumar of NW University by establishing quantum logic gates with "entangled" photons. Please see the link from MIT Tech Review and UoW. The rest should be history in the making…
In my very humble personal opinion, the real- life work in stuff like quantum teleportation (QTP) and quantum non-locality is not really as far away as it seems, especially if the tech-sector biggies ( though IBM and possibly HP are doing some work in their research labs) decide to invest in the research of the same. Besides lack of solid and steady funding, I think development in this area has suffered to some extent because most students of Physics lack a bridge to the realistic world of Communication technology and most telecom engineers lack a thorough insight into Quantum Mecahnics and hence the inspiration to go ahead dissappears quickly.
The Quantum World and manipulation of it perhaps require a bit a of a leg-up in terms of knowledge, insight and application from the world of semiconductor chip based computers that we have known so far. We need to bridge this gap. There is decreasing money in research in this field probably due to scepticism among other reasons ( we did the same with Controlled & Cold Fusion research couple of decades back and almost shut down the most promising source of almost everlasting cheap energy). This is a science that, if successfully implemented, will break all known barriers of technology. But very little Industrial interest is openly exhibited at present time. Physics of subjects like quantum cryptography offers not a "quantum algorithm" or a "quantum software" but a solution based on the quantum system themselves. Technology connected directly to mother nature…
Links:
http://faculty.washington.edu/jcramer/NLS/NL_signal.htm
MIT Tech Review: http://www.technologyreview.com/Infotech/20565/?a=f
References:
1) A. Galindo and M.A. Martin-Delgado: Infomration and Computation: Classical and Quantum Aspects, 74:347-423,
2) Michael A. Nielsen and I. L. Chuang, Quantum Information and Communication. Cambridge University Press, 2001
3) P. H. Eberhard, (1977) Nuovo Cimento 38B, 75.
4) P. H. Eberhard, (1978) Nuovo Cimento 46B, 392
5) Steven Weinberg, (1989) Physical Review Letters 62, 485.
6) Joseph Polchinski, (1991) Physical Review Letters 66, 397.
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