We show that controlled interference of a particle's wavefunction can be used to perform a quantum mechanical measurement in an incomplete basis. This happens because the measurement projects the particle into a lower dimensional subspace of the Hilbert space of the incoming wave. It allows a sender (Alice) to signal the receiver (Bob) nonlocally, by Alice's choosing to measure in a complete or incomplete basis (in general: bases of differing incompleteness). If experimentally confirmed, it furnishes a new quantum communication act: nonlocal transmission of a bit without concomitant causal communication. However, the question of its compatibility with special relativity remains unresolved.
We show that controlled interference of a particle's wavefunction can be used to perform a quantum mechanical measurement in an incomplete basis. This happens because the measurement projects the particle into a lower dimensional subspace of the Hilbert space of the incoming wave. It allows a sender (Alice) to signal the receiver (Bob) nonlocally, by Alice's choosing to measure in a complete or incomplete basis (in general: bases of differing incompleteness). If experimentally confirmed, it furnishes a new quantum communication act: nonlocal transmission of a bit without concomitant causal communication. However, the question of its compatibility with special relativity remains unresolved.
In order to understand whether nonlocality implies information transfer, a quantum optical experimental test, well within the scope of current technology, is proposed. It is essentially a delayed choice experiment as applied to entangled particles. The basic idea is: given two observers sharing position-momentum entangled photons, one party chooses whether she measures position or momentum of her photons after the particles leave the source. The other party should infer her action by checking for the absence or presence of characteristic interference patterns after subjecting his particles to certain optical pre-processing. An occurance of signal transmission is attributed to the breakdown of complementarity in incomplete measurements. Since the result implies that the transferred information is classical, we discuss some propositions for safeguarding causality.
In 1927, at the Solvay conference, Einstein posed a thought experiment with the primary intention of showing the incompleteness of quantum mechanics; to prove it, he employed the instantaneous nonlocal effects caused by the collapse of the wavefunction of a single particle-the spooky action at a distance-, when a measurement is done. This historical event preceded the well-know Einstein-Podolsk-Rosen criticism over the incompleteness of quantum mechanics. Here, by using the Stern-Gerlach experiment, we demonstrate how the instantaneous nonlocal feature of the collapse of the wavefunction together with the single-particle entanglement can be used to produce the nonlocal effect of steering, i.e. the single-particle steering. In the steering process Bob gets a quantum state depending on which observable Alice decides to measure. To accomplish this, we fully exploit the spreading (over large distances) of the entangled wavefunction of the single-particle. In particular, we demonstrate that the nonlocality of the single-particle entangled state allows the particle to "know" about the kind of detector Alice is using to steer Bob's state. Therefore, notwithstanding strong counterarguments, we prove that the single-particle entanglement gives rise to truly nonlocal effects at two faraway places. This opens the possibility of using the single-particle entanglement for implementing truly nonlocal task.
Since our early childhood we know in our bones that in order to interact with an object we have either to go to it or to throw something at it. Yet, contrary to all our daily experience, Nature is nonlocal: there are spatially separated systems that exhibit nonlocal correlations. In recent years this led to new experiments, deeper understanding of the tension between quantum physics and relativity and to proposals for disruptive technologies. Consider two spatially separated quantum systems, one controlled by Alice, the other by Bob, in a pure state . Alice and Bob may perform some measurements x and y on their systems and collect the results a and b, respectively. This situation is described by a conditional probability distribution p (a,bj x,y). In general this correlation doesn’t factorize: p (a,bj x,y) 6 p (aj x) � p (bj y), i.e. the two systems are correlated. At first, this is no surprise, correlations are everywhere. For example, consider two cups of the same color, either both red or both green, one in Alice’s and one in Bob’s hands. If they looks at the color of their cups, Alice and Bob’s results are correlated. In this example the origin of the correlation is obvious, Alice and Bob had only partial information: they knew that both have the same color, but they ignored which color. This differs deeply from the quantum situation, as quantum theory claims that a pure state provides a complete description of the two systems. This led EPR[2] to believe that quantum theory is incomplete in the same sense as the description ”of the same color” provides only an incomplete description of the color state of the cups.
this paper was written around the same time as Bell's landmark paper which addressed the problem of nonlocality [2]. This question of nonlocality had first been raised by Einstein, Podolsky and Rosen (EPR) who claimed that if quantum mechanics were a complete model of reality, then nonlocal interactions between particles had to exist. [3]. Since they felt that nonlocality was impossible, quantum mechanics either had to be wrong or at least incomplete. An experiment was later performed that showed that nonlocal influences do exist once these particles interact and, that one can test the explicit quantum nature of systems via the use of EPR nonlocality [4]. And, as per Feynman, since this nonlocality cannot be duplicated by a classical system, this enables it to be used to test the quantum nature of systems [5]. The experiments performed and the results obtained were as follows
Present understanding of non-possibility of Quantum communication rests on analysis of possibility of signalling within a Quantum system. Is it possible to achieve signalling by a setup where the system is changed from one to another where the act of change serves as the signal and the change in the measurement probabilities would be detected and deciphered at the receiver end? A proposal is made where a variant of Delayed Choice Quantum Eraser experiment setup of Kim et al is used to achieve the same. The setup is also designed such that the speed of communication can be faster than light. NonLocality and Quantum communication Nonlocality is a feature of Quantum Mechanics that has been firmly established through experiments [Aspect et al, 1981] and well accepted now. Entangled particles separated by a distance still are part of one entity and when that entity changes state by a measurement in one part of it the change happens instantaneously across the domain the entity pervades. Is it possible to use this instantaneous state change to transmit information across at high speeds, faster than light (FTL)? This issue has been dealt with from the time the Einstein raised the EPR paradox [Einstein et al,1935] and has been claimed to be impossible and shown to be so by Peres, [Peres and Tenko, 2004] through the no-communication theorem which shows that any amount of signalling by Alice through measurements done at one end of the quantum system would not be sufficient for Bob to detect and decipher at the other end of the system. However the discussions in literature (as above) always deal with information transfer within a stable quantum system by making measurements at one end and analysing the possibility of detecting it at the other end. Peres and Tenko analyse a Quantum System S of mixed-state where Alice and Bob make measurements at their ends of the system. The authors derive the probability that Bob’s outcome is ν as
In 1935, Einstein, Podolsky, and Rosen argued that quantum mechanics is incomplete based on the assumption that local actions cannot influence elements of reality at a distant location (local realism). In this work, using a recently defined quantum reality quantifier, we show that Alice's local quantum operations can be correlated with the erasure of the reality of observables in Bob's causally disconnected laboratory. To this end, we implement a modified optical quantum eraser experiment, ensuring that Alice's and Bob's measurements remain causally disconnected. Using an entangled pair of photons and quantum state tomography, we experimentally verify that, even with the total absence of any form of classical communication, the choice of quantum operation applied by Alice on her photon is correlated with the erasure of a spatial element of reality of Bob's photon. Our results reveal that Bob's photon can entangle two extra non-interacting degrees of freedom, thus confirming that Bob's photon path is not an element of physical reality.