In this work, firstly, we propose how to experimentally demonstrate the single photon steering phenomenon in a simple way. The quantum steering phenomenon was discovered by Erwin Schrodinger, who reason that the laws of quantum mechanics obliges us to admit that by suitable measurements taken on one of two entangled system, then the state of the other system can be determined and steered. On the other hand, the first proposal about the nonlocal property of a single photon focus on showing the Bell nonlocality by using the single-photon path entanglement. Here, we propose a new experimental scheme that, by incorporating and manipulating the internal degree of freedom (IDF) of the photon, easily demonstrate the nonlocal steering phenomenon of single-photon's states. The experimental set-up that we propose differs from those published in the quantum optics' literature to shown the single photon nonlocality, which generally use the path entanglement of photons and homodyne measurements. Secondly, by incorporating its IDF, we show that the single photon steering and the quantum mechanical interaction-free measurement (QM-IFM) are identical phenomena; we will argue that QM-IFM is just a particular instance of the single-photon steering. In short, both the single photon steering and QM-IFM (being identical phenomena) have a common cause: the nonlocality of the wave function. In conclusion, we have demonstrated that it is possible to produce a nonlocal phenomena by manipulating the single photon internal degree of freedom. On the other hand, this unification between steering and QM-IFM could establish a strong support to counterfactual quantum communication and computation.
In this work, firstly, we propose how to experimentally demonstrate the single photon steering phenomenon in a simple way. The quantum steering phenomenon was discovered by Erwin Schrodinger, who reason that the laws of quantum mechanics obliges us to admit that by suitable measurements taken on one of two entangled system, then the state of the other system can be determined and steered. On the other hand, the first proposal about the nonlocal property of a single photon focus on showing the Bell nonlocality by using the single-photon path entanglement. Here, we propose a new experimental scheme that, by incorporating and manipulating the internal degree of freedom (IDF) of the photon, easily demonstrate the nonlocal steering phenomenon of single-photon's states. The experimental set-up that we propose differs from those published in the quantum optics' literature to shown the single photon nonlocality, which generally use the path entanglement of photons and homodyne measurements. Secondly, by incorporating its IDF, we show that the single photon steering and the quantum mechanical interaction-free measurement (QM-IFM) are identical phenomena; we will argue that QM-IFM is just a particular instance of the single-photon steering. In short, both the single photon steering and QM-IFM (being identical phenomena) have a common cause: the nonlocality of the wave function. In conclusion, we have demonstrated that it is possible to produce a nonlocal phenomena by manipulating the single photon internal degree of freedom. On the other hand, this unification between steering and QM-IFM could establish a strong support to counterfactual quantum communication and computation.
According to Schrödinger, the laws of quantum mechanics obliges us to admit that by suitable measurement taken on one of the two system only1 the state of the other system can not only be determined but steered too. That is, it conveys the potential ability to steer the state of another physical system without interacting with it by implementing independent measurements, this nonlocal phenomenon was named steering. On the other hand, the first proposals about the nonlocality of a single photon focus on showing the Bell nonlocality by using a single photon path entanglement. This path entanglement of a single photon was also used for analyzing and experimentally produce the steering of a single photon. However, these established facts have been recently called into question suggesting that single-photon entanglement is not non-local. In this letter, we show that by incorporating and manipulating the internal degrees of freedom of the photon, together with the external path, it is easy to demonstrate the nonlocal effect of steering of a single photon's state. In this sense, the experimental set-up that we propose differs from the one reported in the quantum optics literature which only uses, to the best of our knowledge, the path entanglement of photons for showing the steering phenomenon, i.e. here we exploit the entanglement between the internal and external degrees of freedom of the photon to show this nonlocal effect. The introduction of the photon's internal degrees of freedom in the experimental set-up gives us new insight, advantages and possibilities to control the nonlocal character of the single-photon entangled state.
A single-photon incident on a beam splitter produces an entangled field state, and in principle could be used to violate a Bell inequality, but such an experiment (without postselection) is beyond the reach of current experiments. Here we consider the somewhat simpler task of demonstrating Einstein-Podolsky-Rosen (EPR) steering with a single photon (also without postselection). We demonstrate that Alice's choice of measurement on her portion of the entangled state can affect Bob's portion of the entangled state in his laboratory, in a sense rigorously defined by us and Doherty [Phys. Rev. Lett. 98, 140402 (2007)]. Previous work by Lvovsky and coworkers [Phys. Rev. Lett. 92, 047903 (2004)] has addressed this phenomenon (which they called remote preparation) experimentally using homodyne measurements on a single photon. Here we show that, unfortunately, their experimental parameters do not meet the bounds necessary for a rigorous demonstration of EPR steering with a single photon. However, we also show that modest improvements in the experimental parameters, and the addition of photon counting to the arsenal of Alice's measurements, would be sufficient to allow such a demonstration.
Recently quantum nonlocality has been classified into three distinct types: quantum entanglement, Einstein-Podolsky-Rosen steering, and Bell's nonlocality. Among which, Bell's nonlocality is the strongest type. Bell's nonlocality for quantum states is usually detected by violation of some Bell's inequalities, such as Clause-Horne-Shimony-Holt inequality for two qubits. Steering is a manifestation of nonlocality intermediate between entanglement and Bell's nonlocality. This peculiar feature has led to a curious quantum phenomenon, the one-way Einstein-Podolsky-Rosen steering. The one-way steering was an important open question presented in 2007, and positively answered in 2014 by Bowles et al., who presented a simple class of one-way steerable states in a two-qubit system with at least thirteen projective measurements. The inspiring result for the first time theoretically confirms quantum nonlocality can be fundamentally asymmetric. Here, we propose another curious quantum phenomenon: Bell nonlocal states can be constructed from some steerable states. This novel finding not only offers a distinctive way to study Bell's nonlocality without Bell's inequality but with steering inequality, but also may avoid locality loophole in Bell's tests and make Bell's nonlocality easier for demonstration. Furthermore, a nine-setting steering inequality has also been presented for developing more efficient one-way steering and detecting some Bell nonlocal states.
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.
Within the hierarchy of inseparable quantum correlations, Einstein-Podolsky-Rosen steering is distinguished from both entanglement and Bell nonlocality by its asymmetry-there exist conditions where the steering phenomenon changes from being observable to not observable, simply by exchanging the role of the two measuring parties. While this one-way steering feature has been previously demonstrated for the restricted class of Gaussian measurements, for the general case of positive-operator-valued measures even its theoretical existence has only recently been settled. Here, we prove, and then experimentally observe, the one-way steerability of an experimentally practical class of entangled states in this general setting. As well as its foundational significance, the demonstration of fundamentally asymmetric nonlocality also has practical implications for the distribution of the trust in quantum communication networks.
Quantum mechanics has led to the biggest technological breakthroughs of last century. However, many paradoxes still remain in the foundations of quantum theory. In this research we study one effect that leads to such paradoxes; quantum correlations. Quantum nonlocality refers to the set of quantum correlations that are not explainable using local models. It is quantum nonlocality that has troubled physicists since the inception of quantum theory nearly 100 years ago. However, these nonlocal quantum correlations offer a range of advantages in quantum information tasks. For example, quantum key distribution and quantum computing are two tasks that are powered by quantum correlations. For this reason a wide range of quantum correlations are studied experimentally in this research. These include: entanglement, EPR-steering, Bell nonlocality and bilocality. We study these correlations using the framework of quantum information, using single photons as our chosen quantum information carriers. The first quantum correlation tested in this research is EPR-steering. EPR-steering is the application of the EPR-paradox to a quantum information task. We implement three different experiments on EPR-steering. By studying the properties of the quantum information task we find that: EPR-steering is more robust to depolarisation noise than Bell-nonlocality; EPR-steering is absolutely loss tolerant; and tests of EPR-steering are easier to carry out, than tests of Bell nonlocality, but harder to implement compared to tests of non-separability. As a result of these three findings we experimentally show that EPR-steering can be demonstrated on Belllocal states, we close the detection loophole in a photonic test of quantum nonlocality, and we discover and implement maximally parsimonious tests of non-separability, EPR-steering and Bell nonlocality.