Quantum correlations are central resources for quantum information processing, yet their ability to manipulate dynamical transmission processes remains largely unexplored. Here, we investigate this ability through backflow, a uniquely interference phenomenon in which local probability flow propagates opposite to the momentum direction. We report the first nonlocal manipulation of backflow in double-slit interference using polarization-path-entangled photons. By performing local measurements on one photon, we remotely engineer the relative amplitude and phase of the two paths associated with its partner, manipulating the emergence, spatial distribution, and propagation dynamics of backflow without directly accessing the interfering system. Combining weak measurements to extract the transverse momentum and reconstruct Bohmian trajectories, we provide a direct visualization of the manipulation process with single-pixel spatial resolution. Furthermore, using Werner states with tunable correlation strengths, we reveal a distance-dependent resource requirement for nonlocal backflow manipulation: the minimum correlation strength required to induce backflow increases with propagation distance, progressing from entanglement to EPR-steering and ultimately Bell nonlocality. Our results show quantum correlations as operational resources for manipulating transmission dynamics and open new avenues for non-contact manipulation of fragile or inaccessible systems.
Quantum correlations are central resources for quantum information processing, yet their ability to manipulate dynamical transmission processes remains largely unexplored. Here, we investigate this ability through backflow, a uniquely interference phenomenon in which local probability flow propagates opposite to the momentum direction. We report the first nonlocal manipulation of backflow in double-slit interference using polarization-path-entangled photons. By performing local measurements on one photon, we remotely engineer the relative amplitude and phase of the two paths associated with its partner, manipulating the emergence, spatial distribution, and propagation dynamics of backflow without directly accessing the interfering system. Combining weak measurements to extract the transverse momentum and reconstruct Bohmian trajectories, we provide a direct visualization of the manipulation process with single-pixel spatial resolution. Furthermore, using Werner states with tunable correlation strengths, we reveal a distance-dependent resource requirement for nonlocal backflow manipulation: the minimum correlation strength required to induce backflow increases with propagation distance, progressing from entanglement to EPR-steering and ultimately Bell nonlocality. Our results show quantum correlations as operational resources for manipulating transmission dynamics and open new avenues for non-contact manipulation of fragile or inaccessible systems.
High-dimensional entanglement in the form of transverse spatial correlation between a pair of photons generated via spontaneous parametric downconversion is not only a valuable resource in many academic and real-life applications but also provides access to several intriguing quantum phenomena. One such non-intuitive phenomenon is phase entanglement, in which the biphoton state is correlated in the complex phase of its wavefunction. This state, which emerges during the propagation of the biphoton wavefunction, exhibits neither position nor momentum correlation, yet retains full entanglement. In this work, we experimentally explore this state in two distinct ways. The first is by tracking the vanishing spatial photon number correlation over propagation distances lying in [0,∞), folded into a finite range using single-lens imaging. These observations show excellent agreement with our theoretical predictions based on the Double Gaussian (DG) approximation of the biphoton state. The second approach involves performing a two-photon interference experiment using a double slit and this state, which reveals the correlated phase front. We show, both theoretically and experimentally, that the observed two-photon interference structure is markedly different from that produced by position-correlated photons, as confirmed by computing the joint probability distribution of photons (JPD) and related metrics. Such interference using phase-entangled light has not been attempted before and opens avenues for advanced experiments and applications in the field of spatial entanglement.
Nonlocal quantum correlations, such as quantum entanglement, quantum steering, and Bell nonlocality, are crucial resources for quantum information tasks. How to protect these quantum resources from decoherence is one of the most urgent problems to be solved. Here, we investigate the evolution of these correlations in the correlated squeezed generalized amplitude damping (SGAD) channel and propose a scheme to protect them with weak measurement (WM) and quantum measurement reversal (QMR). Compared with the results of the uncorrelated SGAD channel, we find that when [Formula: see text], correlation and squeezing effects can prolong the survival time of quantum entanglement, Bell nonlocality, and quantum steering by about 152 times, 207 times, and 10 times, respectively. In addition, local WM and QMR can effectively recover the disappeared nonlocal quantum correlations either in uncorrelated or completely correlated SGAD channels. Moreover, we find that these initial nonlocal quantum correlations could be drastically amplified under the correlated channel. And the steering direction can be flexibly manipulated either by changing the channel parameters or the strength of WM and QMR. These results not only make a step forward in suppressing decoherence and enhancing quantum correlation in noise channels, but also help to develop relevant practical applications.
Interpretations of quantum mechanics (QM), or proposals for underlying theories, that attempt to present a definite realist picture, such as Bohmian mechanics, require strong non-local effects. Naively, these effects would violate causality and contradict special relativity. However if the theory agrees with QM the violation cannot be observed directly. Here, we demonstrate experimentally such an effect: we steer the velocity and trajectory of a Bohmian particle using a remote measurement. We use a pair of photons and entangle the spatial transverse position of one with the polarization of the other. The first photon is sent to a double-slit-like apparatus, where its trajectory is measured using the technique of Weak Measurements. The other photon is projected to a linear polarization state. The choice of polarization state, and the result, steer the first photon in the most intuitive sense of the word. The effect is indeed shown to be dramatic, while being easy to visualize. We discuss its strength and what are the conditions for it to occur.
Partially coherent quantum-entangled beams combine quantum correlation with partial coherence, allowing them to maintain quantum characteristics while being more resistant to distortions caused by random media during propagation. In this study, we investigate the effect of coherence variation of such beams on non-local double-slit quantum interference. The spatial coherence variation is achieved by controlling the transverse coherence length of the Gaussian Schell model pump in the spontaneous parametric down conversion process. For a fixed beam size, the momentum correlation width of partially coherent biphotons increases with the decrease in the transverse coherence length. This results in a biphoton beam exhibiting multiple spatial modes, making it more suitable for studying the non-local features of quantum states in imaging, interference, and diffraction experiments. Our findings infer both high-quality and near-unity visibility of nonlocal interference using the partially coherent twin beams, even with the substantial decrease in the coherence of the pump. We believe these results can enhance robustness against the deleterious effects of the medium during propagation and can have potential applications in optical image cryptography, biomedical imaging, quantum lithography, and quantum holography.
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.
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.