We experimentally demonstrate a protocol for entanglement distribution by a separable quantum system. In our experiment, two spatially separated modes of an electromagnetic field get entangled by local operations, classical communication, and transmission of a correlated but separable mode between them. This highlights the utility of quantum correlations beyond entanglement for the establishment of a fundamental quantum information resource and verifies that its distribution by a dual classical and separable quantum communication is possible. [6,7]. Furthermore, it extends our abilities to process information. Here, entanglement is used as a resource which needs to be shared between remote parties. However, entanglement is not the only manifestation of quantum correlations. Notably, separable quantum states can also be used as a shared resource for quantum communication. The experiment presented in this Letter highlights the quantumness of correlations in separable mixed states and the role of classical information in quantum communication by demonstrating entanglement distribution using merely a separable ancilla mode.The role of entanglement in quantum information is nowadays vividly demonstrated in a number of experiments. A pair of entangled quantum systems shared by two observers enables us to teleport [8] quantum states between them with a fidelity beyond the boundary set by classical physics. Concatenated teleportations [9] can further span entanglement over large distances [10] which can be subsequently used for secure communication [11]. An a priori shared entanglement also allows us to double the rate at which information can be sent through a quantum channel [12] or one can fuse bipartite entanglement into larger entangled cluster states that are "hardware" for quantum computing [13]. * contributed equally to this workThe common feature of all entangling methods used so far is that entanglement is either produced by some global operation on the systems that are to be entangled or it results from a direct transmission of entanglement (possibly mediated by a third system) between the systems. Even entanglement swapping [9,14], capable of establishing entanglement between the systems that do not have a common past, is not an exception to the rule because also here entanglement is directly transmitted between the participants.However, quantum mechanics admits conceptually different means of establishing entanglement which are free of transmission of entanglement. Remarkably, the creation of entanglement between two observers can be disassembled into local operations and the communication of a separable quantum system between them [15]. The impossibility of entanglement creation by LOCC is not violated because communication of a quantum system is involved. The corresponding protocol exists only in a mixed-state scenario and obviously utilizes fewer quantum resources in comparison with the previous cases because communication of only a discordant [16][17][18] separable quantum system is required.In this Le...
The spin Hall effect of light (SHEL) is the photonic analogue of the spin Hall effect occurring for charge carriers in solid-state systems. This intriguing phenomenon manifests itself when a light beam refracts at an air-glass interface (conventional SHEL) or when it is projected onto an oblique plane, the latter effect being known as the geometric SHEL. It amounts to a polarization-dependent displacement perpendicular to the plane of incidence. In this work, we experimentally investigate the geometric SHEL for a light beam transmitted across an oblique polarizer. We find that the spatial intensity distribution of the transmitted beam depends on the incident state of polarization and its centroid undergoes a positional displacement exceeding one wavelength. This novel phenomenon is virtually independent from the material properties of the polarizer and, thus, reveals universal features of spin-orbit coupling.
We theoretically investigate the quantum uncertainty in the beam width of transverse optical modes and, for this purpose, define a corresponding quantum operator. Single mode states are studied as well as multimode states with small quantum noise. General relations are derived, and specific examples of different modes and quantum states are examined. For the multimode case, we show that the quantum uncertainty in the beam width can be completely attributed to the amplitude quadrature uncertainty of one specific mode, which is uniquely determined by the field under investigation. This discovery provides us with a strategy for the reduction of the beam width noise by an appropriate choice of the quantum state.
We provide experimental evidence of quantum features in bipartite states classified as entirely classical according to a conventional criterion based on the Glauber P function but possessing nonzero Gaussian quantum discord. Their quantum nature is experimentally revealed by acting locally on one part of the discordant state. We experimentally verify and investigate the effect of discord increase under the action of local loss and link it to the entanglement with the environment. Adding an environmental system purifying the state, we unveil the flow of quantum correlations within a global pure system using the Koashi-Winter inequality. For a discordant state generated by splitting a state in which the initial squeezing is destroyed by random displacements, we demonstrate the recovery of entanglement highlighting the role of system-environment correlations. As quantum information science develops towards quantum information technology, the question of the efficient use and optimization of resources becomes a burning issue. So far, quantum information processing (QIP) has been mostly thought of as entanglement-enabled technology. Quantum cryptography is an exception, but even there the so-called effective entanglement between the parties plays a decisive role [1,2]. With the advent of new quantum computation paradigms [3] interest in more generic and even nonentangled QIP resources has emerged [4]. Unlike entanglement, the new resources, commonly dubbed as quantum correlations, reside in all states which do not diagonalize in any local product basis. Entanglement and quantum correlations are equivalent notions only for pure states. Quantumness of correlations in separable states is fundamentally related to the noncommutativity of observables, nonorthogonality of states, and properties of quantum measurements, whereas entanglement can be seen as a consequence of the quantum superposition principle. Correlated mixed states are a lucid illustration of the fact that the quantum-classical divide is actually purpose-oriented and that such states, long considered unsuitable for QIP, may become a robust and efficient quantum tool.In what follows, we will use quantum discord [5] for quantification of quantum correlations. For two systems A and B, quantum discord is defined as the difference,between quantum mutual information I(AB) = S(A) + S(B) − S(AB) encompassing all correlations present in the system, and the one-way classical correlation, which is operationally related to the amount of perfect classical correlations which can be extracted from the system [6]. Here, S is the von Neumann entropy of the respective state, H {ˆ i } (A|B) is the conditional entropy with measurement on B, and the infimum is taken over all possible measurements {ˆ i }.In this Rapid Communication, we focus on bipartite mixed Gaussian states relevant in the context of continuousvariable quantum information [7]. The respective correlation quantifier is then Gaussian quantum discord [8,9] defined by Eq. (1), where the minimization in J ← (AB) is r...
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