We report results of a high precision phase estimation based on a weak measurements scheme using commercial light-emitting diode. The method is based on a measurement of the imaginary part of the weak value of a polarization operator. The imaginary part of the weak value appeared due to the measurement interaction itself. The sensitivity of our method is equivalent to resolving light pulses of order of attosecond and it is robust against chromatic dispersion.High precision phase measurements play a significant role in modern physics. The standard tool is an interferometer with a balanced homodyne detection [1]. It requires a coherent source and the precision is dominated by the intrinsic quantum noise [2]. To reduce the influence of the noise, quantum metrology technologies [3] including N00N states [4] and squeezed states [5] have been exploited, while white light is usually deemed to be useless in quantum metrology. Recently it has been proposed that white light can be used for a very precise phase estimation [6,7], when weak measurements are performed. Here we experimentally demonstrate such a sensitive method utilizing white light from a commercial light-emitting diode (LED). This opens a new avenue for a high-resolution phase estimation.As in other weak measurement experiments in which the Aharonov-Vaidman-Albert (AAV) amplification effect [8] was demonstrated, we measure the photon polarization operator A with eigenvalues 1 and -1 for the two orthogonal polarizations. The polarization can be preand post-selected with a very good precision. The role of the measuring device is played by the spatial degree of freedom of light. In most weak measurement experiments, the relevant spatial degree of freedom is the position in the transverse direction, i.e. perpendicular to the direction of the light propagation. Here we consider, instead, the longitudinal direction [6,7].The interaction Hamiltonian iswhere g(t) is the coupling strength satisfying g(t)dt = k and P is a component of the momentum of the photon. In the first realization of weak measurement the transversal shift was created by a tilted plate of a birefringent material [9]. In our experiment, the plate is placed perpendicularly to the photon's velocity and leads to a longitudinal shift, see Fig. 1a. We consider a very thin birefringent plate which leads to a time delay of a few attoseconds between the wave packets with different polarizations. The AAV effect with the proper pre-and post-selection of polarization can increase the time delay significantly, but a truly dramatic advantage is obtained for the measurement of the imaginary part of the weak Weak measurement of the photon polarization. Photons emitted from the source are preselected by a polarization beamsplitter (PBS) in a state |ψpre , undergo weak measurement interaction by passing through birefringent plate and are post-selected at a nearly orthogonal state |φpost by a second PBS. a). The wave packets with orthogonal polarizations are delayed after birefringent plate one relative to the othe...

Revivals of quantum correlations in composite open quantum systems are a useful dynamical feature against detrimental effects of the environment. Their occurrence is attributed to flows of quantum information back and forth from systems to quantum environments. However, revivals also show up in models where the environment is classical, thus unable to store quantum correlations, and forbids system-environment back-action. This phenomenon opens basic issues about its interpretation involving the role of classical environments, memory effects, collective effects and system-environment correlations. Moreover, an experimental realization of back-action-free quantum revivals has applicative relevance as it leads to recover quantum resources without resorting to more demanding structured environments and correction procedures. Here we introduce a simple two-qubit model suitable to address these issues. We then report an all-optical experiment which simulates the model and permits us to recover and control, against decoherence, quantum correlations without back-action. We finally give an interpretation of the phenomenon by establishing the roles of the involved parties.

Einstein-Podolsky-Rosen (EPR) steering describes the ability of one observer to nonlocally "steer" the other observer's state through local measurements. EPR steering exhibits a unique asymmetric property, i.e., the steerability can differ between observers, which can lead to one-way EPR steering in which only one observer obtains steerability in the steering process. This property is inherently different from the symmetric concepts of entanglement and Bell nonlocality, and it has attracted increasing interest. Here, we experimentally demonstrate asymmetric EPR steering for a class of two-qubit states in the case of two measurement settings. We propose a practical method to quantify the steerability. We then provide a necessary and sufficient condition for EPR steering and clearly demonstrate one-way EPR steering. Our work provides new insight into the fundamental asymmetry of quantum nonlocality and has potential applications in asymmetric quantum information processing.Quantum nonlocality, which does not have a counterpart in classical physics, is the characteristic feature of quantum mechanics. First noted in the famous paper published by Einstein, Podolsky and Rosen (EPR) in 1935 [1], which aimed to argue the completeness of quantum mechanics, the content of quantum nonlocality has been greatly extended. In 2007, Wiseman et al. summarized the different conditions of quantum nonlocality and reformulated the concept of steering [2] originally introduced by Schrödinger [3] in response to the EPR paper (usually referred to as EPR steering), which stands between entanglement [1] and Bell nonlocality [4] in the hierarchy. In the view of a quantum information task, EPR steering can be regarded as the distribution of entanglement from an untrusted party, whereas entangled states need both parties to trust each other, and Bell nonlocality is presented on the premise that they distrust each other [5,6]. As a result, some entangled states cannot be employed to realize steering, and some steerable states do not violate Bell-like inequalities. EPR steering provides a novel insight into quantum nonlocality, and it exhibits an inherent asymmetric feature that differs from both entanglement and Bell nonlocality. Consider two observers, Alice and Bob, who share entangled states. There are cases in which the ability of Alice to steer Bob's state is not equal to the ability of Bob to steer Alice's state. There are also situations in which Alice can steer Bob's state but Bob cannot steer Alice's state, or vice versa; these situations are referred to as one-way steering [2,7]. Several * These two authors contributed equally to this work. theoretical [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] and experimental studies [23][24][25][26][27][28][29][30][31] have focused on the verification and applications of EPR steering. Experimental demonstrations of one-way steering with the measurements restricted to Gaussian measurements have been reported [24,25]. A class of entangled qubit states that can be used to show th...

Einstein-Podolsky-Rosen (EPR) steering describes the ability of one party to remotely affect another's state through local measurements. One of the most distinguishable properties of EPR steering is its asymmetric aspect. Steering can work in one direction but fail in the opposite direction. This type of one-way steering, which is different from the symmetry concepts of entanglement and Bell nonlocality, has garnered much interest. However, an experimental demonstration of genuine one-way EPR steering in the simplest scenario, i.e., one that employs two-qubit systems, is still lacking. In this Letter, we experimentally demonstrate one-way EPR steering with multimeasurement settings for a class of two-qubit states, which are still one-way steerable even with infinite settings. The steerability is quantified by the steering radius, which represents a necessary and sufficient steering criterion. The demonstrated one-way steering in the simplest bipartite quantum system is of fundamental interest and may provide potential applications in one-way quantum information tasks.