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  • 1. Chen, Geng
    et al.
    Aharon, Nati
    Sun, Yong-Nan
    Zhang, Zi-Huai
    Zhang, Wen-Hao
    He, De-Yong
    Tang, Jian-Shun
    Xu, Xiao-Ye
    Kedem, Yaron
    Stockholm University, Faculty of Science, Department of Physics.
    Li, Chuan-Feng
    Guo, Guang-Can
    Heisenberg-scaling measurement of the single-photon Kerr non-linearity using mixed states2018In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 9, article id 93Article in journal (Refereed)
    Abstract [en]

    Improving the precision of measurements is a significant scientific challenge. Previous works suggest that in a photon-coupling scenario the quantum fisher information shows a quantum-enhanced scaling of N-2, which in theory allows a better-than-classical scaling in practical measurements. In this work, utilizing mixed states with a large uncertainty and a post-selection of an additional pure system, we present a scheme to extract this amount of quantum fisher information and experimentally attain a practical Heisenberg scaling. We performed a measurement of a single-photon's Kerr non-linearity with a Heisenberg scaling, where an ultra-small Kerr phase of. 6 x 10(-8) rad was observed with a precision of similar or equal to 3.6 x 10(-10) rad. From the use of mixed states, the upper bound of quantum fisher information is improved to 2N(2). Moreover, by using an imaginary weak-value the scheme is robust to noise originating from the self-phase modulation.

  • 2.
    Edge, Jonathan M.
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Kedem, Yaron
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Aschauer, Ulrich
    Spaldin, Nicola A.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA.
    Quantum Critical Origin of the Superconducting Dome in SrTiO32015In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 115, no 24, article id 247002Article in journal (Refereed)
    Abstract [en]

    We expand the well-known notion that quantum criticality can induce superconductivity by proposing a concrete mechanism for superconductivity due to quantum ferroelectric fluctuations. To this end, we investigate the origin of superconductivity in doped SrTiO3 using a combination of density functional and strong coupling theories within the framework of quantum criticality. Our density functional calculations of the ferroelectric soft mode frequency as a function of doping reveal a crossover related to quantum paraelectricity at a doping level coincident with the experimentally observed top of the superconducting dome. Thus, we suggest a model in which the soft mode fluctuations provide the pairing interaction for superconductivity carriers. Within our model, the low doping limit of the superconducting dome is explained by the emergence of the Fermi surface, and the high doping limit by departure from the quantum critical regime. We predict that the highest critical temperature will increase and shift to lower carrier doping with increasing O-18 isotope substitution, a scenario that is experimentally verifiable. Our model is applicable to other quantum paraelectrics, such as KTaO3.

  • 3. Ferreiros, Yago
    et al.
    Kedem, Yaron
    Stockholm University, Faculty of Science, Department of Physics.
    Bergholtz, Emil J.
    Stockholm University, Faculty of Science, Department of Physics.
    Bardarson, Jens H.
    Mixed Axial-Torsional Anomaly in Weyl Semimetals2019In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 122, no 5, article id 056601Article in journal (Refereed)
    Abstract [en]

    We show that Weyl semimetals exhibit a mixed axial-torsional anomaly in the presence of axial torsion, a concept exclusive of these materials with no known natural fundamental interpretation in terms of the geometry of spacetime. This anomaly implies a nonconservation of the axial current-the difference in the current of left- and right-handed chiral fermions-when the torsion of the spacetime in which the Weyl fermions move couples with opposite sign to different chiralities. The anomaly is activated by driving transverse sound waves through a Weyl semimetal with a spatially varying tilted dispersion, which can be engineered by applying strain. This leads to a sizable alternating current in the presence of a magnetic field that provides a clear-cut experimental signature of our predictions.

  • 4.
    Kedem, Yaron
    Stockholm University, Faculty of Science, Department of Physics.
    Novel pairing mechanism for superconductivity at a vanishing level of doping driven by critical ferroelectric modes2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 98, no 22, article id 220505Article in journal (Refereed)
    Abstract [en]

    Superconductivity occurring at low densities of mobile electrons is still a mystery since the standard theories do not apply in this regime. We address this problem by using a microscopic model for ferroelectric (FE) modes, which mediate an effective attraction between electrons. When the dispersion of modes, around zero momentum, is steep, forward scattering is the main pairing process and the self-consistent equation for the gap function can be solved analytically. The solutions exhibit unique features: Different momentum components of the gap function are decoupled, and at the critical regime of the FE modes, different frequency components are also decoupled. This leads to effects that can be observed experimentally: The gap function can be nonmonotonic in temperature and the critical temperature can be independent of the chemical potential. The model is applicable to lightly doped polar semiconductors, in particular, strontium titanate.

  • 5.
    Kedem, Yaron
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Zhu, Jian-Xin
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA.
    Unusual superconducting isotope effect in the presence of a quantum criticality2016In: Physical Review B, ISSN 2469-9950, Vol. 93, no 18, article id 184507Article in journal (Refereed)
    Abstract [en]

    The isotope effect in superconductivity (SC) is used to make a concrete connection to a quantum critical point (QCP) that is tunable by isotopic mass substitution. We find a distinct contribution to the isotope exponent in SC and derive an explicit relation to the critical exponent of a QCP. The relation between the two exponents is general and can be used as an experimental signature for the connection between SC and a QCP. We demonstrate it in a scenario where the SC pairing is due to modes related to a structural instability. Within this model the isotope exponent is derived in terms of microscopic parameters.

  • 6. Xiao, Ya
    et al.
    Kedem, Yaron
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Xu, Jin-Shi
    Li, Chuan-Feng
    Guo, Guang-Can
    Experimental nonlocal steering of Bohmian trajectories2017In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 25, no 13, p. 14463-14472Article in journal (Refereed)
    Abstract [en]

    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.

  • 7. Xiao, Ya
    et al.
    Wiseman, Howard M.
    Xu, Jin-Shi
    Kedem, Yaron
    Stockholm University, Faculty of Science, Department of Physics.
    Li, Chuan-Feng
    Guo, Guang-Can
    Observing momentum disturbance in double-slit which-way measurements2019In: Science Advances, E-ISSN 2375-2548, Vol. 5, no 6, article id eaav9547Article in journal (Refereed)
    Abstract [en]

    Making a which-way measurement (WWM) to identify which slit a particle goes through in a double-slit apparatus will reduce the visibility of interference fringes. There has been a long-standing controversy over whether this can be attributed to an uncontrollable momentum transfer. Here, by reconstructing the Bohmian trajectories of single photons, we experimentally obtain the distribution of momentum change. For our WWM, the change we see is not a momentum kick that occurs at the point of the WWM, but rather one that nonclassically accumulates during the propagation of the photons. We further confirm a quantitative relation between the loss of visibility consequent on a WWM and the total (late-time) momentum disturbance. Our results emphasize the role of the Bohmian momentum in giving an intuitive picture of wave-particle duality and complementarity.

  • 8. Xu, Xiao-Ye
    et al.
    Pan, Wei-Wei
    Wang, Qin-Qin
    Dziewior, Jan
    Knips, Lukas
    Kedem, Yaron
    Stockholm University, Faculty of Science, Department of Physics.
    Sun, Kai
    Xu, Jin-Shi
    Han, Yong-Jim
    Li, Chuan-Feng
    Guo, Guang-Can
    Vaidman, Lev
    Measurements of Nonlocal Variables and Demonstration of the Failure of the Product Rule for a Pre- and Postselected Pair of Photons2019In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 122, no 10, article id 100405Article in journal (Refereed)
    Abstract [en]

    We report the first implementation of the von Neumann instantaneous measurements of nonlocal variables, which becomes possible due to technological achievements in creating hyperentangled photons. Tests of reliability and of the nondemolition property of the measurements have been performed with high precision, showing the suitability of the scheme as a basic ingredient of numerous quantum information protocols. The method allows us to demonstrate for the first time with strong measurements a special feature of pre- and postselected quantum systems: the failure of the product rule. It has been verified experimentally that for a particular pre- and postselected pair of particles, a single measurement on particle A yields with certainty sigma(A)(x) = -1, a single measurement on particle B yields with certainty sigma(B)(y) = -1, and a single nonlocal measurement on particles A and B yields with certainty sigma(A)(x) sigma(B)(y) = -1.

  • 9. Zhou, Zong-Quan
    et al.
    Liu, Xiao
    Kedem, Yaron
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Cui, Jin-Min
    Li, Zong-Feng
    Hua, Yi-Lin
    Li, Chuan-Feng
    Guo, Guang-Can
    Experimental observation of anomalous trajectories of single photons2017In: Physical Review A, ISSN 2469-9926, Vol. 95, no 4, article id 042121Article in journal (Refereed)
    Abstract [en]

    A century after its conception, quantum mechanics still hold surprises that contradict many common sense notions. The contradiction is especially sharp in case one consider trajectories of truly quantum objects such as single photons. From a classical point of view, trajectories are well defined for particles, but not for waves. The wave-particle duality forces a breakdown of this dichotomy and quantum mechanics resolves this in a remarkable way: Trajectories can be well defined, but they are utterly different from classical trajectories. Here, we give an operational definition to the trajectory of a single photon by introducing a technique to mark its path using its spectral composition. The method demonstrates that the frequency degree of freedom can be used as a bona fide quantum measurement device (meter). The analysis of a number of setups, using our operational definition, leads to anomalous trajectories which are noncontinuous and in some cases do not even connect the source of the photon to where it is detected. We carried out an experimental demonstration of these anomalous trajectories using a nested interferometer. We show that the two-state vector formalism provides a simple explanation for the results.

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