We demonstrate the conditional reversal of a weak (partial-collapse) quantum measurement on a photonic qubit. The weak quantum measurement causes a nonunitary transformation of a qubit which is subsequently reversed to the original state after a successful reversing operation. Both the weak measurement and the reversal operation are implemented linear optically. The state recovery fidelity, determined by quantum process tomography, is shown to be over 94% for partial-collapse strength up to 0.9. We also experimentally study information gain due to the weak measurement and discuss the role of the reversing operation as an information erasure.PACS numbers: 03.65.Wj, 42.50.Dv, The projection postulate states that measurement of a variable of a quantum system irrevocably collapses the initial state to one of the eigenstates (corresponding to the measurement outcome) of the measurement operator and is one of the basic postulates of the standard quantum theory [1,2]. The initial state can never be recovered after a projection measurement on a quantum system.If the measurement is not sharp (i.e., non-projective measurement), however, the situation is different. It is possible to reverse the measurement-induced state collapse and the unsharpness of a measurement has been shown to be related to the probabilistic nature of the reversing operation which can serve as a probabilistic quantum error correction [3]. In particular, practical schemes for reversing the state collapse due to a weak (or partialcollapse) measurement in a solid-state qubit have been proposed in Ref.[4] and one of the schemes has recently been demonstrated using a superconducting phase qubit in Ref. [5].Since single-photon states and linear optics play important roles in quantum communication and quantum computing research [6,7,8,9], it is of interest and importance to investigate how the measurement-induced state collapse due to a weak measurement can be reversed for a photonic qubit. In this letter, we report a linear optical implementation of conditional reversal of weak (or partial-collapse) quantum measurements on a photonic qubit. We demonstrate experimentally that a nonunitary transformation of a photonic qubit, caused by a weak quantum measurement, can be reversed by applying an appropriately designed reversing operation. We also quantify and experimentally study information gain due to the weak measurement and discuss the role of the reversing operation as an information erasure.Consider the initial state of a qubit represented in the computational basis, |ψ o = α|0 + β|1 , where * Electronic address: yskim25@postech.ac.kr † Electronic address: yoonho@postech.ac.kr |α| 2 + |β| 2 = 1. Ordinary projection measurement in the computational basis would collapse the state into |0 (or |1 ) with the probability equal to |α| 2 (or |β| 2 ). The projection measurement cannot be reversed because the projection operators P 0 = |0 0| = ( 1 0 0 0 ) and P 1 = |1 1| = ( 0 0 0 1 ) do not have mathematical inverse. An unsharp measurement on the qubit, however, ...

We report the presence of defects in CH3NH3PbI3, which is one of the main factors that deteriorates the performance of perovskite solar cells.

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SrTiO 3 is a well-known incipient ferroelectric (FE) material.1 As quantum fluctuation and antiferrodistortion compete with FE instability, SrTiO 3 remains in paraelectric phase even at low temperatures. Many researchers have used the strain engineering concept based on a lattice mismatch with different substrates to improve the FE properties.2,3 We recently successfully fabricated strontium titanate (STO) thin films that exhibited room-temperature ferroelectricity. 4 We introduced vacancy defects in the STO films by adjusting the growth conditions. Although we attributed the occurrence of ferroelectricity to the defects in the STO films, 4 it was unclear what types of defect play important roles in this ferroelectricity.In this paper, we describe the investigation of possible defects and their roles in FE STO films. From optical spectroscopy, we found that the occurrence of ferroelectricity in STO films has a strong correlation with the appearance of an absorption peak at around 1.3 eV. By performing first-principles calculations, we investigated changes in the electronic structure due to several complexes of Sr and O vacancies. We found that the Sr-O-O vacancy complex is the most probable candidate for defect dipoles that provide a localized state in the band gap, resulting in the optical absorption around 1.3 eV and ferroelectricity.For experimental investigation of the electronic structures of FE STO films, we grew 100-nm-thick STO films on both sides of polished SrTiO 3 (001) substrates for optical measurements using the same deposition conditions