Using a basic Mach-Zehnder interferometer, we demonstrate experimentally the measurement of the photonic de Broglie wavelength of entangled photon pairs (biphotons) generated by spontaneous parametric down-conversion. The observed interference manifests the concept of the photonic de Broglie wavelength. We also discuss the phase uncertainty obtained from the experiment.

Entanglement is one of the key features of quantum information and communications technology. The method that has been used most frequently to generate highly entangled pairs of photons is parametric down-conversion. Short-wavelength entangled photons are desirable for generating further entanglement between three or four photons, but it is difficult to use parametric down-conversion to generate suitably energetic entangled photon pairs. One method that is expected to be applicable for the generation of such photons is resonant hyper-parametric scattering (RHPS): a pair of entangled photons is generated in a semiconductor via an electronically resonant third-order nonlinear optical process. Semiconductor-based sources of entangled photons would also be advantageous for practical quantum technologies, but attempts to generate entangled photons in semiconductors have not yet been successful. Here we report experimental evidence for the generation of ultraviolet entangled photon pairs by means of biexciton resonant RHPS in a single crystal of the semiconductor CuCl. We anticipate that our results will open the way to the generation of entangled photons by current injection, analogous to current-driven single photon sources.

The uncertainty principle formulated by Heisenberg in 1927 describes a trade-off between the error of a measurement of one observable and the disturbance caused on another complementary observable such that their product should be no less than the limit set by Planck's constant. However, Ozawa in 1988 showed a model of position measurement that breaks Heisenberg's relation and in 2003 revealed an alternative relation for error and disturbance to be proven universally valid. Here, we report an experimental test of Ozawa's relation for a single-photon polarization qubit, exploiting a more general class of quantum measurements than the class of projective measurements. The test is carried out by linear optical devices and realizes an indirect measurement model that breaks Heisenberg's relation throughout the range of our experimental parameter and yet validates Ozawa's relation.

No abstract

We experimentally test the error-disturbance uncertainty relation (EDR) in generalized, strengthvariable measurement of a single photon polarization qubit, making use of weak measurement that keeps the initial signal state practically unchanged. We demonstrate that Heisenberg's EDR is violated, yet Ozawa's and Branciard's EDRs are valid throughout the range of our measurement strength.

2Optical fibers have been enabling numerous distinguished applications involving the operation and generation of light, such as soliton transmission 1 , light amplification 2 , alloptical switching 3 and supercontinuum generation 4 . The active function of optical fibers in the quantum regime is expected to be applicable to ultralow-power all-optical signal processing 5 and quantum information processing 6 . Here we demonstrate the first experimental observation of optical nonlinearity at the single-photon level in an optical fiber. Taking advantage of large nonlinearity and managed dispersion of a photonic crystal fiber 7,8 , we have successfully measured very small (10 -7 ~ 10 -8 ) conditional phase shifts induced by weak coherent pulses that contain one or less than one photon per pulse on average. In spite of its tininess, the phase shift was measurable using much (~10 6 times) stronger coherent probe pulses than the pump pulses. We discuss the feasibility of quantum information processing using optical fibers, taking into account the observed Kerr nonlinearity accompanied by ultrafast response time and low induced loss.A photon, the light quantum having much less interaction with its environment than do other quanta (e.g., electron spin, superconducting current), is an outstanding carrier of information for quantum communication and thus is called a 'flying qubit.' This also means that photons may not be suited for computations that require strong unitary interaction between qubits. Hence, fabrication of optical nonlinear media that intermediate sufficiently strong interaction between photons has been under intense study. Cavity quantum electrodynamics-based devices have performed nonlinear Kerr phase shifts of a few ten degrees at the single-photon level 9,10 . Another approach to quantum-optical information processing (QOIP) is to apply weak nonlinearity inherent in currently existing media. Recent proposals 11,12 indicated that such moderately weak nonlinearity can mediate the interaction between photons or other qubits through a strong coherent light (known as a qubus).Exploring the availability of single-photon-level nonlinearity in various media is thus an important challenge that provides a test bed for nonlinear optical phenomena that may emerge in the quantum regime of light 13 .In the present experiment, we used a photonic crystal fiber (PCF) as a Kerr medium. PCF has a high capacity for confining light in its silica core by a large core-cladding index 3 contrast 7,8 . Taking advantage of this feature incorporated with its controlled dispersion property, PCF is widely applied in various applications such as supercontinuum generation 4 , entangled photon generation 14 , squeezing light 15 and a test of the event horizon 16 . To measure the expected ultrasmall phase shift at the single-photon level, we adopted a polarizationdivision Sagnac interferometer (SI) 17 . The SI has the advantage of inherent stability; two interfering beams counter-propagate through the same path in the interferometer so tha...

We demonstrate that the superposition of light polarization states is coherently transferred to electron spins in a semiconductor quantum well. By using time-resolved Kerr rotation, we observe the initial phase of Larmor precession of electron spins whose coherence is transferred from light. To break the electron-hole spin entanglement, we utilized the big discrepancy between the transverse g factors of electrons and light-holes. The result encourages us to make a quantum media converter between flying photon qubits and stationary electron-spin qubits in semiconductors.

Spin is a fundamental property of electrons, with an important role in information storage. For spin-based quantum information technology, preparation and read-out of the electron spin state are essential functions. Coherence of the spin state is a manifestation of its quantum nature, so both the preparation and read-out should be spin-coherent. However, the traditional spin measurement technique based on Kerr rotation, which measures spin population using the rotation of the reflected light polarization that is due to the magneto-optical Kerr effect, requires an extra step of spin manipulation or precession to infer the spin coherence. Here we describe a technique that generalizes the traditional Kerr rotation approach to enable us to measure the electron spin coherence directly without needing to manipulate the spin dynamics, which allows for a spin projection measurement on an arbitrary set of basis states. Because this technique enables spin state tomography, we call it tomographic Kerr rotation. We demonstrate that the polarization coherence of light is transferred to the spin coherence of electrons, and confirm this by applying the tomographic Kerr rotation method to semiconductor quantum wells with precessing and non-precessing electrons. Spin state transfer and tomography offers a tool for performing basis-independent preparation and read-out of a spin quantum state in a solid.