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Slow Images and Entangled Photons

Simon P. Swordy

Published Jun 20, 2007• 0 citations
OpenAlex

Abstract

I will discuss some recent experiments using slow light and entangled photons. We recently showed that it was possible to map a two dimensional image onto very low light level signals, slow them down in a hot atomic vapor while preserving the amplitude and phase of the images. If time remains, I will discuss some of our recent work with time-energy entangled photons for quantum cryptography. We were able to show that we could have a measurable state space of over 1000 states for a single pair of entangled photons in fiber.

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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)•May 1, 2013•12 citations
I will discuss some recent experiments using slow light and entangled photons. We recently showed that it was possible to map a two dimensional image onto very low light level signals, slow them down in a hot atomic vapor while preserving the amplitude and phase of the images. If time remains, I will discuss some of our recent work with time-energy entangled photons for quantum cryptography. We were able to show that we could have a measurable state space of over 1000 states for a single pair of entangled photons in fiber.
Summary form only given. Exciting observations of ultra-slow group velocities and very large nonlinearities in coherently driven atomic media open up interesting prospectives for nonlinear optical processes at low light levels. Recent experiments, for instance, demonstrated that very efficient nonlinear phenomena can be induced by laser fields of tiny energies. These developments stimulate the studies of ultimate efficiency limits for nonlinear optics based on electromagnetically induced transparency (EIT) as well as its possible applications for generation and manipulation of quantum entangled states of the light fields and matter. We describe a particular method that allows for two slow light fields of energies corresponding to that of a single photon to interact coherently.
This thesis presents the results of four experiments related to applications of higher dimensional photonic entanglement. 1) We use energy-time entangled biphotons from spontaneous parametric down-conversion (SPDC) to implement a large-alphabet quantum key distribution (QKD) system which securely transmits up to 10 bits of the random key per photon. An advantage over binary alphabet QKD is demonstrated for quantum channels with a single-photon transmissionrate ceiling. The security of the QKD system is based on the measurable reduction of entanglement in the presence of eavesdropping. 2) We demonstrate the preservation of energy-time entanglement in a tunable slow-light medium. The fine-structure resonances of a hot Rubidium vapor are used to slow one photon from an energy-time entangled biphoton generated with non-degenerate SPDC. The slow-light medium is placed in one arm of a Franson interferometer. The observed Franson fringes witness the presence of entanglement and quantify a delay of 1.3 biphoton correlation lengths. 3) We utilize holograms to discriminate between two spatially-coherent single-photon images. Heralded single photons are created with degenerate SPDC and sent through one of two transmission masks to make single-photon images with no spatial overlap. The single-photon images are sent through a previously prepared holographic filter. The filter discriminates the single-photon images with an average confidence level of 95%. 4) We employ polarization entangled biphotons generated from non-collinear SPDC to violate a generalized Leggett-Garg inequality with non-local weak measurements. The weak measurement is implemented with Fresnel reflection of a microscope coverslip on one member of the entangled biphoton. Projective measurement with computer-controlled polarizers on the entangled state after the weak measurement yields a joint probability with three degrees of freedom. Contextual values are then used to determine statistical averages of measurement operations from the joint probability. Correlations between the measured averages are shown to violate the upper bound of three distinct two-object Leggett-Garg inequalities derived from assumptions of macro-realism. A relationship between the violation of two-object Leggett-Garg inequalities and strange non-local weak values is derived and experimentally demonstrated.
These notes are more or less a faithful representation of my talk at the Workshop on ``Quantum Coding and Quantum Computing'' held at the University of Virginia. As such it is an introduction for non-physicists to the topics of the quantum theory of light and entangled states of light. In particular, I discuss the photon concept and what is really entangled in an entangled state of light (it is not the photons). Moreover, I discuss an example that highlights the peculiar behavior of entanglement in an infinite-dimensional Hilbert space.
Summary form only given. In quantum cryptography, non-cloning properties of quantum states are exploited to achieve quantum security. At the same time, typically one optical mode is used, for instance that of a single-mode optical fiber, effectively reducing the problem to a one-dimensional system. Only recently have researchers begun to exploit higher dimensions for quantum cryptography [1]. High dimensional spaces appear naturally in imaging applications.We here experimentally investigate the question if it is possible to recognize an arbitrary pattern in a physical object using much fewer photons than the complexity of the pattern. Theoretically it has already been proposed to use a quantum computer for pattern recognition and that this can be achieved with very few photons [2]. To avoid using a quantum computer, we exploit a spatial light modulator, which can map a single incoming optical mode on thousands outgoing optical modes in a programmable way. In this way, a weak light pulse containing only a few or even a single photon can be distributed over the high number of pixels of a complex 2-dimensional image. The inverse is just as feasible: a known complex pattern can be mapped onto a single mode. We use this principle to analyze the light from a physical object when illuminated with, e.g., a plane wave. After reflection of or transmission through the object under investigation, the response can be projected onto a single mode. The signal in that mode can be analyzed with the standard repertoire of quantum optical measurements, determining, e.g., the field amplitude, the intensity or the number of photons.We use coherent pulses of light which have mean photon numbers much smaller than the thousands of modes of the complex image. We introduce the parameter S=K/n as the ratio of the number of modes, K, of the response field pattern and the number of photons, n, that is used for the illumination. This measurement is in the quantum domain if S>>1. We read out objects with S=4, well in the quantum regime. The method can be extended by preshaping the illumination light, with applications in authentication of physical objects [3,4].