A single-atom quantum memory.

The faithful storage of a quantum bit (qubit) of light is essential for long-distance quantum communication, quantum networking and distributed quantum computing. The required optical quantum memory must be able to receive and recreate the photonic qubit; additionally, it must store an unknown quantum state of light better than any classical device. So far, these two requirements have been met only by ensembles of material particles that store the information in collective excitations. Recent developments, however, have paved the way for an approach in which the information exchange occurs between single quanta of light and matter. This single-particle approach allows the material qubit to be addressed, which has fundamental advantages for realistic implementations. First, it enables a heralding mechanism that signals the successful storage of a photon by means of state detection; this can be used to combat inevitable losses and finite efficiencies. Second, it allows for individual qubit manipulations, opening up avenues for in situ processing of the stored quantum information. Here we demonstrate the most fundamental implementation of such a quantum memory, by mapping arbitrary polarization states of light into and out of a single atom trapped inside an optical cavity. The memory performance is tested with weak coherent pulses and analysed using full quantum process tomography. The average fidelity is measured to be 93%, and low decoherence rates result in qubit coherence times exceeding 180 microseconds. This makes our system a versatile quantum node with excellent prospects for applications in optical quantum gates and quantum repeaters.

Critical flicker-fusion frequency in age-related macular degeneration.

PURPOSE: To assess the influence of age-related macular degeneration (AMD) on critical flicker-fusion frequency (CFF). METHODS: CFF was measured centrally for a red, green and blue target, and in 10 degrees excentricity with a red target. Twenty-eight patients with non-exsudative AMD, 12 patients with exsudative AMD and 45 age-matched healthy eyes were included. RESULTS: CFF decreased in eyes with non-exsudative AMD (red 1.3 Hz, p=0.025; green 1.4 Hz, p=0.053; blue 2.1 Hz, p=0.006) and exsudative AMD (red 2.2 Hz, p=0.02; green 3.3 Hz, p=0.001; blue 2.9 Hz, p=0.02). The difference between central and peripheral CFF increased in non-exsudative AMD (red-red 10 degrees, 0.7 Hz, p=0.024), but was not significantly increased in exsudative AMD (1.3 Hz, p=0.059). There was no difference between eyes with non-exsudative AMD with good visual acuity (VA >20/32, n=18) and healthy eyes, nor between eyes with non-exsudative (n=10) and exsudative AMD (n=9) with VA from 20/100 to 20/40. CONCLUSIONS: CFF decreased in non-exsudative and exsudative AMD. CFF is not able to distinguish between AMD eyes and healthy eyes of equal visual acuity, and therefore is not applicable as a possible diagnostic test.

Combination of transepithelial phototherapeutic keratectomy and autologous serum eyedrops for treatment of recurrent corneal erosions.

PURPOSE: To evaluate the clinical outcomes after transepithelial phototherapeutic keratectomy (t-PTK) using an excimer laser and postoperative administration of autologous serum eyedrops. SETTING: Department of Ophthalmology, Ruprecht-Karls-University of Heidelberg, Heidelberg, Germany. METHODS: Twenty-five eyes of 25 patients with 3 to 12 recurrent corneal erosions were treated with t-PTK using the Schwind Keratom broad-beam excimer laser (Schwind). Routine therapy consisted of 2 pulses x 40 pulses (18.82 +/- 2.56 mum ablation). Postoperatively, autologous serum eyedrops were administered 6 times a day for 6 weeks. Postoperative outcomes and rate of recurrent erosions were evaluated. RESULTS: The median of the follow-up after t-PTK was 15.5 months (range 6 to 20 months), and the mean age of the patients was 40.6 years +/- 12.0 (SD). Twenty of 25 eyes (80%) recovered without further corneal erosion. Five eyes had 1 further erosion, which was treatable in 4 cases with autologous serum eyedrops without additional excimer laser treatment. One patient requested additional t-PTK treatment and recovered without further complications thereafter. CONCLUSIONS: Transepithelial phototherapeutic keratectomy is a safe and effective therapy for recurrent corneal erosions. Additional treatment with autologous serum eyedrops can support the healing process following corneal erosions and t-PTK and can be given as a long-term artificial tear treatment.