A high-storage efficiency and long-lived quantum memory for photons is an essential component in long-distance quantum communication and optical quantum computation. Here, we report a 78% storage efficiency of light pulses in a cold atomic medium based on the effect of electromagnetically induced transparency (EIT). At 50% storage efficiency, we obtain a fractional delay of 74, which is the best up-to-date record. The classical fidelity of the recalled pulse is better than 90% and nearly independent of the storage time, as confirmed by the direct measurement of phase evolution of the output light pulse with a beat-note interferometer. Such excellent phase coherence between the stored and recalled light pulses suggests that the current result may be readily applied to single photon wave packets. Our work significantly advances the technology of EIT-based optical memory and may find practical applications in long-distance quantum communication and optical quantum computation.PACS numbers: 42.50. Gy, 32.80.Qk Quantum memory [1][2][3][4][5][6][7][8] is essential for quantum information processing, including quantum communication [9][10][11] and quantum computation [12]. Using quantum repeaters will be a practical protocol for implementing long-distance quantum communication without suffering from transmission loss [13][14][15]. The scheme proposed in Ref.[13] divides a long distance into shorter elementary channels, stores and retrieves entanglement pairs, and extends the transmission distance via entanglement swapping. Quantum memory, a storage device mapping quantum state between light and matter, is a crucial component of the quantum repeater. Processing a particular task or waiting for the completion of others requires a quantum state to be stored in memory for a long enough time. Therefore, quantum memory with high storage efficiency (SE), which is defined as the ratio of recalled to input photon energies, and long coherence time are the keys to successful operation of long-distance quantum communication and quantum information processing.The fractional delay (FD) or delay-bandwidth product at 50% SE is a possible figure of merit for a memory in the no-cloning limit [16,17] or in the one-way quantum computation [18], where FD is defined as the ratio of storage time to the full-width-half-maximum (FWHM) pulse duration. Several memory devices based on different mechanisms, such as gradient photon echo, Raman interaction, and electromagnetically induced transparency (EIT), have been proposed and experimentally demonstrated. With the gradient echo memory, a maximum SE of 87% as well as the FD of 11 at 50% SE for classical light was demonstrated [6], and the recall fidelity can be as high as 98% for coherent pulses containing around one photon [7]. With a far-off-resonant Raman transition, Ref. [8] showed Raman memory with an SE of 43% and a coherence time of approximately 1 µs for 300-ps coherent light pulses.Slowing and storing light using the EIT effect [19,20] has been intensively explored in the past two dec...

Slow light based on the effect of electromagnetically induced transparency is of great interest due to its applications in low-light-level nonlinear optics and quantum information manipulation. The previous experiments all dealt with the single-component slow light. Here, we report the experimental demonstration of two-component or spinor slow light using a double-tripod atom–light coupling scheme. The scheme involves three atomic ground states coupled to two excited states by six light fields. The oscillation due to the interaction between the two components was observed. On the basis of the stored light, our data showed that the double-tripod scheme behaves like the two outcomes of an interferometer enabling precision measurements of frequency detuning. We experimentally demonstrated a possible application of the double-tripod scheme as quantum memory/rotator for the two-colour qubit. Our study also suggests that the spinor slow light is a better method than a widely used scheme in the nonlinear frequency conversion.

Objective. Retinal ganglion cells (RGCs) represent an attractive target in vision restoration strategies, because they undergo little degeneration compared to other retinal neurons. Here we investigated the temporal and spatial resolution in adult photoreceptor-degenerated (rd10) mouse retinas, where RGCs have been transduced with the optogenetic actuator channelrhodopsin-2 (ChR2). Approach. The RGC spiking activity was recorded continuously with a CMOS-based microelectrode array during a variety of photostimulation protocols. The temporal resolution was assessed through Gaussian white noise stimuli and evaluated using a linear-nonlinear-Poisson model. Spatial sensitivity was assessed upon photostimulation with single rectangular pulses stepped across the retina and upon stimulation with alternating gratings of different spatial frequencies. Spatial sensitivity was estimated using logistic regression or by evaluating the spiking activity of independent RGCs. Main results. The temporal resolution after photostimulation displayed an about ten times faster kinetics as compared to physiological filters in wild-type RGCs. The optimal spatial resolution estimated using the logistic regression model was 10 µm and 87 µm based on the population response. These values correspond to an equivalent acuity of 1.7 or 0.2 cycles per degree, which is better than expected from the size of RGCs’ optogenetic receptive fields. Significance. The high temporal and spatial resolution obtained by photostimulation of optogenetically transduced RGCs indicate that high acuity vision restoration may be obtained by photostimulation of appropriately modified RGCs in photoreceptor-degenerated retinas.

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Objective. Most neuroprosthetic implants employ pulsatile square-wave electrical stimuli, which are significantly different from physiological inter-neuronal communication. In case of retinal neuroprosthetics, which use a certain type of pulsatile stimuli, reliable object and contrast discrimination by implanted blind patients remained challenging. Here we investigated to what extent simple objects can be discriminated from the output of retinal ganglion cells (RGCs) upon sinusoidal stimulation. Approach. Spatially confined objects were formed by different combinations of 1024 stimulating microelectrodes. The RGC activity in the ex vivo retina of photoreceptor-degenerated mouse, of healthy mouse or of primate was recorded simultaneously using an interleaved recording microelectrode array implemented in a CMOS-based chip. Main results. We report that application of sinusoidal electrical stimuli (40 Hz) in epiretinal configuration instantaneously and reliably modulates the RGC activity in spatially confined areas at low stimulation threshold charge densities (40 nC mm−2). Classification of overlapping but spatially displaced objects (1° separation) was achieved by distinct spiking activity of selected RGCs. A classifier (regularized logistic regression) discriminated spatially displaced objects (size: 5.5° or 3.5°) with high accuracy (90% or 62%). Stimulation with low artificial contrast (10%) encoded by different stimulus amplitudes generated RGC activity, which was classified with an accuracy of 80% for large objects (5.5°). Significance. We conclude that time-continuous smooth-wave stimulation provides robust, localized neuronal activation in photoreceptor-degenerated retina, which may enable future artificial vision at high temporal, spatial and contrast resolution.

All-optical switching (AOS) or cross-phase modulation (XPM) based on the effect of electromagnetically induced transparency (EIT) makes one photon switched or phase-modulated by another possible. The existence of four-wave mixing (FWM) process greatly diminishes the switching or phase-modulation efficiency and hinders the single-photon operation. We proposed and experimentally demonstrated an idea that with an optimum detuning the EIT-based AOS can be completely intact even under the influence of FWM. The results of the work can be directly applied to the EIT-based XPM. Our work makes the AOS and XPM schemes more flexible and the single-photon operation possible in FWM-allowed systems.

Slow light based on the effect of electromagnetically induced transparency is of great interest due to its applications in low-light-level nonlinear optics and quantum information manipulation. The previous experiments all dealt with the single-component slow light. Here, we report the experimental demonstration of two-component or spinor slow light using a double-tripod atom-light coupling scheme. The scheme involves three atomic ground states coupled to two excited states by six light fields. The oscillation due to the interaction between the two components was observed. On the basis of the stored light, our data showed that the double-tripod scheme behaves like the two outcomes of an interferometer enabling precision measurements of frequency detuning. We experimentally demonstrated a possible application of the double-tripod scheme as quantum memory/rotator for the two-colour qubit. Our study also suggests that the spinor slow light is a better method than a widely used scheme in the nonlinear frequency conversion.

PURPOSE.It is well known that the gradual loss of axon growth ability of retinal ganglion cells (RGCs) during development is largely determined by extrinsic signals rather than being programmed intrinsically. Spontaneous retinal waves are the major neural activity during retinal development. Thus restoring the developmental environment by providing the proper neural activity may be able to help axon regeneration of RGCs. METHODS.Retinal explants from P5 and P11 C57BL/6 mice were treated pharmacologically or stimulated electrically, and cultured with or without brain-derived neurotrophic factor (BDNF) on coverslips or a multielectrode array for 5 days to examine the neurite outgrowth capacity of RGCs. RESULTS.Here we have demonstrated that neurite outgrowth of retinal explants was not affected when acetylcholine transmission was blocked pharmacologically in retinas that normally display stage II retinal waves. However, short-term induction of globally correlated neural activity at 1-to 2-minute intervals in retinas that normally display stage III retinal waves by blocking inhibitory neural transmission was found to greatly promote neurite outgrowth even in the absence of exogenous neurotrophic factors. Moreover, short-term electrical stimulation with a temporal pattern of 1-to 2-minute intervals rather than simply increasing the neural activity greatly enhanced neurite outgrowth of retinal explants of the same age.CONCLUSIONS. These results suggest that short-term alteration of neural activity with a specific temporal pattern in retinas of later developmental stages is sufficient to enhance neurite outgrowth of retinal explants. This finding could lead to a therapeutic strategy that is able to prevent the gradual loss of the axon growth ability of RGCs in more mature retinas.Keywords: retinal waves, electrical stimulation, retinal ganglion cells, axon regeneration, neurotrophic factors T he immature mammalian central nervous system (CNS) retains the ability to regenerate, 1 but neurons in the adult CNS are difficult to regrow after severe injury. Previous studies have shown that the gradual loss of the ability of axons to grow during the development of retinal ganglion cells (RGCs), one type of CNS neurons, is not intrinsically programmed but rather determined by extrinsic signals. 2 It follows that a restoration of the extrinsic developmental environment of the retina may be able to provide a therapeutic strategy for promoting axon regeneration of RGCs after injury or degeneration.During retinal development, one of the most well-known phenomena is retinal waves. These are characterized as the correlated spontaneous neural activity in RGCs that is accompanied by the propagation of calcium waves, and these appear from the late embryonic stage to eye opening. 3,4 It has been shown that retinal waves are important for the refinement of binocular segregation in the dorsal lateral geniculate nucleus, 5,6 as well as the formation of the retinotopic map in the superior colliculus. 7,8 It has also been reported that a bl...