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Physical Review A

Beat-note interferometer for direct phase measurement of photonic information

Yong‐Fan Chen, Yuchen Liu, Zen-Hsiang Tsai, Shih-Hao Wang, Ite A. Yu

Published Sep 14, 2005• 42 citations
Publisher

Abstract

We describe a simple method of the beat-note interferometer that directly and dynamically measures the phase shift of light pulses induced by highly dispersive samples. Using the method, we show intuitively that the storage and retrieval of light pulses in a medium due to the effect of electromagnetically induced transparency is a coherent process and, quantitatively, that there is no observable phase shift caused by the process. This method is sensitive enough that it can be used to measure the phase of a light pulse with energy on the order of a dozen photons.

DOI
10.1103/physreva.72.033812

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We describe a simple method of the beat-note interferometer that directly and dynamically measures the phase shift of light pulses induced by highly dispersive samples. Using the method, we show intuitively that the storage and retrieval of light pulses in a medium due to the effect of electromagnetically induced transparency is a coherent process and, quantitatively, that there is no observable phase shift caused by the process. This method is sensitive enough that it can be used to measure the phase of a light pulse with energy on the order of a dozen photons.
We report the phase measurement of a fast light pulse in electromagnetically induced absorption (EIA) of the 5S₁/₂ (F = 2)-5P₃/₂ (F' = 3) transition of ⁸⁷Rb atoms. Using a beat-note interferometer method, a stable measurement without phase dithering of the phase of the probe pulse before and after it has passed through the EIA medium was achieved. Comparing the phases of the light pulse in air and that of the fast light pulse though the EIA medium, the phase of the fast light pulse at EIA resonance was not shifted and maintained to be the same as that of the free-space light pulse. The classical fidelity of the fast light pulse according to the advancement of the group velocity by adjusting the atomic density was estimated to be more than 97%.
In quantum memory based on an atomic medium, we may have a question about whether all information on the stored light is preserved. In particular, the phase coherence between the stored and retrieval light pulses is very interesting, because it can indicate the relationship between the coherence time and storage time of the light. In this paper, we investigate the phase coherence time of light stored in a warm atomic vapor, by examining the beat-note interference between the retrieval light pulse and a reference light beam optically delayed using an optical fiber. The beat-note interference fringes are measured for different reference-light optical delays. The observed retrieval-light phase indicates that the phase of the input probe light is preserved in the medium. However, we further confirm that the retrieval-light phase coherence depends on the phase coherence of the coupling light used for retrieval in the storage process.
Storing light based on the effect of electromagnetically induced transparency (EIT) provides a coherent way to transfer wave functions between photons and matters. The storage device is promising for the application of quantum memory. An important figure of merit for optical memory is fidelity, which can be degraded by either a two-photon detuning or an axial magnetic field gradient in the EIT scheme. We systematically studied the phase of readout light pulses with the beat-note interferometer. The detuning induces a nonuniform phase variation during the pulse propagation and a uniform phase shift during the light storage; in addition, the field gradient induces a nonuniform phase variation during the storage. We further demonstrated that the phase measurement can be used to diagnose a two-photon detuning below 10 kHz (or 1/600 of the excited-state natural linewidth) and a magnetic field gradient as low as 2 mG/cm. The fidelity of the readout pulse from the EIT memory can be achieved to 0.94. Our work provides useful information for practical applications of EIT-based quantum memory and advances the knowledge of EIT-related research fields.
Abstract We demonstrate a detection method based on a relative phase detection in two photon Raman excitation. The two Raman beams going through the sample experience a phase change of opposing sign, which appears as a phase shift of their beat note in a fast detector. This signal is compared to a reference one in a microwave interferometer. We show that this way of measuring the phase change in the optical beams displays a very high phase stability that derives from having both beams sharing exactly the same path and being originated from a single laser. This modulated interferometer is at least 18 600 times more immune to temperature variations than a displaced Sagnac interferometer, something that allows for its operation even in the absence of temperature stabilization. The setup maintains full sensitivity to relative phase changes of pairs of beams such as those present in an Electromagnetic Induced Transparency. Using a co-propagating two photon excitation makes it almost insensitive to atomic or molecular motion and gives narrow spectral features.
A deterministic photon-photon quantum logic gate is a long-standing goal. Building such a gate becomes possible if a light pulse containing only one photon imprints a phase shift of π onto another light field. We experimentally demonstrate the generation of such a π phase shift with a single-photon pulse. A first light pulse containing less than one photon on average is stored in an atomic gas. Rydberg blockade combined with electromagnetically induced transparency creates a phase shift for a second light pulse, which propagates through the medium. We measure the π phase shift of the second pulse when we postselect the data upon the detection of a retrieved photon from the first pulse. This demonstrates a crucial step toward a photon-photon gate and offers a variety of applications in the field of quantum information processing.