Atom-light hybrid interferometer for atomic sensing with quantum memory

Abstract

Quantum memories feature a reversible conversion of optical fields into long-lived atomic spin waves, and are therefore ideal for operating as sensitive atomic sensors. However, up to now, atom-light interferometers have lacked an efficient approach to exploit their ultimate atomic sensing performance, since an extra optical delay line is required to compensate for the memory time. Here, we report a new protocol that records the photocurrent via heterodyne mixing with a stable local oscillator. The obtained complex quadrature amplitude that carries information imprinted on its phase by an external magnetic field, is successfully recovered from the interference patterns between the light and the atomic spin wave, without the stringent requirement of having them overlap in time. Our results reveal that the sensitivity scales favorably with the lifetime of the quantum memory. Our work may have important applications in building distributed quantum networks through quantum memory-assisted atom-light interferometers.

Figures (4)

• Figure 1: Schematic setup. (a) An elongated, laser-cooled atomic ensemble serves as an atom-light beam splitter (A-L BS), atomic sensor (AS), and coupler (A-L C). A strong control light denoted as $\Omega_c$ couples the signal field $E$ and atomic coherence denoted as $S$, which accumulates a phase $\phi$ in the presence of an external magnetic field $B_{z}$ in the $z$-direction. The balanced homodyne detector (BHD) with a strong local oscillator (LO) records the quadrature amplitude and phase of the light signal. (b) The raw electronic output of BHD on a digital oscilloscope (DOS) is shown as a solid line. The dashed line is pulse profile recorded by photomultiplier. The purple, green, and orange lines represent the transmitted signal and multiple readouts of the memory.
• Figure 2: (a) The measurement sequence for directly comparing transmitted and retrieved pulses, in which the storage time is $\Delta\tau=5\mu s$. (b)-(g) Beating patterns between the signal and local oscillator in the balanced detector with a frequency difference $\delta\omega/2\pi=5kHz$; the purple and green dots are experimental data for the transmitted $i_T$ and retrieved signal $i_R$, and the solid curve represents a fit to the data. The phase differences for (b): $\pi$, (d): $\frac{\pi}{2}$, and (f): $0$. (c), (e), and (g) are the amplitudes of the addition in blue, corresponding to the filled points in (i). (h) The phase difference between LO and T is plotted as purple dots, and that between LO and R is plotted as green dots. The relative phase between T and R is plotted as yellow dots as a function of the scanned magnetic field. (i) Recovery of the interference fringes using the average power of the addition of the T and R beating patterns. The dots represent experimental data, and the solid line is the fitted curve.
• Figure 3: (a) The measurement sequence with multiple readout pulses. The first and second retrieved signals r1 and r2 are denoted as T and R in Fig. \ref{['fig:fringes']}(a), and their storage time is $\Delta\tau=5\mu s$. (b) The recovered interference fringe pattern obtained using a slow detector, where frequency differences $\delta\omega/2\pi=5kHz$ and $\delta\omega/2\pi=4MHz$ are marked as circles and triangles, respectively; the solid lines are the fitting curves. (c) The recovered interference fringe for the fast detector case, with a frequency difference $\delta\omega/2\pi=4MHz$; the square points and solid line are the experimental data and the fitting curve, respectively. (d) Visibility as a function of the frequency shift of the LO, i.e., $\delta\omega$. The visibility of the curves in (b) and (c) is marked by the corresponding filled data points.
• Figure 4: The magnetic field measurement precision of the interferometer versus storage time $\Delta\tau$. The circle points are the measurement data and the dashed line is the fitting curve with a $\Delta\tau^{-1}$ scaling. Error bars indicate the standard deviation of all measurements. The green and red regions represent atoms trapped in the MOT and the ODT, respectively. The square and triangle points are the optimal measurement times for atoms held in the MOT, and the ODT, from Ref. Cho2016Highly and Ref. Dudin2013Light, respectively.