Quantum-memory-assisted on-demand microwave-optical transduction

Abstract

Microwave-optical transducers and quantum memories are fundamental components of quantum repeaters, essential for developing a quantum internet in which solid-state quantum computers serve as nodes interconnected by optical fibers for data transmission. Although both technologies have made significant advancements, the integration of microwave-optical conversion and quantum memory functionalities remains a challenge. Here, we theoretically propose and experimentally demonstrate a memoryenhanced quantum microwave-optical transduction using a Rydberg ensemble. By utilizing a cascaded electromagnetically induced transparency process, we store microwave photons in a highly excited collective state and subsequently convert them into optical photons during the retrieval process. Taking advantage of the optical depth with order of millions for microwave photons in Rydberg ensemble, combined with a minimal storage dephasing rate at the single-photon level, the transducer achieves an areanormalized storage efficiency greater than 90%, a bandwidth of 2.1 MHz, and a noise equivalent temperature as low as 26 K, even in cavity-free conditions. Our findings pave the way for the practical implementation of quantum repeaters based on atomic ensembles in quantum information processing.

Figures (7)

• Figure 1: OMQT scheme and its application for entanglement generation. (A) A cigar-shaped atomic ensemble trapped on a coplanar waveguide serves as an interface for implementing the OMQT scheme. The dashed line represents the field distribution of waveguide mode. The inset shows the energy-level configuration, consisting of MW photon storage (left) and optical photon retrieval (right). (B) Illustration of the entanglement generation between solid-state qubits Q$_{\rm{a}}$ in node $A$ and qubits Q$_{\rm{b}}$ in node $B$ using OMQTs. (C) Entanglement generation rate $R$ between the remote solid-state qubits as a function of conversion efficiency $\eta$. The solid lines correspond to the OMQT scheme for different noise photon numbers $n_\mathrm{p}$, and the dashed line is that of the direct conversion scheme. The inset illustrates the same plot in a linear scale.
• Figure 2: Proof-of-concept OMQT in cold atoms. (A) Schematic waveforms and time sequences for the input and recalled pulses ($\Omega_{\rm{M}}$ and $\Omega_{\rm{L}}$; bottom), the write and read pulses ($\Omega_{\rm{W}}$ and $\Omega_{\rm{R}}$; middle), and two auxiliary fields ($\Omega_{\rm{P}}$ and $\Omega_{\rm{A}}$; top). (B) Experimental setup, including a cigar-shaped atomic cloud, antennas, lenses, a dichroic mirror (DM), a quarter-wave plate (QWP), a polarization beam splitter (PBS), filters, an avalanche photodiode (APD), and a single-photon counting module (SPCM).
• Figure 3: On-demand MO transduction and its noise characteristics. (A) Temporal waveforms of input MWs, slow-light pulses, retrieved optical pulses, and recalled noises throughout the conversion window. (B) Registered noise components for each pulse are outlined after a 50 ns storage time. (C) Area-normalized efficiency $\eta_\mathrm{I}$ as a function of storage time for $\bar{N}=0.1$. The inset in (C) illustrates the counts of recalled noise photons as a function of storage time. The count data were collected over 200 s. The solid (dashed) line represents fitting results derived from the Gaussian (exponential) decay function. The error bars represent the standard deviation from three measurements. The experimental parameters $\{\Omega_\mathrm{P}, \Omega_\mathrm{A}, \Omega_\mathrm{W}, \Omega_\mathrm{R}, \Gamma_{2}, \gamma_{3}, \Gamma_{4}, \Gamma_{6}\}$= $2\pi \{2.1, 7.6, 1.8, 9.0, 6.0, 0.5, 0.001, 1\}$ MHz.
• Figure 4: Storage efficiency and intensity autocorrelations. (A) Area-normalized efficiency $\eta_\mathrm{I}$ versus $\delta_M$. The data were collected at $\bar{N} = 0.3$, and the solid line represents the results of Lorentzian fitting. (B) Second-order autocorrelation functions of retrieved optical photons at $\bar{N} = 0, 5 \bar{n}_{th}/\eta_\mathrm{I}, 20 \bar{n}_{th}/\eta_\mathrm{I}$. The solid curves represent theoretical predictions without the free parameters, while the error bars represent the standard deviation from three measurements. (C) $\eta_\mathrm{I}$ as a function of $\bar{N}$ at a 50 ns storage time. The shaded area indicates the single-photon-level transduction. The solid curve is obtained by fitting the data ($\bar{N}>0.1$) to Eq.(4), yielding the fitting results of $\eta_0 = 88\%$ and $\gamma_{0}/2\pi = 12.8$ kHz. (D) $\eta_\mathrm{I}$ versus the OD $d_{\text{M}}$ with $\bar{N} = 1$. The solid line illustrates the fitted scaling function for optimal storage, $\eta_\mathrm{I} = 1 - 80796/d_{\text{M}}$. The $d_{\text{M}}$ error bars indicate the standard deviation from sixty measurements, and the $\eta_\mathrm{I}$ uncertainties in (A), (B), and (D) are computed similarly to those in Fig.\ref{['fig:2']} (C).
• Figure S1: Generation of a Gaussian MW pulse. (A) Schematic diagram of the Gaussian MW pulse generation. (B) RF output amplitude as a function of the input voltage of IF port. The RF amplitude is proportional to the microwave field Rabi frequency $\Omega_M$, and calibrated by the cold Rydberg atomic Microwave electrometry. The circle points denote the result of experimental measurements, and the solid line is a Gaussian fit.