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Experimental realization of a multiplexed quantum memory with 225 individually accessible memory cells
Y. -F. Pu, N. Jiang, W. Chang, H. -X. Yang, C. Li, L. -M. Duan
TL;DR
Long-distance quantum communication requires multiplexed memories whose cells are individually addressable and independently accessible. This work realizes a 225-cell DLCZ memory with programmable optical control, demonstrating quantum correlations, low crosstalk, and high-fidelity storage and readout of optical-qubit entanglement.
Problem
Quantum repeater networks require multiplexed quantum memories with many individually addressable cells, independent access, and programmable-time readout.
Method
The experiment divides a macroscopic atomic ensemble into a 15 × 15 array and uses crossed AOD multiplexing and demultiplexing circuits to control cells and photon modes.
Results
The 225-cell memory shows quantum correlations in every cell, neighboring-cell crosstalk well below 1%, and about 90% fidelity for programmable storage and readout of atom-photon entanglement.
Takeaways & Limitations
The demonstrated capacity, programmable addressing, and optical interface constitute an important step toward multiplexed quantum repeater networks.
Abstract
from arXiv · showhide
To realize long-distance quantum communication and quantum network, it is required to have multiplexed quantum memory with many memory cells. Each memory cell needs to be individually addressable and independently accessible. Here we report an experiment that realizes a multiplexed DLCZ-type quantum memory with 225 individually accessible memory cells in a macroscopic atomic ensemble. As a key element for quantum repeaters, we demonstrate that entanglement with flying optical qubits can be stored into any neighboring memory cells and read out after a programmable time with high fidelity. Experimental realization of a multiplexed quantum memory with many individually accessible memory cells and programmable control of its addressing and readout makes an important step for its application in quantum information technology.
INTRODUCTION
The paper addresses the need for many individually accessible, independently controllable quantum-memory cells for quantum repeaters and related photonic quantum technologies. It demonstrates a 225-cell multiplexed DLCZ memory in a macroscopic atomic ensemble, with programmable storage and readout of atom-photon entanglement.
- Quantum repeater networks require multiplexed memories with many individually addressable cells and programmable-time readout.
- Atomic ensembles provide long-lived internal states and an efficient interface with flying photonic qubits.
- 225 individually accessible memory cells are formed by dividing one macroscopic atomic ensemble into a 15 × 15 array of micro-ensembles.
- Crossed AODs implement programmable two-dimensional multiplexing and demultiplexing for addressing cells and coupling their signals into single-mode fibers.
- Entanglement with flying optical qubits is stored in chosen neighboring cells and read out after a programmable delay with about 90% fidelity.
Experimental configuration
The experimental configuration creates a two-dimensional array of memory cells within one atomic ensemble and uses programmable optical routing for write, read, and signal collection. Crossed AOD circuits provide site-independent access while avoiding active phase locking across the optical paths.
- 87Rb atoms carry the quantum information, with DLCZ write and read processes coupling collective atomic excitations to signal and idler photons.
- Experimental configuration: 15 × 15 write and read paths are generated to create many cells with site-independent access time.
- Experimental configuration: Crossed AODs and lenses direct beams and photon modes to selected cells or superpositions through programmable radio-frequency signals.
- Experimental configuration: The common optical apparatus makes the path phases intrinsically stable, removing the need for active phase locking across 225 paths.
Characterization of quantum correlation of 225 memory cells
The memory exhibits nonclassical signal-idler correlations across all 225 cells while maintaining low neighboring-cell crosstalk. Correlations decrease toward the ensemble edge but remain well above the nonclassicality criterion.
- Characterization of quantum correlation of 225 memory cells: All 225 memory cells satisfy the nonclassical correlation criterion gc > 2.
- Characterization of quantum correlation of 225 memory cells: The minimum edge-cell correlation remains well above 10, despite a gradual decrease from the ensemble center toward the edge.
- Characterization of quantum correlation of 225 memory cells: Neighboring-cell crosstalk from both write and read beams has a maximum error rate well below 1%.
- Characterization of quantum correlation of 225 memory cells: The crosstalk measurements scan the read or write beam across a fixed target cell and its neighboring cells using relative coincidence counts.
Storage of atom-photon entanglement in memory cells
The memory stores atom-photon entanglement in selected neighboring cells and retrieves it after programmable delays. Correlations and entanglement fidelity remain above classical limits for tens of microseconds.
- Entanglement generation: The AODs split the write beam into paths L and R, creating an excitation distributed between the corresponding signal-photon and collective-atomic modes.The relative phase and detection-basis weights are programmable through the AOD control signals.
- Storage lifetime: 28 µs: the fitted e^-1 decay time for the cross correlation gc in typical memory cells.Both gc and Fe remain above their classical limits after 35 µs of storage.
- Storage lifetime: 35 µs: cross correlation and entanglement fidelity remain above the classical limits after this storage duration.The observed storage-time limitation is attributed mainly to collective-mode dephasing from atomic motion and residual magnetic-field gradients.
DISCUSSION
The experiment realizes a high-capacity quantum memory with individually programmable cells and location-independent access. The demonstrated capabilities support applications in quantum repeaters, many-particle entanglement, and linear-optics quantum information processing.
- DISCUSSION: 225 individually accessible memory cells are realized in a multiplexed quantum memory with programmable individual-qubit control and location-independent access time.The memory uses a macroscopic atomic ensemble divided into individually addressable cells.
- DISCUSSION: The memory is identified as a critical device for multiplexed quantum repeater networks for long-distance quantum communication.Its relevant features include many memory cells, programmable addressing, and an efficient interface with flying optical qubits.
- DISCUSSION: The architecture can support demonstrations of many-particle entanglement, linear-optics quantum information processing, and use as a quantum-internet node.These applications are stated within the scope of the demonstrated memory capabilities.
Experimental setup
The experiment prepares 87Rb atoms in a magneto-optical trap and applies write, clean, and read sequences after cooling and trap-field shutoff. Longer-storage measurements add polarization-gradient cooling, while residual magnetic fields produce Larmor oscillations.
- Atomic preparation: 87Rb atoms are loaded into a magneto-optical trap, and the experimental sequence begins after the MOT beams and magnetic-gradient coils are shut off.The atomic cloud has a diameter of about 3.5 mm and a temperature of about 300 µK.
- Cooling and preparation: Longer-storage measurements include a 1 ms polarization-gradient-cooling stage before the experimental sequence.A 100 µs repumping pulse is applied afterward to return atoms to the |g⟩ state.
- Magnetic-field effects: 5.8 µs: the Larmor period caused by the uncompensated ambient magnetic field during the experimental sequence.Data for Fig. 4 are taken at integer periods of the Larmor oscillation.
- Experimental sequence: The write-clean sequence repeats until a signal photon is detected, after which the excitation is stored for a controllable delay.Each write pulse lasts 100 ns, the clean pulse follows after 500 ns, and the full sequence lasts 1 µs.
Multiplexing and de-multiplexing optical circuits
Crossed AODs route write, read, signal, and idler modes through a programmable 2D addressing and collection circuit. The setup maintains mode matching across all memory-cell paths and reconstructs entanglement using quantum-state tomography.
- Optical circuits: Four crossed AODs control beam deflection in orthogonal X and Y directions to implement multiplexing and demultiplexing across the 2D array.Lenses and AOD placement produce the addressing configuration used for write/read beams and signal/idler modes.
- Mode matching: Over 70% coupling efficiency is achieved for all 225 optical paths addressing different memory cells.Mode matching uses counter-propagating write/read beams and signal/idler modes.
- RF control: Two four-channel arbitrary-waveform generators provide RF control for directing the four optical modes to selected memory cells.Different RF frequencies address different cells, while matched channels control write, read, signal, and idler paths.
- Alignment: The alignment procedure uses resonant probe and pumping beams to maximize optical depth and satisfy phase matching.The procedure begins by aiming all crossed-AOD pairs at the middle cell of the scanning array.
- State characterization: Quantum-state tomography reconstructs the atomic-memory–signal-photon density matrix by maximum likelihood from measurements in 42 bases.The reconstruction characterizes the generated entanglement.