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arXiv:2609.11344

Quantum-RAM Implementation Using Multiple Interacting Rydberg-Blockaded EIT Systems

Avirup Chakraborty, Shrabana Chakrabarti

quant-phSubmitted Sep 10, 2026• Announced Sep 11, 2026• 0 citations
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Abstract

We propose a novel theoretical architecture for implementing a quantum random access memory (qRAM) based on quantum random walks in a Rydberg blockaded atomic ensemble utilizing multilevel Electromagnetically Induced Transparency (EIT). Unlike previous approaches that rely on geometric phase gates in solid-state or trapped ion systems, our scheme harnesses the strong, coherent dipole dipole interactions between Rydberg atoms to achieve high-fidelity phase control of photonic qubits without the need for cryogenic temperatures. By generating conditional phase shifts through cross-phase modulation in multiple lambda-type EIT systems, we realize the controlled unitary operations requisite for an efficient qRAM. In the proposed architecture, Zeeman splitting is used to engineer a set of parallel lambda systems in a cavity, where pairs of magnetic sublevels of the ground state are coupled to highly excited Rydberg states via circularly polarized laser pulses. These Rydberg excited EIT systems serve as the elementary phase gates that form the nodes of a binary tree enabling quantum random walking. Address and data qubits are encoded into distinct probe fields and coherently mapped into the metastable atomic states, where their interactions within the EIT medium generate conditional phases required for state-selective routing. The system uses $n+m$ layers of cold alkali atoms to form an $n$-level binary tree of Rydberg nodes connected to $2^n$ cavity-trapped memory atoms, operated by $n+m$ laser pulses acting as quantum walkers and address units. Our scheme offers a scalable, reducing operational complexity to $\mathcal{O}(n)$ and highly coherent pathway toward photonic qRAM, exploiting collective Rydberg interactions to realize programmable, parallel entangling operations in an atomic ensemble.

Comments
BibTeX, Version 0.99d (MiKTeX 25.4), 13 pages and 10 figures, submitted to Journal of Physics B

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We propose a novel theoretical architecture for implementing a quantum random access memory (qRAM) based on quantum random walks in a Rydberg blockaded atomic ensemble utilizing multilevel Electromagnetically Induced Transparency (EIT). Unlike previous approaches that rely on geometric phase gates in solid-state or trapped ion systems, our scheme harnesses the strong, coherent dipole dipole interactions between Rydberg atoms to achieve high-fidelity phase control of photonic qubits without the need for cryogenic temperatures. By generating conditional phase shifts through cross-phase modulation in multiple lambda-type EIT systems, we realize the controlled unitary operations requisite for an efficient qRAM. In the proposed architecture, Zeeman splitting is used to engineer a set of parallel lambda systems in a cavity, where pairs of magnetic sublevels of the ground state are coupled to highly excited Rydberg states via circularly polarized laser pulses. These Rydberg excited EIT systems serve as the elementary phase gates that form the nodes of a binary tree enabling quantum random walking. Address and data qubits are encoded into distinct probe fields and coherently mapped into the metastable atomic states, where their interactions within the EIT medium generate conditional phases required for state-selective routing. The system uses n+mn+mn+m layers of cold alkali atoms to form an nnn-level binary tree of Rydberg nodes connected to 2n2^n2n cavity-trapped memory atoms, operated by n+mn+mn+m laser pulses acting as quantum walkers and address units. Our scheme offers a scalable, reducing operational complexity to O(n)\mathcal{O}(n)O(n) and highly coherent pathway toward photonic qRAM, exploiting collective Rydberg interactions to realize programmable, parallel entangling operations in an atomic ensemble.
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Neutral atoms provide an excellent resource for quantum information processing, combining the long atomic coherence times of the hyperfine ground-state with strong dipole-dipole interactions of highly excited Rydberg states for generating deterministic entanglement between qubits separated by < 10 µm [1]. Scalable long-range interactions can be obtained by coupling the atomic array to a superconducting microwave cavity enabling hybrid quantum information processing where the cavity-mediated entanglement allows atoms to be coupled over cm length scales. We present the first steps towards such an experiment incorporating high fidelity readout using an sCMOS camera [2] and the ability to drive fast, optically addressable rotations of the hyperfine-encoded qubits to the Rydberg state. Using our sub-kHz cavitystabilised laser system [3] we demonstrate coherent control of single Rydberg atoms, performing Ramsey spectroscopy to determine coherence time and to generate entanglement between a pair of atoms separated by 6 µm. Combining this excitation scheme with our ground-state Raman lasers we show progress towards the implementation of a mesoscopic Rydberg gate based on electromagnetically induced transparency (EIT) offering robust entanglement of multi-atom ensembles [4].
In this work we propose a hybrid scheme using electromagnetically induced transparency (EIT) and the dipole blockade between Rydberg atoms and their interactions with microwave fields. This scheme makes use of the spatial properties of the blockade phenomenon to realize a controlled-z, photonic phase gate which works with all fields on resonance, where the maximal efficiency is accessible. Photons of the control and target qubits are stored in a cold atomic medium by means of EIT and processed therein with a microwave field. Not taking into account coupling efficiency, which can be over ~90% per-channel, estimated processing fidelities exceeding 90% are achievable, even taking into account motional dephasing and the finite lifetime of the Rydberg levels.
This thesis proposes a means of implementing quantum information processing using photonic qubits as information carriers. Electromagnetically-induced transparency (\textsmaller{EIT}) is used to store information encoded in photons into Rydberg excitations in a cloud of cold atoms, where strong dipole-dipole interactions induce interactions between qubits. After a storage time, information is mapped back into photons collectively emitted from the cloud again via \textsmaller{EIT}. A new experimental apparatus is built to implement non-linear Rydberg quantum optics. A high repetition rate is achieved owing to a \textsmaller{2D-MOT} atom source, and high optical resolution for trapping and probing microscopic atomic ensembles is achieved by the use of aspheric lenses inside the vacuum chamber. A new, high resolution computer control scheme is implemented. This thesis demonstrates that, during the holding time, multiple collective Rydberg excitations at a controlled separation interact with each other to imprint a non-uniform phase gradient resulting in anti-correlation of photon emission. Interactions are observed at up to 151515 times the wavelength of the photonic qubits. These long range interactions offer a promising approach to scaling all-optical quantum computing. Applying resonant microwave fields during the storage time is demonstrated to offer a competitive method of performing sensitive microwave electrometry. A sensitivity of $12 \pm 7 \uV \percm \perRHz$ is found at a frequency of $7.7\GHz$. The high sensitivity is shown to arise from remnant, Rydberg excitations providing an additional source of atom loss from the atomic ensemble, leading to a suppression of photon storage and retrieval efficiency. An additional stage of microwave driving to sanitise the atomic ensemble for recycling is shown to successfully suppress the atom-loss mechanism. The use of successive microwave pulses is shown to provide a feasible approach to interfacing microwave and optical quantum information processing systems. Information encoded as the presence or absence of a microwave field is translated into information encoded as early or late retrieval of single photons, demonstrating proof of principle for an approach to implementing a proposal for an all-optical controlled-zzz gate. Externally driven microwave fields are used to provide rapid, low-loss modulation of the signal retrieved from an atomic ensemble, demonstrating the proof of principle of implementing a probabilistic single photon source that can intensity modulate with low-loss at frequencies of at least $27\MHz$, with evidence that modulation may be achievable at rates in excess of $500\MHz$.
We discuss electromagnetically induced transparency in a cold ensemble of atoms optically excited to the Rydberg states. Strong dipole-dipole or van der Waals interactions between the atomic Rydberg states translate into large nonlocal nonlinearities for a propagating probe field. In the case of weak quantum fields, this can be used to attain a conditional phase shift of π between pairs of single photons realizing a deterministic photonic phase gate. For stronger fields, long-range interactions between the atoms constrain the medium to behave as a collection of superatoms, each comprising a blockade volume that can accommodate at most one Rydberg excitation. The propagation of a probe field is affected by its two-photon correlations within the blockade distance, which are strongly damped due to low saturation threshold of the superatoms. Our calculations reproduce and interpret the results of recent experiments.
One of the critical elements in the realization of the quantum internet are deterministic two-photon gates. This CZ photonic gate also completes a set of universal gates for all-optical quantum information processing. This article discusses an approach to realize a high fidelity CZ photonic gate by storing both control and target photons within an atomic ensemble using non-Rydberg electromagnetically induced transparency (EIT) followed by a fast, single-step Rydberg excitation with global lasers. The proposed scheme operates by relative intensity modulation of two lasers used in Rydberg excitation. Circumventing the conventional π-gap-π schemes, the proposed operation features continuous laser protection of the Rydberg atoms from the environment noise. The complete spatial overlap of stored photons inside the blockade radius optimizes the optical depth and simplifies the experiment. The coherent operation here is performed in the region that was dissipative in the previous Rydberg EIT schemes. Encountering the main imperfection sources, i.e., the spontaneous emission of the Rydberg and intermediate levels, population rotation errors, Doppler broadening of the transition lines, storage/retrieval efficiency, and atomic thermal motion induced decoherence, this article concludes that with realistic experimental parameters 99.7% fidelity is achievable.