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arXiv:2211.06998 · Optics Express

All-optical quantum information processing via a single-step Rydberg blockade gate

Mohammadsadegh Khazali

Published Mar 31, 2023• 14 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1364/oe.481256

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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.
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 $500\MHz$.
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.
In this Letter, we propose a hybrid scheme to implement a photonic controlled-z (CZ) gate using photon storage in highly excited Rydberg states, which controls the effective photon-photon interaction using resonant microwave fields. Our scheme decouples the light propagation from the interaction and exploits the spatial properties of the dipole blockade phenomenon to realize a CZ gate with minimal loss and mode distortion. By excluding the coupling efficiency, fidelities exceeding 95% are achievable and are found to be mainly limited by motional dephasing and the finite lifetime of the Rydberg levels.
We have numerically simulated a two-qubit CZ gate for ultracold neutral atoms based on Rydberg blockade, utilizing three-photon excitation to Rydberg states. The three-photon laser excitation overcomes lack of individual addressability for entangling gates in atomic arrays, which is a key limitation of conventional schemes of quantum information processing with neutral atoms. This is achieved by mitigating sensitivity to laser intensity fluctuations caused by spatial motion of atoms in optical tweezers. Our goal is to investigate the gate performance when three-photon laser excitation is used. We considered Levine-Pichler gate which applies an abrupt phase jump at the center of the control laser pulse. Our simulation accounts for the influence of spontaneous decay of intermediate and Rydberg states and finite Rydberg excitation energies. We obtained the maximum fidelity of the CZ gate around 99.7% for the model based on probability amplitudes and 99.72% for the model based on density matrices. This value exceeds the threshold required for the application of quantum error correction protocols, demonstrating the promise of this gate for experimental realization. This work was supported by BASIS foundation Grant No. № 25-1-1-57-5
This thesis reports the experimental realisation of a single-photon switch and transistor.To this end, Rydberg blockade in electromagnetically induced transparency (EIT) is used to create an interaction between two light pulses at the single photon level.In the experiment, a gate pulse containing one incoming photon on average is stored as a Rydberg excitation in an ultracold atomic gas using EIT.Subsequently a target light pulse is sent through the atomic ensemble.Without the gate pulse, the medium would be transparent due to EIT and the transmission would be high.If a gate excitation has been stored, however, Rydberg blockade suppresses the transmission of the target light pulse.The stored gate photon can be retrieved after the transmission of the target light.A retrieved photon heralds successful storage.In the corresponding postselected subensemble the transmission of the target light is reduced by an extinction factor of 0.05.This reduction is robust if the number of incoming gate photons or target photons is varied.In a conceptually improved version of the experiment, a single-photon transistor is realised that shows a gain above unity.The gain quantifies the change in the transmitted target photon number per incoming gate photon.For one incoming gate photon on average, a gain of 20 is observed.The improved experiment uses a Förster resonance, the benefit of which becomes directly evident in an improved gain and extinction.If a Rydberg excitation is created during the gate pulse, then the transmission of the target light pulse drops substantially.By setting a discrimination threshold for the number of detected target photons, it is possible to detect in a single shot with a fidelity of 86% whether a Rydberg excitation was created during the gate pulse.Since the gate excitation is not retrieved by the target light, this detection method is nondestructive.i iii 6.2.Experimental Sequence . . . . . . .
Electromagnetically induced transparency (EIT) enables coherent light-matter storage, forming the basis of photonic quantum memories that are essential for scalable quantum networks and distributed quantum computing. However, accelerating the storage process violates the adiabatic condition, resulting in the excitation of the lossy intermediate state and a reduction in writing efficiency. We propose and numerically investigate a high-speed, high-fidelity quantum storage scheme by incorporating a shortcut-to-adiabaticity (STA) technique based on counter-diabatic (CD) driving. By introducing a precisely engineered auxiliary field into a conventional EIT system, our protocol significantly shortens the writing time beyond the conventional adiabatic limit while effectively suppressing the transient population of the lossy intermediate state. Furthermore, our scheme demonstrates strong flexibility in pulse design, remaining effective across different temporal profiles of both the control and signal fields. It also exhibits robustness against imperfections in the CD drive. Even with imperfect single-photon writing and non-ideal Rydberg blockade, the scheme retains clear advantages, maintaining high storage performance and overcoming the intrinsic speed-fidelity trade-off of traditional EIT protocols. These features pave the way for fast and robust quantum devices suitable for high-throughput quantum repeaters and advanced quantum information processing.