Photonic controlled-phase gates through Rydberg blockade in optical cavities

Abstract

We propose a novel scheme for high fidelity photonic controlled-phase gates using Rydberg blockade in an ensemble of atoms in an optical cavity. The gate operation is obtained by first storing a photonic pulse in the ensemble and then scattering a second pulse from the cavity, resulting in a phase change depending on whether the first pulse contained a single photon. We show that the combination of Rydberg blockade and optical cavities effectively enhances the optical non-linearity created by the strong Rydberg interaction and makes the gate operation more robust. The resulting gate can be implemented with cavities of moderate finesse allowing for highly efficient processing of quantum information encoded in photons. As an illustration, we show how the gate can be employed to increase the communication rate of quantum repeaters based on atomic ensembles.PACS numbers: Keywords:Large bandwidth, fast propagation and the non-interacting nature of photons, make them ideal for communicating quantum information over long distances [1]. In contrast, strong photon-photon interactions are desirable for processing of quantum information encoded in the photons, especially if both high fidelity and high efficiency are needed. To satisfy these requirements one needs a highly non-linear medium. Typically, the strength of photon-photon interactions mediated by a non-linear medium is very weak at the single-photon level where photonic quantum logic gates are operating [2]. As a consequence, the implementation of photonic quantum gates remains an unsolved challenge and requires novel means of efficient light-matter interaction. To enhance light-matter interactions, a viable solution is to use ensembles of atoms, e.g., configured for electromagnetically induced transparency (EIT) [3]; this can be further improved by placing the ensemble in an optical cavity, but these ensemble based approaches do not increase the essential photonic non-linearity. In recent years, there has been intense efforts to realize light-matter interactions via, non-linear interactions in a variety of medium, ranging from atoms [4-10] and atom like systems [11][12][13][14] to superconducting qubits [15][16][17].A promising approach towards creating strong quantum nonlinearities is to exploit excitation blockade in Rydberg EIT systems [18][19][20][21][22][23][24][25]. Several quantum effects like strong optical non-linearities and control of light by light [22][23][24][25][26][27][28], deterministic single-photon sources [29], and the generation of entanglement and atomic quantum gates [30][31][32][33][34] have been investigated. The strong nonlinearity originates from the fact that the Rydberg interaction prevents multiple excitations within a blockaded radius r b [35,36]. The ensemble then behaves as a two-level superatom consisting of N b atoms within a radius r b [35,36]. If the optical depth d b corresponding to the superatom is sufficiently large, d b 1 [22], a strong optical nonlinearity at the single-photon level can be achieved in the EIT configurati...

Rydberg-atom-based scheme of nonadiabatic geometric quantum computation

“…It is worth noting that Rydberg atoms have been used to realize photonic pulse-based quantum gates in previous works [66][67][68][69], where the computational qubits are encoded by photons and the Rydberg atomic ensemble only acts as mediator. By converting photonic pulses into Rydberg excitations, the logic operations between pulses can be directly performed.…”

Precise single-qubit control of the reflection phase of a photon mediated by a strongly-coupled ancilla–cavity system

“…In this work, we build on earlier proposals that use Rydberg blockade to alter the EIT condition for a large ensemble of atoms interacting with a cavity field, and thus alter the reflection property of the cavity surrounding the atoms [5,8,30]. While retaining the advantages of the collectively enhanced matter-light interaction, we instead explore different control and interaction mechanisms and we show that qubit-photon high-fidelity gates can be achieved with significantly reduced excitation of the Rydberg levels.…”

“…Proposals to use light to interlink quantum degrees of freedom of spatially separated nodes fall broadly in two categories. The first engages direct transmission of non-classical states of light [1][2][3][4][5][6][7][8], while the second heralds non-local quantum correlations by joint measurements on signals that are emitted from or have sequentially interacted with spatially separated quantum systems [9][10][11][12][13][14][15][16][17][18].…”

“…Ensembles of interacting atoms can be employed to create and manipulate non-classical states of light [24][25][26][27] to enable qubit interactions between separate single photon wave packets [5,8]. It is also possible to address collective qubit degrees of freedom via different internal atomic states [28,29], but effective coupling of single photons to single atomic qubits that form part of a local register with computing, memory or sensing capabilities remains a challenge.…”

Limitations of two-level emitters as nonlinearities in two-photon controlled-phase gates

“…Since these requirements were first stated, single-photon sources have steadily improved [7][8][9][10], with the most promising platforms based on few-level emitters, most notably semiconductor quantum dots [11][12][13] which now boast near-unity indistinguishability with (source to first objective) efficiencies above 70%. Generating photon-photon interactions can be achieved by "off-line nonlinearities" consisting of measurements and feed-forward [1][2][3][4]7], or deterministically by using "in-line" nonlinearities based on a nonlinear material through which two or more photons interact [14][15][16][17]. These in-line nonlinearities can in principle also be generated by few-level emitters [18][19][20], suggesting a quantum photonic architecture in which few-level systems act as both photon sources and photon couplers.…”