A photon–photon quantum gate based on a single atom in an optical resonator

Abstract

That two photons pass each other undisturbed in free space is ideal for the faithful transmission of information, but prohibits an interaction between the photons. Such an interaction is, however, required for a plethora of applications in optical quantum information processing. The long-standing challenge here is to realize a deterministic photon-photon gate, that is, a mutually controlled logic operation on the quantum states of the photons. This requires an interaction so strong that each of the two photons can shift the other's phase by π radians. For polarization qubits, this amounts to the conditional flipping of one photon's polarization to an orthogonal state. So far, only probabilistic gates based on linear optics and photon detectors have been realized, because "no known or foreseen material has an optical nonlinearity strong enough to implement this conditional phase shift''. Meanwhile, tremendous progress in the development of quantum-nonlinear systems has opened up new possibilities for single-photon experiments. Platforms range from Rydberg blockade in atomic ensembles to single-atom cavity quantum electrodynamics. Applications such as single-photon switches and transistors, two-photon gateways, nondestructive photon detectors, photon routers and nonlinear phase shifters have been demonstrated, but none of them with the ideal information carriers: optical qubits in discriminable modes. Here we use the strong light-matter coupling provided by a single atom in a high-finesse optical resonator to realize the Duan-Kimble protocol of a universal controlled phase flip (π phase shift) photon-photon quantum gate. We achieve an average gate fidelity of (76.2 ± 3.6) per cent and specifically demonstrate the capability of conditional polarization flipping as well as entanglement generation between independent input photons. This photon-photon quantum gate is a universal quantum logic element, and therefore could perform most existing two-photon operations. The demonstrated feasibility of deterministic protocols for the optical processing of quantum information could lead to new applications in which photons are essential, especially long-distance quantum communication and scalable quantum computing.

Nondestructive photon detection using a single rare-earth ion coupled to a photonic cavity

“…The resulting entanglement between the photon and the ion can be used to detect the photon through readout of the ion's state. These ideas have recently been realized in a series of impressive experiments with single trapped atoms inside free-space high-finesse cavities [8][9][10][11].…”

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.…”

“…Strong in-line nonlinearities and photon switching have been achieved by using Rubidium atoms strongly coupled to optical cavities [17,[24][25][26], quantum dots in photonic crystal cavities [27][28][29][30], and nitrogen vacancy centers in diamond [31]. The potentially deterministic nature of few-photon in-line nonlinearities makes this approach particularly attractive for the realization of photonic gates, and a number of proposals have been put forward to construct controlled-PHASE gates on various platforms and with various degrees of complexity [14,15,[32][33][34][35][36].…”

Quantum phase gate based on electromagnetically induced transparency in optical cavities

“…Thus, optical architectures have been potentially used to implement a variety of quantum devices, such as optical transistor [4], quantum network [5], quantum memory [6] and to implement quantum logic operations on one or two qubits [7][8][9][10]. A fundamental element for these applications in the field of quantum information and quantum computation is the conditional quantum dynamics, where the quantum state of a system can control the measurement result of another quantum system.…”