A quantum gate between a flying optical photon and a single trapped atom

Abstract

The steady increase in control over individual quantum systems has backed the dream of a quantum technology that provides functionalities beyond any classical device. Two particularly promising applications have been explored during the past decade: First, photon-based quantum communication, which guarantees unbreakable encryption 1 but still has to be scaled to high rates over large distances. Second, quantum computation, which will fundamentally enhance computability 2 if it can be scaled to a large number of quantum bits. It was realized early on that a hybrid system of light and matter qubits 3 could solve the scalability problem of both fields-that of communication via quantum repeaters 4 , that of computation via an optical interconnect between smaller quantum processors 5,6 . To this end, the development of a robust two-qubit gate that allows to link distant computational nodes is "a pressing challenge" 6 . Here we demonstrate such a quantum gate between the spin state of a single trapped atom and the polarization state of an optical photon contained in a faint laser pulse. The presented gate mechanism 7 is deterministic, robust and expected to be applicable to almost any matter qubit. It is based on reflecting the photonic qubit from a cavity that provides strong light-matter coupling. To demonstrate its versatility, we use the quantum gate to create atom-photon, atom-photon-photon, and photon-photon entangled states from separable input states.We expect our experiment to break ground for various applications, including the generation of atomic 8 and photonic 9,10 cluster states, Schrödinger-cat states 11 , deterministic photonic Bell-state measurements 12 , and quantum communication using a redundant quantum parity code 13 .Since their infancy, the fields of quantum communication and quantum computation have been largely independent. For communication 1 , optical photons are employed because they allow to transmit quantum states, such as time-bin or polarization qubits, over large distances using existing telecommunication fibre technology. Quantum computation 2 , on the other hand, is typically based on single spins, either in vacuum or in specific solidstate host materials. In addition to the long coherence times these spins can exhibit, they provide deterministic interaction mechanisms that facilitate local two-qubit * gerhard.rempe@mpq.mpg.deAtom-photon quantum gate. a, Atomic level scheme on the D2 line of 87 Rb. The photonic qubit is defined in the basis of left-(| ↓ p ) and right-(| ↑ p ) circular polarization. The atomic qubit is encoded in the atomic |F, mF states | ↓ a ≡ |1, 1 and | ↑ a ≡ |2, 2 . Here, F denotes the atomic hyperfine state and mF its projection onto an external magnetic field. The cavity (blue semi-circles) is resonant with the a.c. Stark-shifted |2, 2 ↔ |3, 3 transition on the D2 line around 780 nm. Upon reflection of a photon from the cavity, the combined atom-photon state | ↑ a ↑ p (green, ⊕) acquires a phase shift of π with respect to all other states (red, ). b, Measured ...

Quantum nonlinear optics — photon by photon

“…Using the first photon to transfer the atom to another ground state switches the transmission or reflection of the atom, thus enabling the first photon to interact with subsequently applied probe photons. Similar approaches have recently allowed researchers to implement a non-destructive photon detector 48 , a quantum phase switch between a single atom and a single photon 49 , and a quantum gate between an atom and a photon 50 . These demonstrations constitute a key enabling technology for quantum networks 6 , where individual, remote quantum bits encoded in atoms are connected and entangled via photonic channels (Fig.…”

“…This possibility has served as a major motivation for the development of quantum nonlinear optics 6 . Recent experimental demonstrations within this context include photon-mediated quantum state transfer between atoms in distant cavities 75 , a quantum phase switch between a single atom and a single photon 49 , and a quantum gate between an atom and a photon 50 . A major goal is to scale these systems up to large numbers of qubits, nodes and operations.…”

A quantum phase switch between a single solid-state spin and a photon

“…Strong interactions between these two systems provide the necessary interface for developing future quantum networks 1 and distributed quantum computers 2 . They also enable a broad range of critical quantum information functionalities such as entanglement distribution 3,4 , non-destructive quantum measurements [5][6][7] , and strong photon-photon interactions [8][9][10] . Realizing spin-photon interactions in a solid-state device is particularly desirable because it opens up the possibility for chip-integrated quantum circuits that support gigahertz bandwidth operation 11 .…”

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

“…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].…”