Zhen-Sheng Yuan

Experimental entanglement of six photons in graph states

Graph states1,2,3—multipartite entangled states that can be represented by mathematical graphs—are important resources for quantum computation4, quantum error correction3, studies of multiparticle entanglement1 and fundamental tests of non-locality5,6,7 and decoherence8. Here, we demonstrate the experimental entanglement of six photons and engineering of multiqubit graph states9,10,11. We have created two important examples of graph states, a six-photon Greenberger–Horne–Zeilinger state5, the largest photonic Schrodinger cat so far, and a six-photon cluster state2, a state-of-the-art ‘one-way quantum computer’4. With small modifications, our method allows us, in principle, to create various further graph states, and therefore could open the way to experimental tests of, for example, quantum algorithms4,12 or loss- and fault-tolerant one-way quantum computation13,14.

Experimental demonstration of a BDCZ quantum repeater node

Quantum communication is a method that offers efficient and secure ways for the exchange of information in a network. Large-scale quantum communication (of the order of 100 km) has been achieved; however, serious problems occur beyond this distance scale, mainly due to inevitable photon loss in the transmission channel. Quantum communication eventually fails when the probability of a dark count in the photon detectors becomes comparable to the probability that a photon is correctly detected. To overcome this problem, Briegel, Dür, Cirac and Zoller (BDCZ) introduced the concept of quantum repeaters, combining entanglement swapping and quantum memory to efficiently extend the achievable distances. Although entanglement swapping has been experimentally demonstrated, the implementation of BDCZ quantum repeaters has proved challenging owing to the difficulty of integrating a quantum memory. Here we realize entanglement swapping with storage and retrieval of light, a building block of the BDCZ quantum repeater. We follow a scheme that incorporates the strategy of BDCZ with atomic quantum memories. Two atomic ensembles, each originally entangled with a single emitted photon, are projected into an entangled state by performing a joint Bell state measurement on the two single photons after they have passed through a 300-m fibre-based communication channel. The entanglement is stored in the atomic ensembles and later verified by converting the atomic excitations into photons. Our method is intrinsically phase insensitive and establishes the essential element needed to realize quantum repeaters with stationary atomic qubits as quantum memories and flying photonic qubits as quantum messengers.

Memory-built-in quantum teleportation with photonic and atomic qubits

The combination of quantum teleportation1 and quantum memory2,3,4,5 of photonic qubits is essential for future implementations of large-scale quantum communication6 and measurement-based quantum computation7,8. Both steps have been achieved separately in many proof-of-principle experiments9,10,11,12,13,14, but the demonstration of memory-built-in teleportation of photonic qubits remains an experimental challenge. Here, we demonstrate teleportation between photonic (flying) and atomic (stationary) qubits. In our experiment, an unknown polarization state of a single photon is teleported over 7 m onto a remote atomic qubit that also serves as a quantum memory. The teleported state can be stored and successfully read out for up to 8 μs. Besides being of fundamental interest, teleportation between photonic and atomic qubits with the direct inclusion of a readable quantum memory represents a step towards an efficient and scalable quantum network2,3,4,5,6,7,8.

A millisecond quantum memory for scalable quantum networks

calculation shows that the expected lifetime is of the order of seconds in this case. Here we report on our investigation of prolonging the storage time of the quantum memory for single excitations. In the experiment, we find that using only the ‘clock state’ is not sufficient toobtain theexpected longstorage time.We furtheranalyse, isolate and identify the distinct decoherence mechanisms, and thoroughly investigate the dephasing of the spin wave (SW) by varying its wavelength. We find that the dephasing of the SW is extremely sensitive to the angle between the write beam and detection mode, especiallyforsmallangles.Onthebasisofthisfinding,byexploiting the ‘clock state’ and increasing the wavelength of the SW to suppress the dephasing, we succeed in extending the storage time from 10s

Deterministic and storable single-photon source based on a quantum memory

A single-photon source is realized with a cold atomic ensemble (87Rb atoms). A single excitation, written in an atomic quantum memory by Raman scattering of a laser pulse, is retrieved deterministically as a single photon at a predetermined time. It is shown that the production rate of single photons can be enhanced considerably by a feedback circuit while the single-photon quality is conserved. Such a single-photon source is well suited for future large-scale realization of quantum communication and linear optical quantum computation.

Demonstration of a stable atom-photon entanglement source for quantum repeaters.

We demonstrate a novel way to efficiently create a robust entanglement between an atomic and a photonic qubit. A single laser beam is used to excite one atomic ensemble and two different modes of Raman fields are collected to generate the atom-photon entanglement. With the help of built-in quantum memory, the entanglement still exists after 20.5 micros storage time which is further proved by the violation of Clauser-Horne-Shimony-Holt type Bells inequality. The entanglement procedure can serve as a building block for a novel robust quantum repeater architecture [Zhao, Phys. Rev. Lett. 98, 240502 (2007)10.1103/PhysRevLett.98.240502] and can be extended to generate high-dimensional atom-photon entanglements.

Quantum teleportation between remote atomic-ensemble quantum memories

Quantum teleportation and quantum memory are two crucial elements for large-scale quantum networks. With the help of prior distributed entanglement as a “quantum channel,” quantum teleportation provides an intriguing means to faithfully transfer quantum states among distant locations without actual transmission of the physical carriers [Bennett CH, et al. (1993) Phys Rev Lett 70(13):1895–1899]. Quantum memory enables controlled storage and retrieval of fast-flying photonic quantum bits with stationary matter systems, which is essential to achieve the scalability required for large-scale quantum networks. Combining these two capabilities, here we realize quantum teleportation between two remote atomic-ensemble quantum memory nodes, each composed of ∼108 rubidium atoms and connected by a 150-m optical fiber. The spin wave state of one atomic ensemble is mapped to a propagating photon and subjected to Bell state measurements with another single photon that is entangled with the spin wave state of the other ensemble. Two-photon detection events herald the success of teleportation with an average fidelity of 88(7)%. Besides its fundamental interest as a teleportation between two remote macroscopic objects, our technique may be useful for quantum information transfer between different nodes in quantum networks and distributed quantum computing.

Heralded generation of an atomic NOON state.

We report the observation of phase-super resolution in a motion-sensitive spin-wave (SW) atom interferometer utilizing a NOON-type entanged state. The SW interferometer is implemented by generating a superposition of two SWs and observing the interference between them, where the interference fringe is sensitive to the atomic collective motion. By heralded generation of a second order NOON state in the SW interferometer, we clearly observe the interference pattern with phase super-resolution. The demonstrated SW interferometer can in principle be scaled up to highly entangled quantum state, and thus is of fundamental importance to implement quantum-enhancedmeasurement.

Generation and detection of atomic spin entanglement in optical lattices

Atoms in optical lattices are interesting for quantum technologies but engineering entanglement between atom pairs is difficult. Using the double-well potentials of a superlattice, the generation and detection of entanglement is more straightforward.

Synchronized independent narrow-band single photons and efficient generation of photonic entanglement.

We create independent, synchronized single-photon sources with built-in quantum memory based on two remote cold atomic ensembles. The synchronized single photons are used to demonstrate efficient generation of entanglement. The resulting entangled photon pairs violate a Bells inequality by 5 standard deviations. Our synchronized single photons with their long coherence time of 25 ns and the efficient creation of entanglement serve as an ideal building block for scalable linear optical quantum information processing.