Title:Quantum Journal Club: Hybrid quantum logic and a test of Bell's inequality using
Speaker: Shuaining Zhang Tsinghua University
Time: 2015-11-17 14:00-2015-11-17 15:00


Hybrid quantum logic and a test of Bell's inequality using two di_erent atomic species:
Entanglement is one of the most fundamental properties of quantum mechanics, and is the key resource for quantum information processing. Bipartite entangled states of identical particles have been generated and studied in several experiments, and post-selected entangled states involving pairs of photons, or single photons and single atoms, have also been produced. Here, we deterministically generate a \hybrid" entangled state of two di erent species of trapped-ion qubit, perform full tomography of the state produced, and make the rst test of Bell's inequality with nonidentical atoms. We use a laser-driven two-qubit quantum logic gate, whose mechanism is insensitive to the qubits' energy splittings, to produce a maximally-entangled state of one 40Ca+ qubit and one 43Ca+ qubit, held in the same ion trap, with 99.8(5)% delity. We make a test of Bell's inequality for this novel entangled state, and nd that it is violated by 15. Mixed-species quantum logic is an essential technique for the construction of a quantum computer based on trapped ions, as it allows protection of memory qubits while other qubits undergo logic operations, or are used as photonic interfaces to other processing units.

Multi-Element Logic Gates for Trapped-Ion Qubits:
Precision control over hybrid physical systems at the quantum level is important for the realization of many quantum-based technologies. For trapped-ions, a hybrid system formed of different species introduces extra degrees of freedom that can be exploited to expand and re ne the control of the system. Here, we demonstrate an entangling gate between two atomic ions of different elements that can serve as an important building block of quantum information processing (QIP), quantum networking, precision spectroscopy, metrology, and quantum simulation. An entangling geometric phase gate between a 9Be+ ion and a 25Mg+ ion is realized through an e ective spin-spin interaction generated by state-dependent forces1{5. Combined with single-qubit gates and same-species entangling gates, this mixed-element entangling gate provides a complete set of gates over such a hybrid system for universal QIP6{8. Using a sequence of such gates, we demonstrate a Controlled-NOT (CNOT) gate and a SWAP gate9. We also perform a CHSH-type Bell-inequality test10 on Bell states composed of di erent ion species.

Short Bio: