Southwest Quantum Information and Technology

Sixteenth Annual Meeting, February 20-22, 2014
Santa Fe, New Mexico

Full Program | Thursday | Friday | Saturday | All Sessions

SESSION 10: Superconducting Qubits
8:30am - 9:15am	Shyam Shankar, Yale University (invited)
	Autonomously stabilized entanglement between two superconducting qubits
	Abstract. Quantum error-correction codes are designed to protect an arbitrary state of a multi-qubit register against decoherence-induced errors, but their implementation is an outstanding challenge for the development of large-scale quantum computers. A first step is to stabilize a non-equilibrium state of a simple quantum system such as a qubit or a cavity mode, in the presence of decoherence. Several groups have recently accomplished this goal using measurement-based feedback schemes. A next step is to prepare and stabilize a state of a composite system. Here we demonstrate the stabilization of an entangled Bell state of a quantum register of two superconducting qubits for an arbitrary time. Our result[1] is achieved by an autonomous feedback scheme which combines continuous drives along with a specifically engineered coupling between the two-qubit register and a dissipative bath. Similar bath engineering techniques have recently been used for qubit reset, single qubit state stabilization, as well as for the creation and stabilization of states of multipartite quantum systems. Unlike conventional, measurement-based schemes, an autonomous approach which uses engineered dissipation to counteract decoherence, obviates the need for a complicated external feedback loop to correct errors. Instead the feedback loop is built into the Hamiltonian such that the steady state of the system in the presence of drives and dissipation is a Bell state, an essential building-block for quantum information processing. Such autonomous schemes, which are broadly applicable to a variety of physical systems, will be an essential tool for the implementation of quantum-error correction. [1] http://dx.doi.org/10.1038/nature12802
9:15am - 9:45am	Pedram Roushan, UCSB
	Mapping the topological phase diagram of superconducting qubit systems
	Abstract. Building a practical quantum simulator requires a scalable architecture suitable for large numbers of qubits. By combining the high coherence Xmon qubits with an adjustable inductance, we have developed a new qubit architecture called g-mon, which has a tunable qubit-qubit interaction. To demonstrate this tunability, we have performed high fidelity single and two-qubit gates. Turning on the qubit-qubit interaction allows for fast multi-qubit operations implemented in less than 30 ns, achieving multi-qubit gate times approaching that of single qubit gates. Furthermore, we show the versatility of this system by mapping the topological phase diagram of interacting Hamiltonians. So far, experimental studies of topological invariants in condensed matter systems have been limited to transport measurements in non-interacting systems. Recently, it was proposed [1] that the topological properties of Hamiltonians can be inferred from quantum dynamics. Using superconducting g-mon qubits, we experimentally measure the Berry curvature, a quantity that reflects the geometrical properties of the eigenstates, for various eigenstates of the Hamiltonian of the system. We will discuss the phase diagram of various topological phases and the robustness of the measured Chern numbers. [1] Gritsev and Polkovnikov, PNAS, 109, 6457 (2012).