L. Childress

High-sensitivity diamond magnetometer with nanoscale resolution

Impurity centres in diamond have recently attracted attention in the context of quantum information processing. Now their use as magnetic-field sensors is explored, promising a fresh approach to single-spin detection and magnetic-field imaging at the nanoscale.

Quantum Register Based on Individual Electronic and Nuclear Spin Qubits in Diamond

The key challenge in experimental quantum information science is to identify isolated quantum mechanical systems with long coherence times that can be manipulated and coupled together in a scalable fashion. We describe the coherent manipulation of an individual electron spin and nearby individual nuclear spins to create a controllable quantum register. Using optical and microwave radiation to control an electron spin associated with the nitrogen vacancy (NV) color center in diamond, we demonstrated robust initialization of electron and nuclear spin quantum bits (qubits) and transfer of arbitrary quantum states between them at room temperature. Moreover, nuclear spin qubits could be well isolated from the electron spin, even during optical polarization and measurement of the electronic state. Finally, coherent interactions between individual nuclear spin qubits were observed and their excellent coherence properties were demonstrated. These registers can be used as a basis for scalable, optically coupled quantum information systems.

Fault-Tolerant Quantum Communication Based on Solid-State Photon Emitters

We describe a novel protocol for a quantum repeater which enables long distance quantum communication through realistic, lossy photonic channels. Contrary to previous proposals, our protocol incorporates active purification of arbitrary errors at each step of the protocol using only two qubits at each repeater station. Because of these minimal physical requirements, the present protocol can be realized in simple physical systems such as solid-state single photon emitters. As an example, we show how nitrogen vacancy color centers in diamond can be used to implement the protocol, using the nuclear and electronic spin to form the two qubits.

Fault-tolerant quantum repeaters with minimal physical resources, and implementations based on single photon emitters

We analyze a method that uses fixed, minimal physical resources to achieve generation and nested purification of quantum entanglement for quantum communication over arbitrarily long distances and discuss its implementation using realistic photon emitters and photonic channels. In this method, we use single-photon emitters with two internal degrees of freedom formed by an electron spin and a nuclear spin to build intermediate nodes in a quantum channel. State-selective fluorescence is used for probabilistic entanglement generation between electron spins in adjacent nodes. We analyze in detail several approaches which are applicable to realistic, homogeneously broadened single-photon emitters. Furthermore, the coupled electron and nuclear spins can be used to efficiently implement entanglement swapping and purification. We show that these techniques can be combined to generate high-fidelity entanglement over arbitrarily long distances. We present a specific protocol that functions in polynomial time and tolerates percent-level errors in entanglement fidelity and local operations. The scheme has the lowest requirements on physical resources of any current scheme for fully fault-tolerant quantum repeaters.

Shaping quantum pulses of light via coherent atomic memory.

We describe proof-of-principle experiments demonstrating a novel approach for generating pulses of light with controllable photon numbers, propagation direction, timing, and pulse shapes. The approach is based on preparation of an atomic ensemble in a state with a desired number of atomic spin excitations, which is later converted into a photon pulse. Spatiotemporal control over the pulses is obtained by exploiting long-lived coherent memory for photon states and Electromagnetically Induced Transparency in an optically dense atomic medium. Using photon counting experiments, we observe Electromagnetically Induced Transparency based generation and shaping of few-photon sub-Poissonian light pulses.

Mesoscopic cavity quantum electrodynamics with quantum dots

We describe an electrodynamic mechanism for coherent, quantum-mechanical coupling between spatially separated quantum dots on a microchip. The technique is based on capacitive interactions between the electron charge and a superconducting transmission line resonator, and is closely related to atomic cavity quantum electrodynamics. We investigate several potential applications of this technique which have varying degrees of complexity. In particular, we demonstrate that this mechanism allows design and investigation of an on-chip double-dot microscopic maser. Moreover, the interaction may be extended to couple spatially separated electron-spin states while only virtually populating fast-decaying superpositions of charge states. This represents an effective, controllable long-range interaction, which may facilitate implementation of quantum information processing with electron-spin qubits and potentially allow coupling to other quantum systems such as atomic or superconducting qubits.

Coherence of an optically illuminated single nuclear spin qubit.

We investigate the coherence properties of individual nuclear spin quantum bits in diamond [Dutt, Science 316, 1312 (2007)10.1126/science.1139831] when a proximal electronic spin associated with a nitrogen-vacancy (N-V) center is being interrogated by optical radiation. The resulting nuclear spin dynamics are governed by time-dependent hyperfine interaction associated with rapid electronic transitions, which can be described by a spin-fluctuator model. We show that due to a process analogous to motional averaging in nuclear magnetic resonance, the nuclear spin coherence can be preserved after a large number of optical excitation cycles. Our theoretical analysis is in good agreement with experimental results. It indicates a novel approach that could potentially isolate the nuclear spin system completely from the electronic environment.

Progress toward generating, storing, and communicating single-photon states using coherent atomic memory

We describe proof-of principle experiments demonstrating a novel approach for generating pulses of light with controllable photon numbers, propagation direction, timing, and pulse shapes. The approach is based on preparation of an atomic ensemble in a state with a desired number of atomic spin excitations, which is later converted into a photon pulse by exploiting long-lived coherent memory for photon states and electromagnetically induced transparency (EIT). We discuss our progress toward applying these techniques to transmit quantum states between atomic memory nodes connected by a photonic channel.

Quantum Control of Light using Coherent Atomic Memory

We describe our recent work using atomic ensembles to store quantum states of light and control light propagation and its quantum properties.

QUANTUM CONTROL OF SPINS AND PHOTONS AT NANOSCALES

P. CAPPELLARO Department of Physics, Harvard University, Cambridge, MA 02138, USA Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA J. M. MAZE Department of Physics, Harvard University, Cambridge, MA 02138, USA L. CHILDRESS Department of Physics, Bates College, Lewiston, ME 04240, USA M. V. G. DUTT Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260, USA J. S. HODGES Department of Physics, Harvard University, Cambridge, MA 02138, USA Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA S. HONG Department of Physics, Harvard University, Cambridge, MA 02138, USA L. JIANG Department of Physics, Harvard University, Cambridge, MA 02138, USA P. L. STANWIX Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA J. M. TAYLOR Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA E. TOGAN Department of Physics, Harvard University, Cambridge, MA 02138, USA A. S. ZIBROV Department of Physics, Harvard University, Cambridge, MA 02138, USA P. HAMMER Department of Electrical and Computer Engineering, Texas A&M University, College Station, TX 77843, USA A. YACOBY Department of Physics, Harvard University, Cambridge, MA 02138, USA R. L. WALSWORTH Department of Physics, Harvard University, Cambridge, MA 02138, USA Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA M. D. LUKIN Department of Physics, Harvard University, Cambridge, MA 02138, USA > > > > Page 1 of 2 QUANTUM CONTROL OF SPINS AND PHOTONS AT NANOSCALES (World Scien...