Extending Quantum Memory Practical applications in quantum communication and quantum computation require the building blocks—quantum bits and quantum memory—to be sufficiently robust and long-lived to allow for manipulation and storage (see the Perspective by Boehme and McCarney ). Steger et al. (p. 1280 ) demonstrate that the nuclear spins of 31 P impurities in an almost isotopically pure sample of 28 Si can have a coherence time of as long as 192 seconds at a temperature of ∼1.7 K. In diamond at room temperature, Maurer et al. (p. 1283 ) show that a spin-based qubit system comprised of an isotopic impurity ( 13 C) in the vicinity of a color defect (a nitrogen-vacancy center) could be manipulated to have a coherence time exceeding one second. Such lifetimes promise to make spin-based architectures feasible building blocks for quantum information science.

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We demonstrate an all-optical method for magnetic sensing of individual molecules in ambient conditions at room temperature. Our approach is based on shallow nitrogen-vacancy (NV) centers near the surface of a diamond crystal, which we use to detect single paramagnetic molecules covalently attached to the diamond surface. The manipulation and readout of the NV centers is alloptical and provides a sensitive probe of the magnetic field fluctuations stemming from the dynamics of the electronic spins of the attached molecules. As a specific example, we demonstrate detection of a single paramagnetic molecule containing a gadolinium (Gd 3+ ) ion. We confirm single-molecule resolution using optical fluorescence and atomic force microscopy to co-localize one NV center and one Gd 3+ -containing molecule. Possible applications include nanoscale and in vivo magnetic spectroscopy and imaging of individual molecules.Precision magnetic sensing is essential to a wide array of technologies such as magnetic resonance imaging (MRI), with important applications in both the physical and life sciences. In particular, in biology and medicine, functional magnetic resonance imaging (fMRI) has emerged as a primary workhorse for obtaining key physiological and pathological information noninvasively, such as blood and tissue oxygen level and redox status [1][2][3]. Developing nanoscale magnetic sensing applicable to individual molecules could enable revolutionary advances in the physical, biological, and medical sciences. Examples include determining the structure of single proteins and other biomolecules as well as in vivo measurements of small concentrations of reactive oxygen species that could lead to insights into cellular signaling, ageing, mutations, and death [4][5][6][7]. The practical realization of these ideas is extremely challenging, however, as it requires sensitive detection of weak magnetic fields associated with individual electronic or nuclear spins at nanometer scale resolution, often under ambient, room-temperature conditions. Many state-of-theart magnetic sensors, including superconducting quantum interference devices (SQUIDs) [8], semiconductor Hall effect sensors [9], and spin exchange relaxation-free atomic magnetometers [10], offer outstanding sensitivity, but their macroscopic nature precludes individual spin sensing. Sensing ensembles of paramagnetic molecules in biological and medical systems is currently performed using bulk electron spin resonance (ESR), which has a limit of roughly 10 7 electron-spins [11]. Magnetic resonance force microscopy has been used to detect individual electronic spins, but operates at cryogenic, milliKelvin temperature [12,13].In this Letter we demonstrate a method for nanoscale magnetic sensing of individual non-fluorescent molecules that employs optical manipulation of nitrogen vacancy (NV) centers in diamond [14][15][16][17][18][19][20][21]. In our approach the target molecules are covalently attached to the diamond surface, and magnetic sensing of these molecules is performed un...

We analyze the evolution of an electromagnetic field inside a double cavity when the difference in length between the two cavities is changed, e.g., by translating the common mirror. We find that this allows photons to be moved deterministically from one cavity to the other. We are able to obtain the conditions for adiabatic transfer by first mapping the Maxwell wave equation for the electric field onto a Schrödinger-like wave equation and then using the Landau-Zener result for the transition probability at an avoided crossing. Our analysis reveals that this mapping only rigorously holds when the two cavities are weakly coupled (i.e., in the regime of a highly reflective common mirror) and that, generally speaking, care is required when attempting a Hamiltonian description of cavity electrodynamics with time-dependent boundary conditions.

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