We experimentally demonstrate that the decoherence of a spin by a spin bath can be completely eliminated by fully polarizing the spin bath. We use electron paramagnetic resonance at 240 GHz and 8 T to study the electron-spin coherence time T 2 of nitrogen-vacancy centers and nitrogen impurities in diamond from room temperature down to 1.3 K. A sharp increase of T 2 is observed below the Zeeman energy (11.5 K). The data are well described by a suppression of the flip-flop induced spin bath fluctuations due to thermal electron-spin polarization. T 2 saturates at 250 s below 2 K, where the polarization of the electron-spin bath exceeds 99%. DOI: 10.1103/PhysRevLett.101.047601 PACS numbers: 76.30.Mi, 03.65.Yz Overcoming spin decoherence is critical to spintronics and spin-based quantum information processing devices [1,2]. For spins in the solid state, a coupling to a fluctuating spin bath is a major source of the decoherence. Therefore, several recent theoretical and experimental efforts have aimed at suppressing spin bath fluctuations [3][4][5][6][7][8][9]. One approach is to bring the spin bath into a well-known quantum state that exhibits little or no fluctuations [10,11]. A prime example is the case of a fully polarized spin bath. The spin bath fluctuations are fully eliminated when all spins are in the ground state. In quantum dots, nuclear spin bath polarizations of up to 60% have been achieved [12,13]. However, a polarization above 90% is needed to significantly increase the spin coherence time [14]. Moreover, thermal polarization of the nuclear spin bath is experimentally challenging due to the small nuclear magnetic moment. Electron-spin baths, however, may be fully polarized thermally at a few degrees of Kelvin under an applied magnetic field of 8 T.Here we investigate the relationship between the spin coherence of nitrogen-vacancy (N-V) centers in diamond and the polarization of the surrounding spin bath consisting of nitrogen (N) electron spins. N-V centers consist of a substitutional nitrogen atom adjoining to a vacancy in the diamond lattice. The N-V center, which has long spin coherence times at room temperature [15,16], is an excellent candidate for quantum information processing applications as well as conducting fundamental studies of interactions with nearby electronic spins [16 -18] and nuclear spins [19,20]. In the case of type-Ib diamond, as studied here, the coupling to a bath of N electron spins is the main source of decoherence for an N-V center spin [15,21]. We have measured the spin coherence time (T 2 ) and spin-lattice relaxation time (T 1 ) in spin ensembles of N-V centers and single N impurity centers (P1 centers) using pulsed electron paramagnetic resonance (EPR) spectroscopy at 240 GHz. By comparing the values of T 1 and T 2 at different temperatures, we verify that the mechanism determining T 2 is different from that of T 1 . Next, we investigate the temperature dependence of T 2 .At 240 GHz and 8.6 T where the Zeeman energy of the N centers corresponds to 11.5 K, the polariza...

Quantum decoherence is a central concept in physics. Applications such as quantum information processing depend on understanding it; there are even fundamental theories proposed that go beyond quantum mechanics, in which the breakdown of quantum theory would appear as an 'intrinsic' decoherence, mimicking the more familiar environmental decoherence processes. Such applications cannot be optimized, and such theories cannot be tested, until we have a firm handle on ordinary environmental decoherence processes. Here we show that the theory for insulating electronic spin systems can make accurate and testable predictions for environmental decoherence in molecular-based quantum magnets. Experiments on molecular magnets have successfully demonstrated quantum-coherent phenomena but the decoherence processes that ultimately limit such behaviour were not well constrained. For molecular magnets, theory predicts three principal contributions to environmental decoherence: from phonons, from nuclear spins and from intermolecular dipolar interactions. We use high magnetic fields on single crystals of Fe(8) molecular magnets (in which the Fe ions are surrounded by organic ligands) to suppress dipolar and nuclear-spin decoherence. In these high-field experiments, we find that the decoherence time varies strongly as a function of temperature and magnetic field. The theoretical predictions are fully verified experimentally, and there are no other visible decoherence sources. In these high fields, we obtain a maximum decoherence quality-factor of 1.49 × 10(6); our investigation suggests that the environmental decoherence time can be extended up to about 500 microseconds, with a decoherence quality factor of ∼6 × 10(7), by optimizing the temperature, magnetic field and nuclear isotopic concentrations.

We report coherent manipulation of S=10 Fe8 single-molecule magnets. The temperature dependence of the spin decoherence time T2 measured by high-frequency pulsed electron paramagnetic resonance indicates that strong spin decoherence is dominated by Fe8 spin bath fluctuations. By polarizing the spin bath in Fe8 single-molecule magnets at magnetic field B=4.6 T and temperature T=1.3 K, spin decoherence is significantly suppressed and extends the spin decoherence time T2 to as long as 712 ns. A second decoherence source is likely due to fluctuations of the nuclear spin bath. This hints that the spin decoherence time can be further extended via isotopic substitution to smaller nuclear magnetic moments.

We theoretically investigate spin decoherence of a single nitrogen-vacancy (NV) center in diamond. Using the spin coherent state P-representation method, coherence evolution of the NV center surrounded by nitrogen electron spins (N) is simulated. We find that spin decoherence time as well as free-induction decay of the NV center depend on the spatial configuration of N spins.Both the spin decoherence rate (1/T 2 ) and dephasing rate (1/T * 2 ) of the NV center increase linearly with the concentration of the N spins. Using the P-representation method, we also demonstrate extracting noise spectrum of the N spin bath, which will provide promising pathways for designing an optimum pulse sequence to suppress the decoherence in diamond.

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The cavity perturbation technique is an extremely powerful method for measuring the electrodynamic response of a material in the millimeter-and sub-millimeter spectral range (10 GHz to 1 THz), particularly in the case of high-field/frequency magnetic resonance spectroscopy. However, the application of such techniques within the limited space of a high-field magnet presents significant technical challenges. We describe a 7.62 mm × 7.62 mm (diameter × length) rotating cylindrical cavity which overcomes these problems. The cylinder is mounted transverse to the bore of the magnet, coupling is achieved through the side walls of the cavity, and the end plate is then rotated (by means of an external drive) instead of the body of the cavity itself. Therefore, rotation does not affect the cylindrical geometry, or the mechanical connections to the incoming waveguides. The TE011 mode frequency of the loaded cavity is 51.863 GHz, with the possibility to work on higher-order modes to frequencies of order 350 GHz. Neither the quality factor (∼ 22, 000 for the fundamental mode) or the coupling to the cavity are significantly affected for full 360 • of rotation. The rotation mechanism provides excellent angle resolution (< 0.1 • ), and is compact enough to enable measurements in the high-field (up to 45 T) magnets at the National High Magnetic Field Laboratory. Two-axis rotation capabilities are also possible in conjunction with split-pair magnet configurations. We present examples of angle-dependent measurements which illustrate the unique capabilities of this rotating cavity, including: high-field angle-dependent measurements of a novel form of cyclotron resonance in anisotropic organic conductors; and angle-dependent high-frequency single-crystal electron paramagnetic resonance (EPR) measurements in single-molecule magnets.