We demonstrate high-quality, highly fluorescent, ZnSe colloidal nanocrystals (or quantum dots) that are doped with paramagnetic Mn 2+ impurities. We present luminescence, magnetic circular dichroism (MCD), and electron paramagnetic resonance (EPR) measurements to confirm that the Mn impurities are embedded inside the nanocrystal. Optical measurements show that by exciting the nanocrystal, efficient emission from Mn is obtained, with a quantum yield of 22% at 295 K and 75% below 50 K (relative to Stilbene 420). MCD spectra reveal an experimental Zeeman splitting in the first excited state that is large (28 meV at 2.5 T), depends on doping concentration, and saturates at modest fields. In the low field limit, the magnitude of the effective g factor is 430 times larger than in undoped nanocrystals. EPR experiments exhibit a six-line spectrum with a hyperfine splitting of 60.4 × 10 -4 cm -1 , consistent with Mn substituted at Zn sites in the cubic ZnSe lattice.Nanometer-scale semiconductor crystallites, also referred to as nanocrystals or quantum dots, have been extensively studied to explore their unique properties and potential applications. 1 Interesting behavior arises in these materials due to the confinement of optically excited electron-hole pairs by the crystallite boundary. However, while the basic explanation of this phenomenon, known as the quantum size effect, was provided early in the investigation of these materials, 2-4 a detailed understanding required the advent of high-quality colloidal nanocrystals, which were uniform in size, shape, crystallinity, and surface passivation. Once such materials became available, 5 tremendous progress was made in a variety of physical studies. Consequently, many of the properties of semiconductor nanocrystals are now understood in detail. 1 In addition, high-quality crystallites have led to more complicated nanocrystal-based structures, such as quantum-dot solids, 6 light-emitting devices, 7 and even photonic crystals. 8 These successes have encouraged researchers to go beyond pure nanocrystals and investigate particles that are intention-*

Electron spin echoes were performed on nitrogen-vacancy (N-V) centers in diamond using optical polarization and detection and 35 GHz microwave control. The experiments demonstrate an approach to quantum information in the solid state. A phase memory time of 3.6 s was measured, and coupling of the electronic spin to the 14 N nuclear spin was observed. Because of the favorable properties of the N-V center, interesting extensions of these single-qubit operations can be proposed.

A concept combining optics and microwave pulses with the negative charge-state of the nitrogenvacancy (NV --) center in diamond is demonstrated through experiments that are equivalent to single-qubit gates, and decoherence for this qubit is examined. The spin levels of the ground state provide the two-level system. Optical excitation provides polarization of these states. The polarized state is operated coherently by 35 GHz microwave pulses. The final state is read out through the photoluminescence intensity. Decoherence arises from different sources for different samples. For high-pressure, high-temperature synthetic diamonds, the high concentration of substitutional N limits the phase-memory to a few ms. In a single-crystal CVD diamond, the phase memory time is at least 32 ms at 100 K. 14 N is tightly coupled to the electronic spin and produces modulation of the electron-spin echo decay under certain conditions. A two-qubit gate is proposed using this nuclear spin. This demonstration provides a great deal of insight into quantum devices in the solid state with some possibility for real application. IntroductionThe nitrogen-vacancy center in its negative charge-state (NV --) in diamond is a defect/host system with remarkable properties. Its optical transition and spin were established through optical studies and EPR [1,2]. A long spin lifetime and more details of the energy levels were obtained through Optically Detected Magnetic Resonance (ODMR) and Raman heterodyne spectroscopy at zero and small magnetic fields [3,4]. These studies and others confirmed that the center has a strong optical transition and a long-lived spin with S ¼ 1 in the ground state. Many further experiments have been performed including room temperature ODMR of single NV --centers [5].The strong optical transition, long spin lifetimes, and stability of NV --in diamond have led naturally to concepts and demonstrations aimed at quantum information technology. One approach employs the 13 C nuclear spins as qubits with radio-frequency pulses as gates to implement a quantum computer [6]. A second approach makes use of cavity dark states for quantum computing [7]. Other works use the NV --center as a single-photon source for application in quantum communication [8,9].In this work, a concept is demonstrated that uses the electronic spin of the NV --in diamond as the qubit with optical polarization, microwave pulses as operators, and optical emission for readout. Gates involving a single qubit are demonstrated with ensembles of NV --centers. Previous work with one sample [10] has been extended to a set of samples, and some of the causes of decoherence can now be given. The involvement of

A time-resolved Faraday rotation magnetometer using externally triggered pulsed diode lasers is described. This device permits measurement of the dynamic properties of polarized electronic spins in semiconductors. A nonequilibrium spin polarization is created in the conduction band electrons of n-type GaAs using a circularly polarized laser pulse generated by a pulsed laser diode. A subsequent linearly polarized pulse from a second laser diode probes the time evolved electronic polarization via the Faraday effect. Since two different laser diodes are used for the pump and probe process, the dynamics of optically pumped spins can be directly observed at arbitrarily long pump-probe delays with a temporal resolution of 75 ps and a spatial resolution of 25 m. The signal-to-noise of the laser diodes is sufficient to achieve a sensitivity on the order of 3000 spins.Time-resolved Faraday rotation ͑TRFR͒ is a powerful technique for studying the dynamics of spin-polarized electrons in the conduction band of semiconductors. 1,2 With this technique, two laser pulses are used to, respectively, pump and probe electronic spins in a material. First, a circularly polarized pump pulse generates a nonequilibrium population of spin-polarized carriers in the conduction band of the sample. Subsequently, a linearly polarized probe pulse tuned to the absorption band edge is transmitted through the sample. Due to the circular birefringence induced by the spin-polarized conduction band, the polarization axis of the probe pulse rotates in proportion to the magnitude and direction of electronic spin polarization.Most TRFR magnetometers rely upon mode-locked Ti:sapphire lasers to provide the pump and probe pulses. 3 Recently, Bauer et al. developed a time-resolved magnetometer for studying ferromagnetic materials that used a single pulsed diode laser synchronized with magnetic field pulses. 4 We have extended this concept and developed a TRFR magnetometer that uses two externally triggered diode lasers to study magnetization dynamics in paramagnetic materials. Our technique has the advantages of reduced cost, smaller size, and improved flexibility.We examined spin dynamics in bulk n-type GaAs ͓2 ϫ 10 16 Si/ cm 3 , ͑100͒ surface͔. Similar materials have been extensively studied with TRFR and other optical orientation methods, 5,6 making it an excellent reference material to assess instrument performance. The 1 cmϫ 1 cm sample was bonded to a glass substrate with ultraviolet-cured epoxy and mechanically polished to a thickness of Ϸ75 m.The sample was mounted in a continuous flow liquid helium cryostat that cooled the sample down to 4.3 K ͑see Fig. 1͒. An external magnetic field was applied in the sample plane along the x direction using a conventional air-cooled C-frame electromagnet. The field inhomogeneity was less than 0.5% over the entire area of the sample and essentially zero over the optically sampled area. The applied field range was ±0.3 T. Due to the relative positions of the gaussmeter and sample, there is a 0.5% uncertainty in...

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