Similar coherence between heavy-hole and light-hole excitons has been reported in (33).These are examples of quantum coherence involving a nonradiative superposition of states, which leads to many novel phenomena such as the Hanle effect, dark states, lasing without inversion, electromagnetically induced transparency, and population trapping [see (33) and references therein]. However. unlike the QD system, atomic Zeeman coherence does not necessarily involve two differentiable electrons, in contrast to the experiment we present here.Finally, it is critical to note that while the simple discussion related to Fig. 1 shows the origin of the entanglement, the discussion is considerably over-simplified; it completely disregards the possibility that fast dephasing of the Zeeman coherence could lead to an unobservable effect, even if the single-exciton states themselves were long-lived. In the language of NMR, this is saying that a long T, and T2 associated with the single-exciton states does not guarantee a comparable T2 for the Zeeman coherence. The maptude of the decoherence rate, yij (l/T,), in Eq. 2 can be determined by comparing the relative strength of the coherent and incoherent contribution. The dephasing rate of the radiative coherence was already shown in (24) to be -20 ps and is comparable to the energy relaxation rate, r (lIT,). This measurement fiuther gives the decay rate of the Zeeman coherence as -20 ps, which again is similar to the energy relaxation rate and shows that the pure dephasing of the two-exciton coherence is not significant in QDs. By exchanging the spectral position of the pump and the probe (the polarization has to be changed accordingly, too), the relative time scale of incoherent spin relaxation can also be estimated. As expected, the Zeeman splitting has reduced this process to an unobservable level (>lo0 ps).In summary, we have inferred from our measurements entanglement of an excitonic system in single GaAs QDs and shown the importance of exciton-exciton Coulomb interaction for this observation. Our results are explained by a three-level model in a two-exciton basis. The next step, though more challenging, is to recover the same type of entanglement between coupled QDs [as proposed, for example, in (13, 14)]. This would allow for scaling the experiment to larger systems.

Electrodes constructed from single-crystal sections of rare earth fluorides respond to fluoride ion activity over more than five orders of magnitude and show a high selectivity for fluoride over other common anions. These electrodes can be used for either direct measuremnent of fluoride ion activity or detection of the end point in titration.

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A new potassium electrode, made from valinomycin, which uses commercially available electrode configurations, responds to potassium ion concentrations from 1 mole per liter to below 10(-6) mole per liter. It is highly selective for potassium ion over sodium ion and divalent ions and can be used for the direct determination of potassium ion in human serumn. No significant complexing of potassiium appears to occur in normal seruin.

melts, are susceptible to attack by metals such as lead and arsenic that can drastically reduce the life of the crucible. Either graphite crucibles (#A2260 Fusion Crucible, Ultra Carbon Corp., Bay City, Mich.) or vitreous carbon crucibles (#170 Crucible, Beckwith Carbon Corp., 16140 Raymer St., Van Nuys, Calif.) are well suited for this flux-fusion technique. Vitreous carbon crucibles are employed when reduction of sample components is a problem or when melts tend to wet graphite crucibles. Although vitreous carbon crucibles are rather expensive, 20 to 25 melts can be made in one crucible before it is destroyed by oxidation.Mixtures consisting of 90 wt % flux and 10 wt % sample give good characteristic X-ray yields for the lighter elements. The flux/sample ratio can be changed within reasonable limits to suit particular needs. All samples should be ground to pass at least -100 mesh.Preparation of Flux-Fusion Glasses. After mixing the flux and sample, the mixture is transferred to the proper crucible and fused according to the schedule depicted in Figure 1. An initial 5-minute fusion at 1100 °C will completely dissolve all but the most refractory samples, in which case the initial fusion time may be increased or the melt swirled in the crucible to promote solution of the sample. A mediumsized muffle furnace is satisfactory for either single or multiple fusions. After the initial fusion, a short carbon rod is placed on top of the melt to flatten the surface and an additional 5-minute heating period is begun (no carbon rod is used with vitreous carbon crucibles.) The crucible, melt, and flattening rod are then removed from the furnace intact and are cooled for 10 or 15 minutes to room temperature. The glass disks can be removed easily by lifting off the flattening rod and tapping the inverted crucible against a flat surface. Machining about a 2°t aper on graphite crucibles which

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