A metastable state m subjected to a couple of pulses transits to an excited level α of an atom with probabilities p1 and p2, respectively. One photon is emitted in the transition from the level α to a lower atomic level η. A second photon is emitted in the transition from the level η to a lowest atomic level β. The first photon must be emitted before than the second one so they are both time ordered (time-bin) and entangled. It is calculated that the concurrence of the two produced time-bin photons as a function of p1, p2 and the branching ratios of the two transitions α→η and (η→β). It is found that the two produced photons are maximally entangled if the branching ratios are greater than 80%. It is also found that certain values of p1, p2 and the branching ratios preclude entanglement between the two produced photons. Intervals of values for p1 and p2 where the concurrence is large enough and then two-qubit quantum information processing protocols can be optimally implemented are found.
A metastable state m subjected to a couple of pulses transits to an excited level α of an atom with probabilities p1 and p2, respectively. One photon is emitted in the transition from the level α to a lower atomic level η. A second photon is emitted in the transition from the level η to a lowest atomic level β. The first photon must be emitted before than the second one so they are both time ordered (time-bin) and entangled. It is calculated that the concurrence of the two produced time-bin photons as a function of p1, p2 and the branching ratios of the two transitions α→η and (η→β). It is found that the two produced photons are maximally entangled if the branching ratios are greater than 80%. It is also found that certain values of p1, p2 and the branching ratios preclude entanglement between the two produced photons. Intervals of values for p1 and p2 where the concurrence is large enough and then two-qubit quantum information processing protocols can be optimally implemented are found.
We report a study on coherence of excitation of single quantum dots. We address the coherent excitation of biexcitons, the process that is indispensable for deterministic photon pair generation in quantum dots. Based on theoretical modeling we optimized the duration of the excitation pulse in our experiment to minimize the laser-induced dephasing and increase the biexciton-to-background single-exciton occupation probability. An additional effect of this approach is a high degree of time-bin entanglement with a concurrence of up to 0.78(6) and a 0.88(3) overlap with a maximally entangled state.
When a single emitter is excited by two phase-coherent pulses with a time delay, each of the pulses can lead to the emission of a photon pair, thus creating a "time-bin-entangled" state. Double pair emission can be avoided by initially preparing the emitter in a metastable state. We show how photons from separate emissions can be made indistinguishable, permitting their use for multiphoton interference. Possible realizations are discussed. The method might also allow the direct creation of n-photon entangled states (n > 2).
The problem of the relationship between entanglement and two-qubit systems in which it is embedded is central to the quantum information theory. This paper suggests that the concurrence hierarchy as an entanglement measure provides an alternative view of how to think about this problem. We consider mixed states of two qubits and obtain an exact solution of the time-dependent master equation that describes the evolution of two two-level qubits (or atoms) within a perfect cavity for the case of multiphoton transition. We consider the situation for which the field may start from a binomial state. Employing this solution, the significant features of the entanglement when a second qubit is weakly coupled to the field and becomes entangled with the first qubit, is investigated. We also describe the response of the atomic system as it varies between the Rabi oscillations and the collapse–revival mode and investigate the atomic inversion and the Q -function. We identify and numerically demonstrate the region of parameters where significantly large entanglement can be obtained. Most interestingly, it is shown that features of the entanglement are influenced significantly when the multi-photon process is involved. Finally, we obtain illustrative examples of some novel aspects of this system and show how the off-resonant case can sensitize entanglement to the role of initial state setting.
In this paper we study the production of entanglement between two atoms which are far from each other. We consider a system including eight two-level atoms (1, 2, ⋯, 8) such that any atom with its adjacent atom is in an atomic Bell state, so that we have four separate pairs of maximally entangled states where . Our purpose is to produce entanglement between the atomic pair (1, 8), while these two distant atoms have no interaction. By performing the interaction between adjacent non-entangled atomic pairs (2, 3) as well as (6, 7), each pair with a two-mode quantized field, the entanglement is produced between atoms (1, 4) and (5, 8), respectively. Finally, by applying an appropriate Bell state measurement (BSM) on atoms (4, 5) or performing an interaction between them with a single-mode field (quantum electrodynamic: QED method), the qubit pair (1, 8) becomes entangled and so the quantum repeater is successfully achieved. This swapped entanglement is then quantified via concurrence measure and the effects of coupling coefficients and detuning on the concurrence and success probability are numerically investigated. The maxima of concurrence and success probability and the corresponding time periods have been decreased by increasing the detuning in asymmetric condition in the BSM method. Also, the effects of detuning, initial interaction time and coupling coefficient on the produced entanglement by the QED method are considered. Increasing (decreasing) of the detuning (interaction time) has a destructive effect on the swapped entanglement in asymmetric condition.
We found that the most feasible naturally produced quantum entanglement between two distant particles from astrophysical regions is the two-photon spontaneous transition of the hydrogen 2 2 S 1/2 metastable level, which is known to be one of the components of the continuous spectra of ionized regions.We calculate the two-photon emission rate the Orion Nebula, two nearby planetary nebulae IC 2149 and NGC 7293, and the Solar Corona; whose production of entangled pairs per second is 5.80 × 10 48 , 9.39 × 10 45 , 9.77 × 10 44 , and 1.46 × 10 16 respectively.The distribution of the propagation directions of both emitted photons does not vanish at any angle; therefore it is possible to observe the entangled pair at an angles θ ≈ 0 • .Because the number of two-photon coincidences goes as the fourth power of the ratio between the detector size and the distance from the astrophysical object, coincidences are scarce; for its detection we required receivers with a size much larger than those currently available.
We have measured the lifetime of the 3P1 metastable state of Mg and obtained a value of 2.2 \ifmmode\pm\else\textpm\fi{} 0.2 msec. The level was excited with a dye laser tuned to the intercombination line ($3s3p^{3}P_{1}\ensuremath{\leftrightarrow}3{s}^{2}^{1}S_{0}$). In addition to the 3P1 state, the 3P2 and 3P0 levels were populated by mixing collisions owing to the presence of an inert buffer gas. The populations of all three metastable levels were monitored as a function of time by observing the absorption of Mg resonance radiation ($^{3}P_{0,1,2}\ensuremath{\rightarrow}^{3}S_{1}$) out of each level. The mixing time was short compared to the 3P1 lifetime, so that the populations of the three metastable levels were in thermal equilibrium with each other. This produced an apparent lengthening of the 3P1 lifetime, which would introduce a large systematic error if the degree of mixing were not known. Cenventionally, one observes the fluorescence from the excited levels to obtain their lifetimes. Because the lifetimes of the 3P2 and 3P0 states are too long, fluorescence from these states cannot be observed, and the observation of fluorescence from the 3P1 state alone provides insufficient information to determine its lifetime.