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arXiv:cond-mat/0502550 · Physical Review Letters

Emission of Polarization-Entangled Microwave Photons from a Pair of Quantum Dots

Clive Emary, Björn Trauzettel, C. W. J. Beenakker

Published Sep 12, 2005• 36 citations
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Abstract

We describe a mechanism for the production of polarization-entangled microwaves using intraband transitions in a pair of quantum dots. This proposal relies neither on spin-orbit coupling nor on control over electron-electron interactions. The quantum correlation of microwave polarizations is obtained from orbital degrees of freedom in an external magnetic field. We calculate the concurrence of emitted microwave photon pairs and show that a maximally entangled Bell pair is obtained in the limit of weak interdot coupling.

DOI
10.1103/physrevlett.95.127401

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Building on an earlier proposal for the production of polarization-entangled microwaves by means of intraband transitions in a pair of quantum dots, we show how this device can be used to transfer an unknown single-qubit state from electronic charge to photonic polarization degrees of freedom. No postselection is required, meaning that the quantum state transfer happens deterministically. Decoherence of the charge qubit causes a nonmonotonic decay of the fidelity of the transferred state with an increasing decoherence rate.
Semiconductor quantum dots are among the best sources of on-demand entangled photon pairs. The degree of entanglement, however, is generally limited by the fine structure splitting of exciton states. In this paper, we theoretically investigate the generation of polarization-entangled photon pairs under two-photon excitation and cavity-assisted two-photon emission, both in the weak and strong cavity coupling regimes. We demonstrate and clarify that cavity coupling together with an excitation pulse reduces the degree of entanglement in three different ways. First, in a strong coupling regime, the cavity introduces the unequal ac-Stark shift of horizontally and vertically polarized exciton states, which results in the effective splitting of exciton states. Second, it induces the cross-coupling between the exciton states even in the weak coupling regime, causing the creation of unfavorable two-photon states. Finally, higher excited states of the cavity modes also contribute to the reduction of entanglement. Therefore, in the setting considered here, cavity coupling, which is generally required for the efficient collection of emitted photons, degrades the entanglement both in weak and strong coupling regimes.
We demonstrate the emission of polarization-entangled photons from a single semiconductor quantum dot in the telecom C-band (1530 nm–1565 nm). To reach this telecommunication window, the well-established material system of InAs quantum dots embedded in InGaAs barriers is utilized with an additional insertion of an InGaAs metamorphic buffer to spectrally shift the system to the desired wavelengths. For the observation of polarization-entangled photon pairs, the biexciton-exciton cascade of a quantum dot displaying an intrinsically low fine-structure splitting is investigated by means of polarization-dependent cross-correlation measurements. A complete set of tomography measurements enables us to reconstruct the two-photon density matrix and therefore to calculate a corresponding fidelity f+ to the maximally entangled Bell state Ψ+ of 0.61 ± 0.07, a concurrence of 0.74 ± 0.11, a tangle of 0.55 ± 0.14, and a negativity of 0.63 ± 0.12, clearly proving the entanglement of the states. Finally, the development of the concurrence is studied in dependency of the post-selected time-gate of the emission events and the progression of the time-delay dependent fidelity to distinct Bell states is displayed.
The polarization entanglement of photons emitted by semiconductor quantum dots is unavoidably limited by the spin fluctuations of the host lattice nuclei. To overcome this limitation, we develop a theory of entangled photon pair generation by a symmetric colloidal quantum dot mediated by a triplet exciton. We derive general analytical expressions for the concurrence as a function of the hyperfine interaction strength and show that it is intrinsically higher than that in conventional doublet-exciton systems such as self-assembled quantum dots. The concurrence sensitively depends on the shape anisotropy and the strain applied to a nanocrystal. In particular, we uncover a possibility of completely suppressing the detrimental effect of the hyperfine interaction due to the interplay between nanocrystal anisotropy and electron-hole exchange interaction. We argue that this represents the ultimate limit for the generation of entangled photon pairs by semiconductor quantum dots.