Abstract Long-distance quantum communication and computation is based on the exchange of information via photons as flying qubits. In all foreseen implementations, from quantum relays and networks to remote quantum computing, the photons must be able to propagate over long distances, in silica fibers, with limited absorption and wave packet dispersion. When propagating into silica fibers, photons in the so-called telecom C-band (i.e. wavelength around 1550 nm) will experience the absolute minimum of absorption, together with a limited photon wave packet dispersion. This implies that losses of information will be minimized and additionally, the capability of the photons to quantum interfere (the so-called Hong–Ou–Mandel effect) will be negligibly affected. This motivated the search for efficient non-classical light sources in this wavelength range. In the present review, we discuss the approaches followed to red-shift the emission wavelength from the near-infrared (NIR) to telecom wavelengths. In particular, the use of metamorphic buffers (MMBs) enabled the use of highly developed InAs/GaAs systems, engineering the dots to emit single and entangled photons in the telecom C-band. The main advantage of this approach is set by the choice of the material system: being the same as state-of-the-art structures emitting in the NIR range, it opens the possibility of achieving comparable performances even at telecommunication wavelengths. Here we will discuss the current state-of-the-art in the generation of non-classical photons, comparing the properties and performances of MMB-based results with other competing quantum dot (QD) platforms. We report in particular that very low fine-structure splitting can be observed with conservative values, on average well below 10 μ eV. This allowed for the observation of post-selected entanglement fidelity higher than 0.6. Additionally, on-demand single-photon emission with g ( 0 ) 2 = 0.184 ± 0.002 was observed in addition to a very low emitter density of ≈ 10 7 cm − 2 and a decay time of τ ≈ 1.2 ns comparable to standard InAs QDs emitting in the NIR range. A final highlight on approaches to further improve the current device performances will also be reported.
Abstract Long-distance quantum communication and computation is based on the exchange of information via photons as flying qubits. In all foreseen implementations, from quantum relays and networks to remote quantum computing, the photons must be able to propagate over long distances, in silica fibers, with limited absorption and wave packet dispersion. When propagating into silica fibers, photons in the so-called telecom C-band (i.e. wavelength around 1550 nm) will experience the absolute minimum of absorption, together with a limited photon wave packet dispersion. This implies that losses of information will be minimized and additionally, the capability of the photons to quantum interfere (the so-called Hong–Ou–Mandel effect) will be negligibly affected. This motivated the search for efficient non-classical light sources in this wavelength range. In the present review, we discuss the approaches followed to red-shift the emission wavelength from the near-infrared (NIR) to telecom wavelengths. In particular, the use of metamorphic buffers (MMBs) enabled the use of highly developed InAs/GaAs systems, engineering the dots to emit single and entangled photons in the telecom C-band. The main advantage of this approach is set by the choice of the material system: being the same as state-of-the-art structures emitting in the NIR range, it opens the possibility of achieving comparable performances even at telecommunication wavelengths. Here we will discuss the current state-of-the-art in the generation of non-classical photons, comparing the properties and performances of MMB-based results with other competing quantum dot (QD) platforms. We report in particular that very low fine-structure splitting can be observed with conservative values, on average well below 10 μ eV. This allowed for the observation of post-selected entanglement fidelity higher than 0.6. Additionally, on-demand single-photon emission with g ( 0 ) 2 = 0.184 ± 0.002 was observed in addition to a very low emitter density of ≈ 10 7 cm − 2 and a decay time of τ ≈ 1.2 ns comparable to standard InAs QDs emitting in the NIR range. A final highlight on approaches to further improve the current device performances will also be reported.
Semiconductor quantum dots (QDs) are well established as sub-Poissonian sources of entangled photon pairs. To improve the utility of a QD light source, it would be advantageous to extend their emission further into the near infrared, into the low absorption wavelength windows utilised in long-haul optical telecommunication. Initial experiments succeeded in interfering O-band (1260—1360 nm) photons from an InAs/GaAs QD with dissimilar photons from a laser, an important mechanism for quantum teleportation. Interference visibilities as high as 60 ± 6 % were recorded, surpassing the 50 % threshold imposed by classical electrodynamics. Later, polarisation-entanglement of a similar QD was observed, with pairs of telecom-wavelength photons from the radiative cascade of the biexciton state exhibiting fidelities of 92.0 ± 0.2 % to the Bell state. Subsequently, an O-band telecom-wavelength quantum relay was realised. Again using an InAs/GaAs QD device, this represents the first implementation of a sub-Poissonian telecom-wavelength quantum relay, to the best knowledge of the author. The relay proved capable of implementing the famous four-state BB84 protocol, with a mean teleportation fidelity as high as 94.5 ± 2.2 %, which would contribute 0.385 secure bits per teleported qubit. After characterisation by way of quantum process tomography, the performance of the relay was also evaluated to be capable of implementing a six-state QKD protocol. In an effort to further extend the emitted light from a QD into the telecom C-band (1530—1565 nm), alternative material systems were investigated. InAs QDs on a substrate of InP were shown to emit much more readily in the fibre-telecom O- and C-bands than their InAs/GaAs counterparts, largely due to the reduced lattice mismatch between the QD and substrate for InAs/InP (~3 %) compared to InAs/GaAs (~7 %). Additionally, to minimize the fine structure splitting (FSS) of the exciton level, which deteriorates the observed polarisation-entanglement, a new mode of dot growth was investigated. Known as droplet epitaxy (D-E), QDs grown in this mode showed a fourfold reduction in the FSS compared to dots grown in the Stranski-Krastanow mode. This improvement would allow observation of polarisation-entanglement in the telecom C-band. In subsequent work performed by colleagues at the Toshiba Cambridge Research Labs, these D-E QDs were embedded in a p-i-n doped optical cavity, processed with electrical contacts, and found to emit entangled pairs of photons under electrical excitation. The work of this thesis provides considerable technological advances to the field of entangled-light sources, that in the near future may allow for deterministic quantum repeaters operating at megahertz rates, and in the further future could facilitate the distribution of coherent multipartite states across a distributed quantum network.
High Resolution Image Download MS PowerPoint Slide Entangled photons are an integral part in quantum optics experiments and a key resource in quantum imaging, quantum communication, and photonic quantum information processing. Making this resource available on-demand has been an ongoing scientific challenge with enormous progress in recent years. Of particular interest is the potential to transmit quantum information over long distances, making photons the only reliable flying qubit. Entangled photons at the telecom C-band could be directly launched into single-mode optical fibers, enabling worldwide quantum communication via existing telecommunication infrastructure. However, the on-demand generation of entangled photons at this desired wavelength window has been elusive. Here, we show a photon pair generation efficiency of 69.9 ± 3.6% in the telecom C-band by an InAs/GaAs semiconductor quantum dot on a metamorphic buffer layer. Using a robust phonon-assisted two-photon excitation scheme we measure a maximum concurrence of 91.4 ± 3.8% and a peak fidelity to the Φ + state of 95.2 ± 1.1%, verifying on-demand generation of strongly entangled photon pairs and marking an important milestone for interfacing quantum light sources with our classical fiber networks.
Quantum dots reaching the best performances as non-classical light emitters are based on the GaAs platform. The main drawback is their natural emission wavelength, generally around 800-900 nm, so not suitable for fibre-based experiments. In this talk we will discuss two approaches to reach the telecom C-band, within the maximum transmission window of silica fibers. On the one hand, the use of quantum frequency conversion will enable the use of non-linear processes to transfer the high quality NIR photons to any other wavelength, here 1550 nm [1]. On the other hand, growth engineering can be utilized to modify size and strain conditions of the quantum dot, enabling the emission at telecom O- and C-band with the same dot stoichiometry as for the NIR ones [2]. [1] J. H. Weber, et al., Nat. Nanotechnol. 14, 23 (2019). [2] C. Nawrath et al., Appl. Phys. Lett. 115, 023103 (2019).
Non-classical photon sources made of semiconductor quantum dots (QDs) emitting in the telecommunication C-band are crucial components for low-loss, long-distance photonic quantum communication networks. Here we designed and fabricated strain--free In0.7Ga0.3As/In0.7Al0.3As QDs grown on GaAs(111)A substrates working as single-photon emitters in the 1550 nm window. The QDs were grown via local droplet etching method in a molecular beam epitaxy environment, employing a thin In0.7Al0.3As metamorphic buffer layer with the same lattice constant of the QD material, thus allowing for a completely strain--free self-assembly of the QDs. The QDs exhibit a C3v symmetry with a ground state emission in the 1400--1600 nm range. The exciton lifetimes of ≈ 1.3--1.9 ns and linewidths as low as ≈ 300 μeV show the good quality of the fabricated QDs. Second-order autocorrelation measurements under pulsed excitation confirmed the single-photon purity of the emitters, yielding a g(2)(0) value of 0.141±0.027
Semiconductor quantum dots emitting in the telecommunication O- and C-bands are promising candidates as single-photon sources for fiber-based quantum communication. In this work, we show how coherent quasi-resonant and phonon-assisted excitation help reveal the intrinsic optical properties of InAs quantum dots embedded in relaxed InGaAs matrices on top of metamorphic InGaAs layers for achieving emission in the telecom bands. By systematically comparing excitation schemes, we separate excitation-induced effects from intrinsic emitter properties. This allows us to observe linewidths as narrow as 21 μeV, the disappearance of slow decay channels, and single-photon purity reaching second-order autocorrelation values in the order of 10−2. Photoluminescence excitation reveals higher excited states, enabling coherent control of trion transitions and the observation of Rabi-like oscillations in both telecom bands. These results confirm the promising potential of these quantum dots for applications in quantum technologies.