Coherence and indistinguishability of highly pure single photons from non-resonantly and resonantly excited telecom C-band quantum dots

Abstract

In the present work, the effect of resonant pumping schemes in improving the photon coherence is investigated on InAs/InGaAs/GaAs quantum dots emitting in the telecom C-band. The linewidths of transitions of multiple exemplary quantum dots are determined under above-band pumping and resonance fluorescence via Fourier-transform spectroscopy and resonance scans, respectively. The average linewidth is reduced from 9.74 GHz in above-band excitation to 3.50 GHz in resonance fluorescence underlining its superior coherence properties. Furthermore, the feasibility of coherent state preparation with a fidelity of 49.2 % is demonstrated, constituting a step towards on-demand generation of coherent, single C-band photons from quantum dots. Finally, two-photon excitation of the biexciton is investigated as a resonant pumping scheme. A deconvoluted single-photon purity value of gHBT (0) = 0.072 ± 0.104 and a degree of indistinguishability of V HOM = 0.894 ± 0.109 are determined for the biexciton transition. This represents an important step towards fulfilling the prerequisites for quantum communication applications like quantum repeater schemes at telecom wavelength.Over the past two decades, semiconductor quantum dots (QDs) have received unceasing attention from researchers in the field of quantum optics due to their outstanding properties in terms of non-classical light emission 1-5 , i.e. bright singlephoton emission, entanglement fidelity, indistinguishability and the simultaneous combination of the aforementioned 4,5 . This designates them as promising candidates for applications like quantum computing and quantum communication 6 . The best performances are currently achieved with GaAs-based dots emitting in the near infrared 7 . However, especially regarding quantum communication schemes, an emission wavelength around 1550 nm (Telecom C-band) is much soughtafter both for satellite-based quantum communication due to an atmospheric transmission window and the possibility to perform it in broad daylight 8 , as well as for its fiber-based counterpart due to the global absorption minimum and low dispersion of standard glass fibers forming the existing global fiber network 9 . However, to extend the range of quantum communication applications such as quantum key distribution 10,11 , quantum relays 12,13 or quantum repeaters 14-17 are needed. The ideal light source for such applications combines bright single-photon and entangled-photon pair emission with a high degree of indistinguishability at 1550 nm.The emission of single and entangled photons in the telecom C-band has been demonstrated in two material systems, namely InAs/InP 18,19 and InAs/InGaAs/GaAs 20,21 . The last requirement, i.e. the indistinguishability of photons, is of major importance because it is necessary for two-photon interference (TPI), enabling linear-optic Bell state measurements and, therefore, entanglement swapping 22,23 in quantum repeater schemes. An experimental demonstration at this wavelength has been elusive in both material systems up to now.Ho...

Coherence in single photon emission from droplet epitaxy and Stranski–Krastanov quantum dots in the telecom C-band

“…While the raw measured two-photon intensities give a maximum visibility of 71.3 ± 1.6 %, deconvolving the fit to the data with the detector response increases the visibility to 98.6 ± 1.6. This value compares well with the recently reported indistinguishability measured on GaAs-based C-band quantum QDs, even though those results were achieved under resonant excitation 16 . In conclusion, we report a comparison between the coherence properties of SK and DE QDs, where the DE growth mode is shown to provide a superior material system and a calmer environment to the QD spins.…”

“…Most promisingly, these are InAs QDs relying on either strain-relaxing techniques in GaAs-based host materials 9 , or using InP substrates with a smaller lattice mismatch with respect to the QD material 10 . Both approaches can provide single and entangled photons [11][12][13][14][15] , however they differ in the coherence of emission, where the InP material system has proven superior so far 16,17 and photons with coherence times up to 0.3 of the Fourier limit have been observed 17 even under non-resonant excitation. Long coherence times leading to highly indisdinguishable photons are crucial for the efficient quantum communications protocols used in quantum networks, and it is important to understand the sources of noise limiting the performance of the QDs.…”

High‐Purity Triggered Single‐Photon Emission from Symmetric Single InAs/InP Quantum Dots around the Telecom C‐Band Window

“…In this approach, the growth procedure is demanding and causes severe technological complications deteriorating the optical quality of the material . Despite that, a single photon emission in the telecom C‐band range from InAs QDs on InGaAs metamorphic buffer layers on a GaAs substrate has been recently presented showing high fidelity of entanglement, precise wavelength tunability, and coherent control of a QD state combined with single‐photon indistinguishability . Although these dots are under impressive development, they still struggle with some inherent difficulties, including non‐zero fine structure splitting being a prerequisite for high entanglement fidelity of photons originating from biexciton–exciton cascade relevant for quantum communication schemes or only partial suppression of multiphoton emission events under non‐resonant pulsed excitation .…”

“…Although these dots are under impressive development, they still struggle with some inherent difficulties, including non‐zero fine structure splitting being a prerequisite for high entanglement fidelity of photons originating from biexciton–exciton cascade relevant for quantum communication schemes or only partial suppression of multiphoton emission events under non‐resonant pulsed excitation . Although selective excitation schemes, resonant or quasi‐resonant, can help in accessing the target QD only and suppressing the dephasing processes, and therefore greatly reduce the background emission and enhance the purity and coherence properties of single photons, they remain more challenging to realize and require tunable laser sources. For applications, where the purity of single photons is crucial, but high coherence is not indispensable, for example, quantum key distribution protocols (BB84), the non‐resonant excitation is much easier to realize and more flexible.…”

InP-Substrate-Based Quantum Dashes on a DBR as Single-Photon Emitters at the Third Telecommunication Window

“…At least several approaches have been used, e.g., utilizing the strain-reducing layer [35][36][37][38], growing on a metamorphic buffer layer [34,39,40], multistacking of QDs [41], or growth on a seeding layer [42]. For GaAs-based QD structures, single-photon emission at the third telecommunication window has been demonstrated only using a special metamorphic buffer layer grown by Metalorganic chemical vapor deposition (MOCVD) [43], with reported emission of entangled photons [44] and emission of indistinguishable photons [45], also with the possibility of precise piezo-tuning [46] and generation of single-photons on demand [47]. However, the growth process of such QDs is very demanding and prone to technological complications, deteriorating the optical quality of the final material [48], which is probably the reason for still lacking results on high-quality and high-brightness photonic structures out of that material system at 1.55 µm [11,40].…”