Quantum technologies have the potential to spur revolutions in computing, sensing, cryptography and beyond. Quantum computing and quantum communication are believed to be the future of information technology. Single-photon sources are important for current research in quantum information, cryptography, and the production of truly random numbers.
The possibility of totally secure communication through the application of quantum mechanical properties was introduced in 1984 by Bennett and Brassard in their paper, “Quantum Cryptography: Public Key Distribution and Coin Tossing”. As Bennett and Brassard outlined, quantum communication makes it impossible for an eavesdropper to intercept a message without detection by the desired parties. This phenomenon paves the way for a future of indecipherably secure communication, even with the advent of more powerful and even quantum computers. Quantum Key Distribution, or QKD, enables two remote parties, “Alice” and “Bob”, who are connected by a passive optical link to securely generate secret key material.
Quantum communication not only opens up opportunities for secure communication and the teleportation of quantum states but also is an important ingredient of the quantum internet, which enables the distribution of entanglement over long distances. Since such a quantum internet will be useful for distributed quantum computing, distributed cryptographic protocols and dramatically lowering communication complexity, its realization is an important long-term scientific and technological goal.
Thanks to the long coherence time of photons, photonic channels, for example, optical fibres, are often used for quantum communication. Nonetheless, owing to loss—which is the dominant noise for photons—the probability of successful transmission of a photon through an optical fibre decays exponentially. Consequently, the efficiency of this kind of quantum communication decreases exponentially with the communication distance, which is limited to hundreds of kilometres.
Owing to photon losses, robust quantum communication over long lossy channels requires quantum repeaters. It is widely believed that a necessary and highly demanding requirement for quantum repeaters is the existence of matter quantum memories.
Single-photon sources (SPSs) and single-photon detectors (SPDs) are key devices for enabling practical quantum key distributions (QKDs). Single-photon sources are used in quantum information and cryptography for encoding of information and communication. A
single photon can be detected at long distances away from the source and still contain its information. With the use of single photons, communication can be completely secured as any intercept would not be able to decrypt the message and the communicators would be able to see that the single-photon had been intercepted
Single-photon emission from semiconductor quantum dots (QDs) has been shown to be a pure and efficient non-classical light source with a high degree of indistinguishability. However, the total internal reflection (TIR) as a result of the high semiconductor-to-air refractive index contrast severely limits the single-photon extraction efficiency.
Quantum dots (QD) are very small semiconductor particles, that have a radius of a few nanometres (1 nm = a billionth of a metre). They are a central theme in nanotechnology. Many types of quantum dot will emit light of specific frequencies if electricity or light is applied to them, and these frequencies can be precisely tuned by changing the dots’ size, shape and material, giving rise to many applications.
Another crucial step in the development of practical quantum networks is the implementation of quantum repeater protocols, which enable long-distance quantum communication via optical fibre channels. These protocols rely on the use of highly indistinguishable, entangled photons, which require the use of single-mode fibres. Thus, an efficient on-chip single-mode fibre-coupled quantum light source is a key element in the realisation of a QD-based real-world quantum communication network.
Fibre-coupled standalone quantum dot device
In a new paper published in Light Science & Application, a team of scientists, led by, Professor Harald Giessen and Professor Peter Michler from the 4th Physics Institute and the Institut für Halbleiteroptik und Funktionelle Grenzflächen, University of Stuttgart, Germany, and co-workers have worked on enhancing the extraction efficiency of semiconductor QDs by optimising micrometre-sized solid-immersion lens (SIL) designs.
Two state-of-the-art technologies, i.e., low-temperature deterministic lithography and femtosecond 3D direct laser writing, are used in combination to deterministically fabricate micro-lenses on pre-selected QDs. Because of the high flexibility of 3D direct laser writing, various SIL designs, including hemispherical SILs (h-SILs), Weierstrass SILs (W-SILs), and total internal reflection SILs (TIR-SILs), can be produced and compared with respect to single-photon extraction enhancement. The experimentally obtained values are compared with analytical calculations, and the role of misalignment between SIL and QD as an error source is discussed in detail.
Furthermore, they highlight the implementation of an integrated single-mode fibre-coupled single-photon source based on 3D printed micro-optics. A 3D printed fibre chuck is used to precisely position an optical single-mode fibre onto a QD with a micro-lens printed on top. This fibre is equipped with another specifically designed 3D printed in-coupling lens to efficiently guide light from the TIR-SIL into the fibre core.
The main results presented in this paper are two-fold:
A reproducible method to enhance the collection efficiency of single QDs based on 3D printed micro-lenses is presented. For all lens geometries, an increase in the collection efficiency was confirmed. The simplest geometry, namely h-SIL, resulted in an intensity enhancement of approximately 2.1. A further increase of up to approximately 3.9 in collection efficiency is promised by the hyper hemispherical Weierstrass geometry. The highest values were achieved for the total internal reflection geometries which reliably provide a PL intensity ratio between 6 and 10.
A standalone fibre-coupled standalone quantum dot device was realised. The validation of the approach for fibre in-coupling, that is the use of a QD provided with a TIR-SIL and a fibre with an additional focusing lens, was performed, employing a setup capable of precisely aligning the fibre with respect to the emitter. A value of up to 26?±?5% was shown, opening the route to a stable stand-alone, fibre-coupled device.
In the future, this technology can be combined with a QD single-photon source based on circular Bragg gratings, NV centres, defects, and a variety of other quantum emitters. In addition, a highly efficient combination with single quantum detectors should be feasible.
Quantum dots as potential sources of strongly entangled photons
The generation and long-haul transmission of highly entangled photon pairs is a cornerstone of emerging photonic quantum technologies with key applications such as quantum key distribution and distributed quantum computing. However, a natural limit for the maximum transmission distance is inevitably set by attenuation in the medium. A network of quantum repeaters containing multiple sources of entangled photons would allow overcoming this limit.
The success or failure of quantum light sources in advanced commercial photonic quantum-network applications is strongly dependent on integrability of these sources with current infrastructure and technology. Regarding networks, operation at telecom wavelength is as essential as wavelength tuneability, required for interfacing and multiplexing with other sources of classical or quantum light over the same optical fibre. In terms of quantum light emitters, scalable manufacturing techniques for their production as well as compliance with low voltage driving electronics for safe and long-term reliable operation in remote non-laboratory environments are most desirable.
Shared entanglement between distant network users is an important resource for quantum-network applications going beyond conventional quantum key distribution (QKD)1 that makes use of weak coherent laser pulses. Greater robustness to photon number splitting attacks an increase of communication distances and the prospect to link remote quantum processors are some of the immediate benefits.
As currently employed sources are mostly based on spontaneous processes, intrinsically limiting efficiencies, further improvement is expected from sub-Poissonian photon-pair sources such as semiconductor quantum dots (QD), with the prospect for deterministic operation. Such sources can also benefit highest level applications for scalable network architectures such as quantum relays and repeaters based on teleportation and entanglement swapping protocols, posing further constraints on photon indistinguishability.
Quantum light sources based on III–V compound semiconductor QDs embedded in a positive-intrinsic-negative (p-i-n) diode are considered a promising approach in terms of scalable manufacturability as they are sharing the same material platform as standard laser diodes. These devices generate single entangled pairs of photons on the so-called biexciton cascade after electrical injection of carriers, which is why they are also referred to as entangled light emitting diodes (ELED).
Another approach for carrier injection is via optical excitation where one mainly distinguishes between non-resonant excitation above the bandgap of the surrounding semiconductor matrix and resonant excitation of the biexciton state. The latter has recently become very popular, achieving good photon indistinguishability with high purity and most importantly, the potential for fully deterministic operation. Recent progress in the field culminated in first demonstrations of entanglement swapping from this kind of source, an important step towards scalable quantum networks.
However, these experiments require a sophisticated pulsed laser system combined with careful suppression of exciting laser light, making them at the moment not the first choice when it comes to field deployment of QD photon-pair sources in non-laboratory environments. In contrast, non-resonant excitation schemes are much simpler to implement and integrate, making them a welcome testbed for first steps into network integration of quantum light emitters. Of course, this comes at the expense of lower efficiency and photon purities, but with performances still good enough for experiments such as quantum teleportation with electrically and optically injected sources.
For quantum communication applications, careful engineering of material compositions and wafer growth conditions have enabled the fabrication of QDs emitting in the standard telecom wavelength bands, recently enabling for the first time the transmission of photons from a single quantum emitter over a deployed standard network fibre
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