An on-demand source of bright entangled photon pairs is desirable for quantum key distribution (QKD) and quantum repeaters. The leading candidate to generate entangled photon pairs is based on spontaneous parametric down-conversion (SPDC) in a non-linear crystal. However, a fundamental trade-off exists between entanglement fidelity and efficiency in SPDC sources due to multiphoton emission at high brightness, which limits the pair extraction efficiency to 0.1% when operating at near-unity fidelity. Quantum dots in photonic nanostructures can in principle overcome this trade-off; however, the quantum dots that have achieved an en- entanglement fidelity on par with an SPDC source (99%) have poor pair extraction efficiency of 0.01%. Here, we show a measured peak concurrence of 95.3% ± 0.5% and pair extraction efficiency of 0.65% from an InAsP quantum dot in an InP photonic nanowire waveguide. Additionally, we show that an oscillating two-photon Bell state generated by a semiconductor quantum dot can establish a secure key for peer-to-peer QKD while using all generated photon pairs. Using our time-resolved QKD scheme alleviates the need to remove the exciton fine structure splitting.
Highly entangled photon sources play a crucial role in advancing the capabilities of quantum networks. In this context, we introduce an advanced scheme aimed at improving entanglement of photons emitted from quantum dots based on the framework proposed by Fognini et al. (2018). We propose a setup with reduced physical footprint which employs one electro-optic modulator strategically to enhance entanglement, mitigating the detrimental effects of fine structure splitting (FSS) observed in quantum dots that contribute to the degradation of entanglement.
In this paper, we review the research and development of the fractal superconducting nanowire single-photon detectors (SNSPDs), including our demonstrations of high-performance devices and systems with over 80% system detection efficiency, negligibly low residual polarization sensitivity, and low timing jitter. Using the fractal SNSPDs, we demonstrate full-Stokes polarimetric imaging LiDAR.
Large-scale quantum photonics requires the integration of several elements on the same chip, including quantum emitters and memories, active photonics, and single-photon detectors. In this talk, I will report on i) our recent work integrating superconducting nanowire single-photon detectors (SNSPD) with mechanically reconfigurable integrated photonics, and ii) our recently developed method for integration of SNSPDs onto arbitrary photonic substrates.
A primary constraint in the major photonic integration platform of Silica-on-Silicon, especially when combined with fabrication approaches like Direct Laser Writing is the optical waveguides' low refractive index contrast, leading thus to limitations for efficient coupling with currently available state-of-the-art single photon emitters such as semiconductor nanowires with quantum dots (NWQD). We propose and demonstrate a novel approach to drastically enhance the light coupling between silica based Laser-written channel waveguides and NWQDs, by incorporating an optical microsphere in their intermediate space. It is demonstrated that the induced photonic nanojet action of a suitably designed microsphere illuminated by the NWQD, excites efficiently the channel waveguide's modes and can enable light coupling to a degree even above 50%. The proposed method is reasonably robust to imperfections and misalignments and could be implemented by current state-of-the-art micro/nano patterning techniques. It is anticipated that the practical implementation of the method will allow the integration of multiple quantum emitters in silica based hybrid integrated circuits thus enabling their scalability towards for quantum computing and sensing applications.
Photonic integrated quantum circuits could fully unlock quantum systems' potential. However,monololithic platforms face several challenges. We explore hybrid quantum photonic integration, combining different photonic materials to mitigate monolithic approaches disadvantages . Our work introduces hybrid circuits integrating different quantum sources and materials.
We present a technique integrating nanowire single photon sources with silicon nitride waveguides, enabling circuit reconfiguration and emission wavelength control using heat, and strain tuning which is compatible with superconducting single photon detectors . We realize a full quantum transceiver on chip using on demand single photon sources and SNSPDS, in additio to dispersion engineering of superconducting transmission lines for SNSPDs multi-pixel integration.
We discuss progress towards room-temperature quantum integrated photonics using hexagonal boron nitride quantum emitters.
Finally, we delve into scalable generation and quantum state reconstruction of W-states and construction using phase retrieval algorithms, and ongoing research on topological photonics, and photonic lattices.
Quantum dots (QDs) embedded inside indium-phosphide (InP) nanowires have the potential to be bright, on-demand sources of perfect polarization-entangled photon pairs fabricated with near-unity yields. However, to date very high degrees of entanglement have not yet been measured from such devices. By performing quantum state tomography with state-of-the-art superconducting nanowire single-photon detectors (SNSPDs) and two-photon resonant excitation of the QD, we show that these sources are indeed capable of producing near-unity entangled photon pairs. We measure a raw peak fidelity of 97.5% +/- 0.8% and a lifetime-weighted fidelity of 0.94% +/- 0.04%. These results conclusively demonstrate that the majority of the degradation from unity-measured entanglement fidelity in earlier studies was not due to spin-spin dephasing from the large 9/2 nuclear spin of indium. These results solidify InP nanowire QDs as a promising platform for future quantum photonics applications.
Entangled photons are an important resource for quantum optics. Quantum dots are a source of on-demand and highly entangled photon pairs at a high repetition rate. However, fine structure splitting (FSS) in the biexciton-exciton cascade causes the photons to be emitted in a time-dependant state instead of an ideal Bell state. Current techniques to remove the FSS include applying a strain, electric, or magnetic field and require post-processing of the quantum dots which reduces device yield. We use a novel all-optical approach implemented by emulating a fast-rotating half-wave plate in a Lithium Niobate waveguide using a electro-optic modulator. This method allows us to frequency shift single photons and produce a time-independant entangled photon.
We demonstrate proof-of-principle carbon dioxide (CO2) sensing at 2.05μm wavelength using a commercial infrared superconducting nanowire single photon detector (SNSPD). We verify the absorption peaks within the tuning range of the illumination source and perform a differential measurement within one of these peaks to detect a changing concentration of carbon dioxide within a gas cell. We demonstrate that SNSPDs are a promising enabling technology for short wave and mid wave infrared gas remote sensing.
We develop quantum devices for the generation, manipulation and detection of light at the single photon level. Quantum dot devices allow for the generation of single and entangled photons at various frequencies including telecom wavelengths. We also develop single photon detectors with high efficiency based on NbTIN superconducting nanostructures, low noise and high time resolution to enable a range of applications including quantum communication, quantum sensing and integrated photonics. To allow for complex systems, integrated quantum optics circuits where we combine quantum sources and superconducting detectors are under development.
Superconducting single-photon detectors (SSPDs) have developed into a mature device technology and excel due outstanding performance metrics, in particular high detection efficiency combined with high time resolution and low dark count rate for a wide wavelength range from the visible to the mid-infrared. In addition to commercially available systems with devices coupled to optical fibers, SSPDs can be integrated with photonic circuits using scalable nanofabrication technologies.
Here, we will present recent progress on SSPDs based on NbTiN thin films and their integration on different photonic material platforms. Our process for NbTiN growth at room temperature will be described, using magnetron reactive co-sputtering to achieve high-quality superconducting layers down to thicknesses of few nanometres. Optimized SSPD devices are realized by tuning the superconducting properties of NbTiN thin films, adjusting the material composition and nanocrystalline structure. The realization of different types of detectors and geometries will be shown, including nanofabrication techniques for achieving fully suspended nanowire structures. Furthermore, we will discuss challenges and prospects for scaling-up SSPD device technology as well as detector systems. Multiplexing schemes such as dispersion engineering of superconducting transmission lines will be highlighted as powerful approach to address multiple detectors and reduce the number of required feedthroughs and electrical lines in the cryostat. Eventually, exemplary applications of SSPDs for photon counting in quantum optics and light detection and ranging (LIDAR) will be outlined.
All envisaged practical implementations of cryogenic processors, including quantum computers and classical processors based on single flux quantum (SFQ) signals, require massive data transfer from and to classical high performance computers (HPCs). Cryogenic computing has recently become a very hot topic, including superconducting quantum computers (QCs), and classical processors based on single flux quantum (SFQ) signals. All envisaged practical implementations of cryogenic processors require massive data transfer from and to classical HPCs. The project aCryComm aims to develop building blocks for cryogenic photonics interconnects and eventually enable this challenging data transfer. The long-term goal is the development of an open-access platform to integrate classical optical interfaces based on low-loss silicon photonics, plasmonics, and nano light sources together with superconducting photonic and electronic devices, including SFQ-based co-processors for HPCs and for QCs.
Fiber-based quantum networks require on-demand sources of entangled photons in the telecom C-band for long distance information transfer. Historically, the field of in-fiber entanglement distribution has been dominated by photons provided via spontaneous processes. In recent years, semiconductor quantum dots have emerged as strong competitors in terms of generating single and entangled photons due to their promise of deterministic qubit generation in the NIR wavelength region. Here, we show the on-demand generation of polarization entangled photons in the telecom C-band based on InAs/GaAs quantum dots grown via metal-organic vapor-phase epitaxy. By employing a robust phonon-assisted two-photon excitation scheme, we are able to generate pairs of entangled photons with a concurrence of 91.4 ± 3.8 % and a maximum fidelity to the Bell state Φ+ of 95.2 ± 1.1 %.
In recent years, two-dimensional semiconductor quantum emitters have gotten substantial attention from the solid-state quantum photonics community. Their potential for on-chip integration in silicon-based photonics makes them an ideal candidate to realize large-scale hybrid quantum photonic circuits.
Given the strain-induced quantum emitter formation in two-dimensional WSe2, coupling of such quantum emitters into a SiN photonic waveguides is very promising. However, demonstration of single-photon emission into a waveguide has been elusive so far. Here, we show single-photon emission of strain-induced quantum emitters in a 2D flake integrated into a SiN waveguide. We take advantage of the waveguide edges as nucleation sites for quantum emitters. We observe single-photon emission coupled into the waveguide with a g(2)(0) = 0.15±0.09. This result opens up the way towards large-scale 2D emitter integration in on-chip quantum photonic circuits.
We investigate the indistinguishability of single photons generated from strain-free GaAs/AlGaAs quantum dots using pulsed resonance fluorescence techniques. In pulsed two-photon interference measurements we observe a single photon indistinguishability with a raw visibility of 95%. This can be traced back to the short intrinsic lifetime of excitons and trions confined in the GaAs quantum dots and demonstrates that for this material system the generation of single photons is possible with near-unity indistinguishability even without Purcell enhancement.
2D materials offer a wide range of perspectives for hosting highly localized 0D states, e.g. vacancy defects, that offer great potential for integrated quantum photonic applications. Here, we create individual defects that act as our single-photon emitters by highly local He-ion irradiation in a monolayer MoS2 van der Waals heterostructure. The defects show anti-bunched light emission at a characteristic energy of ~ 1.75 eV. The emission is highly homogeneous and background free due to the hBN encapsulation with a creation yield of > 70%. Spectroscopic investigation of individual single-photon emitters reveals a strongly asymmetric line shape resembling interaction with acoustic phonons in excellent agreement with an independent boson model. Moreover, emitters are spatially integrated and electrically controlled in field-switchable van der Waals devices. Our work firmly establishes 2D materials as a highly scalable material platform for integrated quantum photonics.
We investigate the indistinguishability of single photons generated from strain-free GaAs/AlGaAs quantum dots using pulsed resonance fluorescence techniques. In pulsed two-photon interference measurements we observe a single photon indistinguishability with a raw visibility of 95%. This can be traced back to the short intrinsic lifetime of excitons and trions confined in the GaAs quantum dots and demonstrates that for this material system the generation of single photons is possible with near-unity indistinguishability even without Purcell enhancement.
We develop schemes to generate, manipulate and detect single photons at various frequencies including telecom wavelengths. With detectors based on superconducting nanowires we combine very high detection efficiency with high time resolution and very low noise levels. We demonstrate on-chip implementation of single photon techniques as well as long distance implementations using deployed optical fibers.
Future quantum optical networks will require an integrated solution to multiplex suitable sources and detectors on a low-loss platform. Here we combined superconducting single-photon detectors with colloidal PbS/CdS quantum dots (QDs) and low-loss silicon nitride passive photonic components to show their combined operation at cryogenic temperatures. Using a planar concave grating spectrometer, we performed wavelength-resolved measurements of the photoluminescence decay of QDs, which were deterministically placed in the gap of plasmonic antennas, in order to improve their emission rate. We observed a Purcell enhancement matching the antenna simulations, with a concurrent increase of the count rate on the superconducting detectors.
Superconducting nanowire single photon detectors (SNSPDs) have demonstrated advantages over traditional detectors in many fields [1]. Most fiber-coupled SNSPDs are coupled to single mode fibers, limiting their usability for applications where large surface area detectors are needed, for example fluorescence detection and satellite-based quantum communication [2][3]. Other important requirements for many applications are broadband detection efficiency, and low timing jitter [4]. So far, the increased meander length of multimode detectors, and the therefore increased kinetic inductance and number of imperfections due to film inhomogeneities have limited the timing jitter [5]. Moreover, combining low timing jitter with high detection efficiency and low dark count rate in one device is challenging due to the tradeoffs between different properties of SNSPDs [6].
In this paper, we achieved high efficiency and strong saturation over a broad wavelength range with a low timing jitter of 16.99 ps while maintaining a low dark count rate of < 0.2 Hz for an SNSPD coupled to a 50 µm core multimode fiber. To enhance the broadband absorption from 405 nm to 830 nm, detectors were fabricated on an optimized SiO2 cavity and Aluminum mirror. The geometry of the nanowire was also tuned to reach a good internal saturation of efficiency over the visible/NIR range but also to carry high current to get a large signal. Furthermore, a cryogenic readout amplifier was optimized to improve the signal to noise ratio and thus lead to high time resolution. Our devices can be readily used to enable higher resolution and faster quantum optics, bio-imaging, laser ranging and other optical experiments.
In this paper, we review theoretical and experimental research progress on timing properties of superconducting nanowire single-photon detectors, including six possible mechanisms that induce timing jitter and experiments towards ultra-low timing jitter.
This paper reviews some recent research progress in superconducting nanowire single-photon detectors (SNSPDs) at the infrared spectrum range, with particular emphasis on detection efficiency and timing jitter. For detection efficiency, we present fractal SNSPDs with reduced polarization sensitivity; for timing jitter, we present two mechanisms of device timing jitter – vortex-crossing-induced timing jitter and spatial-inhomogeneity-induced timing jitter.
Nanowires offer new opportunities for nanoscale and integrated quantum optics; the quantum dot geometry in semiconducting nanowires as well as the material composition and environment can be engineered with unprecedented freedom to improve the light extraction efficiency. Quantum dots in nanowires are shown to be efficient single photon sources, in addition because of the very small fine structure splitting, we demonstrate the generation of entangled pairs of photons from a nanowire and discuss the limits to entanglement fidelity. Another type of nanowire under study in our group is superconducting nanowires for single photon detection, reaching efficiencies, time resolution and dark counts beyond currently available detectors. We will discuss our first attempts at combining semiconducting nanowire based single photon emitters and superconducting nanowire single photon detectors on a chip to realize integrated quantum circuits.
Quantum communication applications require a scalable approach to integrate bright sources of entangled photon-pairs in complex on-chip quantum circuits. Currently, the most promising sources are based on III/V semiconductor quantum dots. However, complex photonic circuitry is mainly achieved in silicon photonics due to the tremendous technological challenges in circuit fabrication. We take the best of both worlds by developing a new hybrid on-chip nanofabrication approach. We demonstrate for the first time on-chip generation, spectral filtering, and routing of single-photons from selected single and multiple III/V semiconductor nanowire quantum emitters all deterministically integrated in a CMOS compatible silicon nitride photonic circuit.
Nanowires offer new opportunities for nanoscale quantum optics; the quantum dot geometry in semiconducting nanowires as well as the material composition and environment can be engineered with unprecedented freedom to improve the light extraction efficiency.
Quantum dots in nanowires are shown to be efficient single photon sources, in addition because of the very small fine structure splitting, we demonstrate the generation of entangled pairs of photons from a nanowire.
Another type of nanowire under study in our group is superconducting nanowires for single photon detection, reaching efficiencies, time resolution and dark counts beyond currently available detectors. We will discuss our first attempts at combining semiconducting nanowire based single photon emitters and superconducting nanowire single photon detectors on a chip to realize integrated quantum circuits.
Nanowires offer new opportunities for nanoscale quantum optics; the quantum dot geometry in semiconducting nanowires as well as the material composition and environment can be engineered with unprecedented freedom to improve the light extraction efficiency.
Quantum dots in nanowires are shown to be efficient single photon sources, in addition because of the very small fine structure splitting, we demonstrate the generation of entangled pairs of photons from a nanowire.
By doping a nanowire and making ohmic contacts on both sides, a nanowire light emitting diode can be obtained with a single quantum dot as the active region. Under forward bias, this will act as an electrically pumped source of single photons. Under reverse bias, an avalanche effect can multiply photocurrent and enables the detection of single photons.
Another type of nanowire under study in our group is superconducting nanowires for single photon detection, reaching efficiencies, time resolution and dark counts beyond currently available detectors. We will discuss our first attempts at combining semiconducting nanowire based single photon emitters and superconducting nanowire single photon detectors on a chip to realize integrated quantum circuits.
The performance of superconducting-nanowire single-photon detectors depends on the efficiency of light absorption in the ultrathin (3-8 nm) superconducting nanowire. In this work, we will discuss two approaches to boost light absorption: coupling the nanowire to the evanescent field propagating in a waveguide and enclosing the nanowire in an optical cavity. The latter method is the most widely used, but it is intrinsically very sensitive to the polarization of light. To overcome this issue, we propose some innovative cavity designs which make use of high-index (n >2) dielectrics. With this technique, highly-efficient polarization-insensitive devices can be easily implemented.
We demonstrate the generation of single photons as well as pairs of entangled photons with quantum dots in semiconducting nanowires, we show applications to quantum optics including generation, manipulation and detection of light at the nanoscale.
Single quantum dots embedded in tapered nanowire waveguides have emerged as leading candidates for designing high efficiency single-photon and entangled photon sources, with efficiencies exceeding 90%. Here we have developed a bottom-up growth approach that allows for independent control of boththe quantum dot size, and position, as well as the nanowire shape. Importantly, by design, the single quantum dot is always found perfectly on the nanowire axis. By integrating a gold mirror at the base of a tapered nanowire waveguide we obtain a 20-fold enhancement in the single-photon flux in comparison to no waveguide. The 20-fold enhancement is accompanied by a shortening of the exciton lifetime as the quantum emitter couples to the fundamental waveguide mode with an increased rate.
Finally, the optical quality of the emitter is drastically improved by removing the nanowire stacking faults in the vicinity of the quantum dot. As a result, we demonstrate very pure single-photon emission with a probability of multi-photon emission below 1%, and an emission line width that is reduced by at least an order of magnitude (<30 μeV) as compared to when stacking faults were present in the nanowire (as high as 10-100 per micron). The demonstrated brightness of our single-photon source (42 % efficiency), combined with the very pure single photon emission and high spectral purity is encouraging in development of future quantum technologies based on nanowires, such as interfacing remote quantum bits or constructing a secure quantum network.
KEYWORDS: Imaging systems, Single photon, Signal to noise ratio, Sensors, Profiling, Picosecond phenomena, Single photon detectors, Avalanche photodiodes, LIDAR, Nanowires
Active depth imaging approaches are being used in a number of emerging applications, for example in
environmental sensing, manufacturing and defense. The high sensitivity and picosecond timing resolution of the
time-correlated single-photon counting technique can provide distinct advantages in the trade-offs between
required illumination power, range, depth resolution and data acquisition durations. These considerations must
also address requirements for eye-safety, especially in applications requiring outdoor, kilometer range sensing.
We present a scanning time-of-flight imager based on MHz repetition-rate pulsed illumination operating with
sub-milliwatt average power. The use of a scanning mechanism permits operation with an individual, high-performance
single-photon detector. The system has been used with a number of non-cooperative targets, in
different weather conditions and various ambient light conditions. We consider a number of system issues,
including the range ambiguity issue and scattering from multiple surfaces. The initial work was performed at
wavelengths around 850 nm for convenient use with Si-based single photon avalanche diode detectors, however
we will also discuss the performance at a wavelength of 1560 nm, made using superconducting nanowire single
photon detectors. The use of the latter wavelength band allows access to a low-loss atmospheric window, as well
as greatly reduced solar background contribution and less stringent eye safety considerations. We consider a
range of optical design configurations and discuss the performance trade-offs and future directions in more
detail.
Michael Reimer, Maarten van Kouwen, Maria Barkelid, Moïra Hocevar, Maarten van Weert, Rienk Algra, Erik Bakkers, Mikael Björk, Heinz Schmid, Heike Riel, Leo Kouwenhoven, Valery Zwiller
We report recent progress toward on-chip single photon emission and detection in the near infrared utilizing semiconductor nanowires. Our single photon emitter is based on a single InAsP quantum dot embedded in a p-n junction defined along the growth axis of an InP nanowire. Under forward bias, light is emitted from the single quantum dot by electrical injection of electrons and holes. The optical quality of the quantum dot emission is shown to improve when surrounding the dot material by a small intrinsic section of InP. Finally, we report large multiplication factors in excess of 1000 from a single-Si-nanowire avalanche photodiode comprised of p-doped, intrinsic, and n-doped sections. The large multiplication factor obtained from a single Si nanowire opens up the possibility to detect a single photon at the nanoscale.
Michael Reimer, Maarten van Kouwen, Maria Barkelid, Moïra Hocevar, Maarten van Weert, Rienk Algra, Erik Bakkers, Mikael Björk, Heinz Schmid, Heike Riel, Leo Kouwenhoven, Val Zwiller
We report recent progress toward on-chip single photon emission and detection in the near infrared utilizing semiconductor nanowires. Our single photon emitter is based on a single InAsP quantum dot embedded in a p-n junction defined along the growth axis of an InP nanowire. Under forward bias, light is emitted from the single quantum dot by electrical injection of electrons and holes. The optical quality of the quantum dot emission is shown to improve when surrounding the dot material by a small intrinsic section of InP. Finally, we report large multiplication factors in excess of 1000 from a single Si nanowire avalanche photodiode comprised of p-doped, intrinsic, and n-doped sections. The large multiplication factor obtained from a single Si nanowire opens up the possibility to detect a single photon at the nanoscale.
In this paper we present our recent developments in control and manipulation of individual spins and photons in a single
nanowire quantum dot. Specific examples include demonstration of optical excitation of single spin states, charge
tunable quantum devices and single photon sources. We will also discuss our recent discovery of a new type of charge
confinement - crystal phase quantum dots. They are formed from the same material with different crystal structure, and
today can only be realized in nanowires.
The catalyst-assisted growth of semiconductor nanowires heterostructures offers a very flexible way to design and
fabricate single photon emitters. The nanowires can be positioned by organizing the catalyst prior to growth. Single
quantum dots can be formed in the core of single nanowires which can then be easily isolated and addressed to generate
single photons. Diameter and height of the dots can be controlled and their emission wavelength can be tuned at the
optical telecommunication wavelengths by the material composition. The final morphology of a wire can be shaped by
the radial/axial growth ratio, offering the possibility to form single mode optical waveguides with a tapered end for
efficient photon collection.
We have fabricated fiber-coupled superconducting single-photon detectors (SSPDs), designed for quantum-correlationtype
experiments. The SSPDs are nanostructured (~100-nm wide and 4-nm thick) NbN superconducting meandering
stripes, operated in the 2 to 4.2 K temperature range, and known for ultrafast and efficient detection of visible to nearinfrared
photons with almost negligible dark counts. Our latest devices are pigtailed structures with coupling between
the SSPD structure and a single-mode optical fiber achieved using a micromechanical photoresist ring placed directly
over the meander. The above arrangement withstands repetitive thermal cycling between liquid helium and room
temperature, and we can reach the coupling efficiency of up to ~33%. The system quantum efficiency, measured as the
ratio of the photons counted by SSPD to the total number of photons coupled into the fiber, in our early devices was
found to be around 0.3 % and 1% for 1.55 &mgr;m and 0.9 &mgr;m photon wavelengths, respectively. The photon counting rate
exceeded 250 MHz. The receiver with two SSPDs, each individually biased, was placed inside a transport, 60-liter
liquid helium Dewar, assuring uninterrupted operation for over 2 months. Since the receiver's optical and electrical
connections are at room temperature, the set-up is suitable for any applications, where single-photon counting capability
and fast count rates are desired. In our case, it was implemented for photon correlation experiments. The receiver
response time, measured as a second-order photon cross-correlation function, was found to be below 400 ps, with
timing jitter of less than 40 ps.
We present the design and performance of a novel, two-channel single-photon receiver, based on two fiber-coupled NbN superconducting single-photon detectors (SSPDs). The SSPDs are nanostructured superconducting meanders covering an area of 100 μm2 and are known for ultrafast and efficient counting of single, visible-to-infrared photons. Their operation has been explained within a phenomenological hot-electron photoresponse model. Our receiver is intended for fiber-based quantum cryptography and communication systems, operational at near-infrared (NIR) telecommunication wavelengths, λ = 1.3 μm and λ = 1.55 μm. Coupling between the NbN detector and a single-mode optical fiber was achieved using a specially designed, micromechanical photoresist ring, positioned directly over the SSPD active area. The positioning accuracy of the ring was below 1 μm. The receiver with SSPDs was placed (immersed) in a standard liquid-helium transport Dewar and kept without interruption for over two months at 4.2 K. At the same time, the optical fiber inputs and electrical outputs were kept at room temperature. Our best system reached a system quantum efficiency of up to 0.3 % in the NIR radiation range, with the detector coupling efficiency of about 30 %. The response time was measured to be about 250 ps and was limited by our read-out electronics. The measured jitter was close to 35 ps. The presented performance parameters show that our NIR single photon detectors are suitable for practical quantum cryptography and for applications in quantum-correlation experiments.
High-efficient single-photon sources are important for fundamental experiments as well as for modern applications in the field of quantum information processing. Therefore, both the overall collection efficiency as well as the photon generation rate are important parameters. In this article, we use cascaded two-photon emission from a single quantum dot in order to double the efficient transmission rate in a quantum key distribution protocol by multiplexing on a single photon level. The energetically non-degenerate photons are separated with a single photon add/drop filter based on a Michelson interferometer. For optimizing the collection efficiency, coupling of quantum emitters to microcavities is advantageous. We also describe preliminary results towards coupling of a single quantum dot grown on a micrometer-sized tip to the whispering gallery modes of a microsphere cavity.
We describe the realization and characterization of nanoscale light emitters comprising one or few semiconductor quantum dots (QDs) in the active region. These devices are intended for use as single-photon sources in fiber-based quantum cryptography systems. The epitaxial growth of low-density QDs emitting at 1300 nm is described, and emission from single QDs is demonstrated through micro-photoluminescence measurements. The realization of QD light-emitting diodes (LEDs) having a submicrometer active area is reported, based on oxidized current apertures. Finally, the integration of nanoscale current injection with high-quality factor optical cavities is described, in view of obtaining efficient single-photon LEDs.
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