Single photon sources for quantum information applications

Based on the quantum dot-photon molecular coupling system, we proposed an efficient and stable single photon source. The system uses photon molecules formed by the coupling of two optical microcavities A and B, one of which contains a quantum dot. The excitation laser act on the quantum dot in cavity A, and through the interaction between the light field in cavity A and cavity B, single photon is generated in cavity B and output extraction is performed therein. The system maintains good sub-Poisson characteristics over a wide range of dot-cavity coupling strength variation (0.25κ<;g<;κ) and at low cavity tunneling strength (J≥0.5κ) as well. It also has excellent robustness and is easy to construct. Under the optimal experimental parameters that can be realized, the zero-delay second-order correlation function g <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (0) of the system can be as low as 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> with a relatively large intra cavity photon number. We theoretically explains the reason why the system produces sub-Poisson light, and analyzed the impact of system parameters change. Meanwhile, the system spatially separates the output light from the excitation light, making it easy to extract single photons, and has potential application value for easier integration. The model proposed in this paper will bring important guiding value to the efficient and integrated single photon sources in future quantum information processing.

A single photon source is demonstrated using a novel oxide-apertured micropillar cavity embedded with InGaAs quantum dots. A bright 80 MHz count rate is enabled by the Purcell effect and charge-tuning of the quantum dots.

A key ingredient for quantum photonic technologies is an on-demand source of indistinguishable single photons. State-of-the-art indistinguishable single-photon sources typically employ resonant excitation pulses with fixed repetition rates, creating a string of single photons with predetermined arrival times. However, in future applications, an independent electronic signal from a larger quantum circuit or network will trigger the generation of an indistinguishable photon. Further, operating the photon source up to the limit imposed by its lifetime is desirable. Here, we report on the application of a true on-demand approach in which we can electronically trigger the precise arrival time of a single photon as well as control the excitation pulse duration based on resonance fluorescence from a single InAs/GaAs quantum dot. We investigate in detail the effect of the finite duration of an excitation $\pi$ pulse on the degree of photon antibunching. Finally, we demonstrate that highly indistinguishable single photons can be generated using this on-demand approach, enabling maximum flexibility for future applications.

Single photon sources, which are compatible with quantum memories, are important components of quantum networks. In this article, we show optical investigations on isolated GaAs/Al0.25Ga0.75As quantum dots grown via droplet epitaxy, which emit single photons on resonance with the Rb-87-D2 line (780 nm). Under continuous wave resonant excitation conditions, we observe bright, clean, and narrowband resonance fluorescence emission from such a droplet quantum dot. Furthermore, the second-order correlation measurement clearly demonstrates the single photon emission from this resonantly driven transition. Spectrally resolved resonance fluorescence of a similar quantum dot yields a linewidth as narrow as 660 MHz (2.7 μeV), which corresponds to a coherence time of 0.482 ns. The observed linewidth is the smallest reported so far for strain free GaAs quantum dots grown via the droplet method. We believe that this single photon source can be a prime candidate for applications in optical quantum networks.

As currently implemented, single-photon sources cannot be made to produce single photons with high probability, while simultaneously suppressing the probability of yielding two or more photons. Because of this, single photon sources cannot really produce single photons on demand. We describe a multiplexed system that allows the probabilities of producing one and more photons to be adjusted independently, enabling a much better approximation of a source of single photons on demand. The scheme uses a heralded photon source based on parametric downconversion, but by effectively breaking the trigger detector area into multiple regions, we are able to extract more information about a heralded photon than is possible with a conventional arrangement. This scheme allows photons to be produced along with a quantitative “certification ” that they are single photons. Some of the single-photon certifications can be significantly better than what is possible with conventional downconversion sources (using a unified trigger detector region), as well as being better than faint laser sources. With such a source of more tightly certified single photons, it should be possible to improve the maximum secure bit rate possible over a quantum cryptographic link. We present an analysis of the relative merits of this method over the conventional arrangement.

The future of quantum photonic technology depends on the realization of efficient sources of single photons, the ideal carriers of quantum information. Parametric downconversion (PDC) is a promising route to create highly coherent, spectrally pure single photons for quantum photonics using versatile group velocity matching (GVM) and tailored nonlinearities. However, the functionality to actively control the poling period of nonlinear crystals used in PDC is currently missing, yet would enable to dynamically modify the wavelength of single photons produced in the PDC process. Herein, a detailed GVM study is presented for functional PMN‐0.38PT material which can be dynamically repolled at ambient conditions with fields as low as 0.4 kV mm −1 . The study reveals phase‐matching conditions for spectrally pure single photon creation at 5–6 µm. Further, a practical approach is proposed for on‐flight wavelength switching of the created single photons. The reported reconfigurable functionality benefits a wide range of emerging quantum‐enhanced applications in the mid‐IR spectral region where the choice of single photon sources is currently limited.

A high-fidelity source of identical single photons is essential for numerous\nquantum technologies such as quantum repeaters and optical quantum information\nprocessing. Hallmarks thereof are a near-unity single-photon purity, near-unity\nindistinguishability of consecutively emitted photons, and high brightness\nthrough a near-unity number of photons per time bin. In order to embed such\nsources in quantum networks, optical fiber integration is essential but\ncomplicated by cryogenic compatability and noise. Here we demonstrate a\nresonantly pumped, quantum dot (QD) based, transmission operated, single-mode\nfiber coupled single photon source with a purity of 97%, indistinguishability\nof 90%, and a brightness of 17%. This is achieved by deploying a unique\nmicropillar cavity design in a closed-cycle cryostat, which is operated using a\nthrough-fiber cross-polarization technique to remove the pump laser light from\nthe resonantly scattered single photons. These results pave the way for fully\nfiber integrated photonic quantum networks, as our technology is equally\napplicable for cavity-QED based photonic quantum gates.\n

We describe the simplest single–photon encoding schemes and introduce the concept of a quantum optical gate. These gates can be used to build up arbitrary entangled states of many initially separate single photons. Experimental realization of such a gate involves the development of nonlinear phase shift elements sensitice at the single quantum level. We report our experimental efforts using linear optical elements and non–classical sources where we are able to demonstrate interference and entanglement of initially separate single–photon pulses.

In biology as well as in quantum information protocols, efficient light sources based on nanoemitters are of great interest. For quantum cryptography, single photons sources, with high quantum yield but also efficient emission collection, are needed. Colloidal CdSe nanocrystals (NCx) are very good candidates for these purposes, as they are versatile and efficient single photon sources at the individual scale. For quantum gates, coherence time of the emission is crucial but its measurement, at cryogenic temperature, remains challenging for single CdSe NCx as they suffer of phase diffusion. A recent experiment proved that their coherence time is larger than 200 ps. But the measurement was limited by the collection efficiency, especially low when the sample is inside a cryostat, and an objective with a quite low numerical aperture has to be used. This paper proposes a configuration for which emission efficiency and far field collection of single NCx can be increased. A confocal microscope is used to collect part of the fluorescence of single quantum dots located at various distances from the gold surface and to measure the corresponding far field intensity. The electromagnetic interaction between a single NCx and its optical environment is highlighted by time-resolved fluorescence measurements at room temperature. Not only the possibility of emission control by coupling to surface plasmon of a metallic structure is demonstrated but also optimization of the far field collection efficiency, which is of great importance for applications in biology and quantum information.

Bright sources of indistinguishable single photons are strongly needed for the scalability of quantum information processing. Semiconductor quantum dots are promising systems to build such sources. Several works demonstrated emission of indistinguishable photons while others proposed various approaches to efficiently collect them. Here we combine both properties and report on the fabrication of ultrabright sources of indistinguishable single photons, thanks to deterministic positioning of single quantum dots in well-designed pillar cavities. Brightness as high as 0.79±0.08 collected photon per pulse is demonstrated. The indistinguishability of the photons is investigated as a function of the source brightness and the excitation conditions. We show that a two-laser excitation scheme allows reducing the fluctuations of the quantum dot electrostatic environment under high pumping conditions. With this method, we obtain 82±10% indistinguishability for a brightness as large as 0.65±0.06 collected photon per pulse.

Micropillar cavities are versatile nanophotonic structures that locally enhance the light-matter interaction due to their high Q-factors, low mode volumes. Moreover, they have a consistent clean free space optical interface. These qualities have facilitated the successful application of micropillar cavities as efficient single photon sources and for cavity QED strong coupling [1]. In all these realizations, however, the cavity resonance is stationary in time during the lifetime of the embedded quantum dots. We propose to bring micropillars into a new dynamic regime where the cavity resonance is switched on a time scale faster than the emitter lifetime. To this aim we have studied the emission intensity for quantum dots with a time dependent decay rate [2] and have performed an experimental investigation of the ultrafast dynamics of switched micropillar cavities.

We demonstrate a high-purity source of indistinguishable single photons using a quantum dot embedded in a nanophotonic waveguide. The source features a near-unity internal coupling efficiency and the collected photons are efficiently coupled off-chip by implementing a taper that adiabatically couples the photons to an optical fiber. By quasi-resonant excitation of the quantum dot, we measure a single-photon purity larger than 99.4% and a photon indistinguishability of up to 94+-1% by using p-shell excitation combined with spectral filtering to reduce photon jitter. A temperature-dependent study allows pinpointing the residual decoherence processes notably the effect of phonon broadening. Strict resonant excitation is implemented as well as another mean of suppressing photon jitter, and the additional complexity of suppressing the excitation laser source is addressed. The study opens a clear pathway towards the long-standing goal of a fully deterministic source of indistinguishable photons, which is integrated on a planar photonic chip.

Semiconductor quantum dots embedded in optical cavities are promising on-demand sources of single photons. Here, we theoretically study single photon emission from an optically driven two-photon Raman transition between the biexciton and the ground state of a quantum dot. The advantage of this process is that it allows all-optical control of the properties of the emitted single photon with a laser pulse. However, with the presence of other decay channels and excitation-induced quantum interference, on-demand emission of the single Raman photon is generally difficult to achieve. Here we show that laser pulses with non-trivial shapes can be used to maintain excitation conditions for which with increasing pulse intensities the on-demand regime is reached. To provide a realistic picture of the achievable system performance, we include phonon-mediated processes in the theoretical caluclations. While preserving both high photon purity and indistinguishability, we find that although based on a higher-order emission process, for realistic system parameters on-demand Raman photon emission is indeed achievable with suitably tailored laser pulses.

Single-photon emitters are crucial for the applications in quantum communication, random number generation and quantum information processing. Self-assembled InAs/GaAs quantum dots (QDs) have demonstrated to have singlephoton emission with high extraction efficiency, single-photon purity, and photon indistinguishability. Thus they are considered as the promising deterministic single-photon emitters. To extend the emission wavelength of InAs/GaAs QDs to telecom band, several methods have been developed, such as the strain engineered metamorphic quantum dots, the use of strain reducing layers and the strain-coupled bilayer of QDs. In fact, it is reported on single-photon emissions based on InAs/InP QDs with an emission wavelength of 1.55μm, but it is difficult to combine such QDs with a high-quality distributed Bragg reflector (DBR) cavity because the refractive index difference between InP and InGaAsP is too small to obtain a DBR cavity with high quality factor. Here we investigate 1.3μm single-photon emissions based on selfassembled strain-coupled bilayer of InAs QDs embedded in micropillar cavities. The studied InAs/GaAs self-assembled QDs are grown by molecular beam epitaxy on a semi-insulating (100) GaAs substrate through strain-coupled bilayer of InAs QDs, where the active QDs are formed on the seed QDs capped with an InGaAs layer, and two-layer QDs are vertically coupled with each other. In such a structure the emission wavelength of QDs can be extended to 1.3μm. The QDs with a low density of about 6×10&lt;sup&gt;8&lt;/sup&gt; cm&lt;sup&gt;-2&lt;/sup&gt; are embedded inside a planar 1-&lt;sup&gt;λ&lt;/sup&gt; GaAs microcavity sandwiched between 20 and 8 pairs of Al&lt;sup&gt;0.9&lt;/sup&gt;Ga&lt;sup&gt;0.1&lt;/sup&gt;As/GaAs as the bottom and top mirror of a DBR planar cavity, respectively. Then the QD samples are etched into 3μm diameter micropillar by photolithography and dry etching. The measured quality factor of studied pillar cavity has a typical value of approximately 300. Photoluminescence (PL) spectra of QDs at a temperature of 5 K are examined by using a micro-photoluminescence setup equipped with a 300 mm monochromator and an InGaAs linear photodiode array detector. A diode laser with a continuous wave or a pulsed excitation repetition rate of 80 MHz and an excitation wavelength of 640 nm is used to excite QDs through an near-infrared objective (&lt;sup&gt;NA&lt;/sup&gt; 0.5), and the PL emission is collected by the same objective. The time-resolved PL of the QDs is obtained by a time-correlated single photon counting. The second-order correlation function is checked by a Hanbury-Brown and Twiss setup through using ID 230 infrared single-photon detectors.&lt;br/&gt;In summary, we find that the 1.3μm QD exciton lifetime at 5 K is measured to be approximately 1 ns, which has the same value as the 920 nm QD exciton lifetime. The second-order correlation function is measured to be 0.015, showing a good characteristic of 1.3μm single photon emission. To measure the coherence time, i.e., to perform highresolution linewidth measurements, of the QDs emitted at the wavelength of 920 and 1300 nm, we insert a Michelson interferometer in front of the spectrometer. The obtained coherence time for 1.3μm QDs is 22 ps, corresponding to a linewidth of approximately 30μeV. Whereas, the coherence time is 216 ps for 920 nm QDs, corresponding to a linewidth of approximately 3μeV. Furthermore, both emission spectral lineshapes are different. The former is of Gaussian-like type, while the latter is of Lorentzian type.

This thesis covers the development and the characterization of a single photon detector based on a superconducting microcalorimeter. The detector development is motivated by the Any Light Particle Search II (ALPS II) experiment at DESY in Hamburg, which searches for weakly interacting sub-eV particles (WISPs). There- fore, a detection of low-fluxes of 1064 nm light is required. The work is divided in three analyses: the characterization of a milli-kelvin (mK) cryostat, the character- ization of superconducting sensors for single photon detection, and the determina- tion of dark count rates concerning 1064 nm signals. Firstly, an adiabatic demagnetization refrigerator (ADR) is characterized, which allows to reach mK-temperatures. During commissioning, the ADR cryostat is op- timized and prepared to stably cool superconducting sensors at 80 mK 25 K. It is found that sensors can be continuously operated for 20 h before recharging the system in < 2 h. Furthermore, the adiabatic system reaches a chance of success of 80 % for a recharge without technical problems. Secondly, superconducting sensors are analyzed. The focus is on microcalori- metric transition-edge sensors (TESs) based on 20 nm Tungsten (W) films fabri- cated by the U.S. National Institute of Standards and Technology (NIST). NIST TESs have a near unity detection e ciency for 1064 nm light (literature value). The energy resolution for 1064 nm signals is measured to be < 8 %. The expo- nential falling time of a photon pulse is 1.5 s. Furthermore, by determining TES parameters, it is found that the linear TES theory describes measured photon pulses well. The TES response is read out by a superconducting quantum interference device (SQUID) fabricated by Physikalisch-Technische Bundesanstalt (PTB). The system bandwidth is measured to be 0.9 MHz. Finally, the operation in the ADR cryostat as well as the ALPS II laboratory is optimized. This setup forms the ALPS TES detector. Thirdly, the background is measured to obtain a dark count rate for 1064 nm signals. The ALPS TES detector is calibrated by a 1064 nm single photon source and methods are developed to analyze signals. In long-term measurements, back- ground events are measured by using di erent optical setups. By operating the TES without an optical link outside mK-environment, intrinsic background components are observed and classified. This results in an intrinsic dark count rate for 1064 nm signals of 1 : 0 10