Quantum technologies harness nonclassical features of particles, here, photons, to develop novel, efficient, and precise devices for information processing applications. Superposition, entanglement, as well as the coherent manipulation of quantum states are at the heart of the second quantum revolution (quantum 2.0) which targets the development of secure cryptographic systems, complex computation protocols, and more. Emerging quantum architectures rely on the realistic implementation of photonic schemes which are scalable, resource-efficient, and compatible with CMOS technologies as well as fiber networks. This work demonstrates current schemes utilized for time-/frequency-bin entanglement generation and processing by leveraging existing telecommunications and integrated photonics infrastructures.
Spectral Hong-Ou-Mandel effect is experimentally revealed between a thermal field and a heralded state. We demonstrate that multi-photon components in heralded state get engaged in quantum interference which is manifested in improved visibilities for certain mean photon numbers of the thermal field. A theoretical relationship is derived for the visibility, showing its compound dependency on the mean photon number per pulse period within the thermal field and the thermal part of the heralded state. Relying on pulsed excitation, we demonstrate an experimental approach for time-efficient acquisition of visibility. The results of this research are of potential application to high-speed remote secret key-sharing and combating the security gaps arising from multi-photon contamination.
We review our work on implementing integrated QFC sources based on microring resonators for on-chip generation of two- and multi-photon time-bin entangled states, d-level frequency-entangled photon pairs, and multipartite d-level cluster states. We also present our recent progress on telecom-compatible, scalable, time-entangled two-photon qubits using on-chip Mach-Zehnder interferometers (MZI) in combination with spiral waveguides. Both approaches are highly cost-effective, efficient, and practical, since we coherently manipulate the time and frequency modes through standard fiber-linked components that are compatible with off-the-shelf telecommunications infrastructures. Our work paves the way for robust sources and processors of complex photon states for future quantum technologies.
We have optimised the design and fabrication of low-loss type-I femtosecond-laser-written waveguides in PPLN that are single-transverse mode at 780 nm and 1560 nm and mode-matched to single-mode fibres. Spontaneous parametric downconversion (SPDC) has been demonstrated at 1560 nm when pumped with a 780 nm DFB laser and has been characterised with measurement of the second-order cross-correlation g(2) using superconducting nanowire detectors. This novel approach to waveguide fabrication in PPLN offers routes to high levels of integration and high generation rates which is important for many quantum-information applications.
We demonstrate two-photon interference and polarization entanglement at 2090 nm, constituting a crucial leap towards
free-space mid-infrared quantum communication systems in a spectral region with high atmospheric transparency and
reduced solar background.
Quantum-enhanced optical technologies operating within the 2- to 2.5-μm spectral region have the potential to revolutionize emerging applications in communications, sensing, and metrology. Currently, sources of entangled photons are available at visible, near-infrared and telecom wavelengths but are strongly underdeveloped at longer wavelengths. Here, using custom-designed lithium niobate crystals for spontaneous parametric down-conversion and tailored superconducting nanowire single-photon detectors, we demonstrate two-photon interference and polarization-entangled photon pairs at 2090 nm. These results open the 2- to 2.5-μm mid-infrared window for the development of optical quantum technologies such as quantum key distribution in next-generation mid-infrared fiber communication systems and future Earth-to-satellite communications.
A key challenge in today’s quantum science is the realization of large-scale complex non-classical systems to enable e.g. ultra-secure communications, quantum-enhanced measurements, and computations faster than classical approaches. Optical frequency combs represent a powerful approach towards this, since they provide a very high number of temporal and frequency modes which can result in large-scale quantum systems. Here, we discuss the recent progress on the realization of integrated quantum frequency combs and reveal how their use in combination with on-chip and fiber-optic telecommunications components can enable quantum state control with new functionalities, yielding unprecedented capability.
Quantum Key Distribution (QKD) directly exploits the quantum phenomenon of entanglement to allow the secure sharing of a cryptographic key for information encoding. The current generation of QKD devices typically operate over dedicated and expensive private ‘dark fiber’ networks, where they are limited in transmission range to 200-300km due to the lack of quantum repeaters. This paper is concerned with an alternative approach that can lift this range limit by exploiting QKD over free-space links between satellites. Typically, commercial QKD systems rely on phase encoding of information on single photons, and more recently on continuously variable schemes with more powerful lasers. However, these protocols are not suitable for communications through atmosphere. On the other hand, QKD by polarization-entanglement holds great promise for satellite-based QKD encoded communications links if the entangledphoton source can be packaged in a compact, robust and commercially-viable form. This paper will describe the development and packaging of an entangled-photon source utilizing space-qualified telecoms packaging techniques, resulting in a compact device that targets satellite deployment. The key design choices that impact performance in a space environment will be discussed and the results of device characterization in the laboratory environment will be shared.
Complex optical quantum states based on entangled photons are essential for investigations of fundamental physics and are the heart of applications in quantum information science. Recently, integrated photonics has become a leading platform for the compact, cost-efficient, and stable generation and processing of optical quantum states. However, onchip sources are currently limited to basic two-dimensional (qubit) two-photon states, whereas scaling the state complexity requires access to states composed of several (<2) photons and/or exhibiting high photon dimensionality. Here we show that the use of integrated frequency combs (on-chip light sources with a broad spectrum of evenly-spaced frequency modes) based on high-Q nonlinear microring resonators can provide solutions for such scalable complex quantum state sources. In particular, by using spontaneous four-wave mixing within the resonators, we demonstrate the generation of bi- and multi-photon entangled qubit states over a broad comb of channels spanning the S, C, and L telecommunications bands, and control these states coherently to perform quantum interference measurements and state tomography. Furthermore, we demonstrate the on-chip generation of entangled high-dimensional (quDit) states, where the photons are created in a coherent superposition of multiple pure frequency modes. Specifically, we confirm the realization of a quantum system with at least one hundred dimensions. Moreover, using off-the-shelf telecommunications components, we introduce a platform for the coherent manipulation and control of frequencyentangled quDit states. Our results suggest that microcavity-based entangled photon state generation and the coherent control of states using accessible telecommunications infrastructure introduce a powerful and scalable platform for quantum information science.
Coherent control of nonlinear and ultrafast plasmon-polariton mediated interactions has attracted wide attention for its potential for enhancing functionality in nano-scale photonic devices and applications. Contemporary research in ultrafast and nonlinear plasmonics primarily utilizes noble metals, such as gold and silver, as material platforms because of their high performance both in linear and nonlinear optical properties. Unfortunately, noble metals possess numerous drawbacks including low melting points, chemical instabilities, and an incompatibility with standard CMOS processing techniques, all of which hamper their incorporation into functional plasmonic devices. Here we investigate the mid-infrared ultrafast and nonlinear properties of the alternative plasmonic material, aluminum-doped zinc oxide (AZO). By performing time-resolved pump-probe spectroscopy, we observe an unprecedentedly large and ultrafast (sub-picosecond) response in AZO thin films for both intra- and inter-band pumping frequencies. These two nonlinearities arise from distinct electron excitation dynamics and, as such, can be controlled simultaneously and independently to provide a novel method of dynamic tunability. We demonstrate this phenomenon with two-color excitation and find our AZO films exhibit a THz modulation bandwidth. We also probed the nonlinear response of AZO films at the epsilon-near-zero (ENZ) frequency and observed a dramatic increase in the Kerr nonlinearity with an induced refractive index change on the order of unity. In summary, our ultrafast and nonlinear studies strongly support AZO as an alternative plasmonic material with qualities pertinent to the development and realization of practical plasmonic technologies.
The on-chip generation of optical quantum states will enable accessible advances for quantum technologies. We demonstrate that integrated quantum frequency combs (based on high-Q microring resonators made from a CMOS-compatible, high refractive-index doped-glass platform) can enable the generation of pure heralded single photons, cross-polarized photon pairs, as well as bi- and multi-photon entangled qubit states over a broad frequency comb covering the S, C, L telecommunications band, with photon frequencies corresponding to standard telecommunication channels spaced by 200 GHz.
Exploiting a self-locked, intra-cavity excitation configuration, a highly-stable source of multiplexed heralded single photons is demonstrated, operating continuously for several weeks with less than 5% fluctuations. The photon bandwidth of 110 MHz is compatible with quantum memories, and high photon purity was confirmed through single-photon auto-correlation measurements. In turn, by simultaneously exciting two orthogonal polarization mode resonances, we demonstrate the first realization of type-II spontaneous FWM (in analogy to type-II spontaneous parametric down-conversion), allowing the direct generation of orthogonally-polarized photon pairs on a chip.
By using a double-pulse excitation, we demonstrate the generation of time-bin entangled photon pairs. We measure qubit entanglement with visibilities above 90%, enabling the implementation of quantum information processing protocols. Finally, the excitation field and the generated photons are intrinsically bandwidth-matched due to the resonant characteristics of the ring cavity, enabling the multiplication of Bell states and the generation of a four-photon time-bin entangled state. We confirm the generation of this four-photon entangled state through four-photon quantum interference.
We investigate the spatio-temporal structure of the biphoton entanglement in Parametric Down Conversion
(PDC). In particular we study the biphoton amplitude at the output face of the nonlinear crystal (near-field)
and we demonstrate its X-shaped geometry in the space-time dimensions, i.e. the non-factorability of the state
with respect to spatial and temporal variables. Our analysis provides a precise and quantitative characterization
of this structure in various regimes and types of phase matching of PDC. The key elements of novelty emerging
from our analysis are the non-factorability of the state with respect to spatial and temporal variables, and the
extreme relative localization of the entangled photons, both in space (few microns) and time (few femtoseconds).
This extreme localization is connected to our ability to resolve the photon positions in the source near-field. The
non factorability opens the possibility of tailoring the temporal entanglement by acting on the spatial degrees of
freedom of twin photons.
We review and compare the results of quantum spatial correlation measurements in parametric down-conversion
in the high-gain pulsed regime, with preliminary measurements performed in the low-gain cw-pumped regime.
The diagnostics is based on a high quantum efficiency CCD camera, and in the second case the radiation pattern
is recorded after temporal integration of the "single-photon" spatial distribution. The effect of the detected
number of temporal modes on the accessibility of the sub-shot noise regime is discussed, together with the
identification of suitable regimes for weak image detection.
We propose an imaging scheme based on the quantum spatial correlation of twin beams generated by PDC,
and we show that it provides a substantial enhancement of the signal-to-noise ratio with respect to classical
schemes.
We propose an imaging scheme based on the quantum spatial correlation of twin beams generated by PDC,
and we show that it provides a substantial enhancement of the signal-to-noise ratio with respect to classical
schemes.
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