Piotr Roztocki

Generation of multiphoton entangled quantum states by means of integrated frequency combs

Entangled frequency combs The ability to generate optical frequency combs in which the output light is made up of millions of sharp lines precisely spaced apart has been important for optical applications and for fundamental science. Reimer et al. now show that frequency combs can be taken into the quantum regime. They took individual teeth of the combs and quantum-mechanically entangled them to form complex optical states. Because the method is compatible with existing fiber and semiconductor technology, the results demonstrate a possible scalable and practical platform for quantum technologies. Science, this issue p. 1176 Optical frequency combs are taken into the quantum regime. [Also see Perspective by Peacock and Steel] Complex optical photon states with entanglement shared among several modes are critical to improving our fundamental understanding of quantum mechanics and have applications for quantum information processing, imaging, and microscopy. We demonstrate that optical integrated Kerr frequency combs can be used to generate several bi- and multiphoton entangled qubits, with direct applications for quantum communication and computation. Our method is compatible with contemporary fiber and quantum memory infrastructures and with chip-scale semiconductor technology, enabling compact, low-cost, and scalable implementations. The exploitation of integrated Kerr frequency combs, with their ability to generate multiple, customizable, and complex quantum states, can provide a scalable, practical, and compact platform for quantum technologies.

On-chip generation of high-dimensional entangled quantum states and their coherent control

Optical quantum states based on entangled photons are essential for solving questions in fundamental physics and are at the heart of quantum information science. Specifically, the realization of high-dimensional states (D-level quantum systems, that is, qudits, with D > 2) and their control are necessary for fundamental investigations of quantum mechanics, for increasing the sensitivity of quantum imaging schemes, for improving the robustness and key rate of quantum communication protocols, for enabling a richer variety of quantum simulations, and for achieving more efficient and error-tolerant quantum computation. Integrated photonics has recently become a leading platform for the compact, cost-efficient, and stable generation and processing of non-classical optical states. However, so far, integrated entangled quantum sources have been limited to qubits (D = 2). Here we demonstrate on-chip generation of entangled qudit states, where the photons are created in a coherent superposition of multiple high-purity frequency modes. In particular, we confirm the realization of a quantum system with at least one hundred dimensions, formed by two entangled qudits with D = 10. Furthermore, using state-of-the-art, yet off-the-shelf telecommunications components, we introduce a coherent manipulation platform with which to control frequency-entangled states, capable of performing deterministic high-dimensional gate operations. We validate this platform by measuring Bell inequality violations and performing quantum state tomography. Our work enables the generation and processing of high-dimensional quantum states in a single spatial mode.

Cross-polarized photon-pair generation and bi-chromatically pumped optical parametric oscillation on a chip

Nonlinear optical processes are one of the most important tools in modern optics with a broad spectrum of applications in, for example, frequency conversion, spectroscopy, signal processing and quantum optics. For practical and ultimately widespread implementation, on-chip devices compatible with electronic integrated circuit technology offer great advantages in terms of low cost, small footprint, high performance and low energy consumption. While many on-chip key components have been realized, to date polarization has not been fully exploited as a degree of freedom for integrated nonlinear devices. In particular, frequency conversion based on orthogonally polarized beams has not yet been demonstrated on chip. Here we show frequency mixing between orthogonal polarization modes in a compact integrated microring resonator and demonstrate a bi-chromatically pumped optical parametric oscillator. Operating the device above and below threshold, we directly generate orthogonally polarized beams, as well as photon pairs, respectively, that can find applications, for example, in optical communication and quantum optics.

Multifrequency sources of quantum correlated photon pairs on-chip: a path toward integrated Quantum Frequency Combs

Recent development in quantum photonics allowed to start the process of bringing photonic-quantum-based systems out of the lab into real world applications. As an example, devices for the exchange of a cryptographic key secured by the law of quantum mechanics are currently commercially available. In order to further boost this process, the next step is to migrate the results achieved by means of bulky and expensive setups to miniaturized and affordable devices. Integrated quantum photonics is exactly addressing this issue. In this paper we briefly review the most recent advancements in the generation of quantum states of light (at the core of quantum cryptography and computing) on chip. In particular, we focus on optical microcavities, as they can offer a solution to the issue of low efficiency (low number of photons generated) typical of the materials mostly used in integrated platforms. In addition, we show that specifically designed microcavities can also offer further advantages, such as compatibility with existing telecom standard (thus allowing to exploit the existing fiber network) and quantum memories (necessary in turns to extend the communication distance), as well as longitudinal multimode character. This last property (i.e. the increased dimensionality necessary for describing the quantum state of a photon) is achieved thanks to the generating multiple photon pairs on a frequency comb corresponding to the microcavity resonances. Further achievements include the possibility to fully exploit the polarization degree of freedom also for integrated devices. These results pave the way to the generation of integrated quantum frequency combs, that in turn may find application as quantum computing platform.Abstract Recent developments in quantum photonics have initiated the process of bringing photonic-quantumbased systems out-of-the-lab and into real-world applications. As an example, devices to enable the exchange of a cryptographic key secured by the laws of quantum mechanics are already commercially available. In order to further boost this process, the next step is to transfer the results achieved by means of bulky and expensive setups into miniaturized and affordable devices. Integrated quantum photonics is exactly addressing this issue. In this paper, we briefly review the most recent advancements in the generation of quantum states of light on-chip. In particular, we focus on optical microcavities, as they can offer a solution to the problem of low efficiency that is characteristic of the materials typically used in integrated platforms. In addition, we show that specifically designed microcavities can also offer further advantages, such as compatibility with telecom standards (for exploiting existing fibre networks) and quantum memories (necessary to extend the communication distance), as well as giving a longitudinal multimode character for larger information transfer and processing. This last property (i.e., the increased dimensionality of the photon quantum state) is achieved through the ability to generate multiple photon pairs on a frequency comb, corresponding to the microcavity resonances. Further achievements include the possibility of fully exploiting the polarization degree of freedom, even for integrated devices. These results pave the way for the generation of integrated quantum frequency combs that, in turn, may find important applications toward the realization of a compact quantum-computing platform.

Practical system for the generation of pulsed quantum frequency combs

The on-chip generation of large and complex optical quantum states will enable low-cost and accessible advances for quantum technologies, such as secure communications and quantum computation. Integrated frequency combs are on-chip light sources with a broad spectrum of evenly-spaced frequency modes, commonly generated by four-wave mixing in optically-excited nonlinear micro-cavities, whose recent use for quantum state generation has provided a solution for scalable and multi-mode quantum light sources. Pulsed quantum frequency combs are of particular interest, since they allow the generation of single-frequency-mode photons, required for scaling state complexity towards, e.g., multi-photon states, and for quantum information applications. However, generation schemes for such pulsed combs have, to date, relied on micro-cavity excitation via lasers external to the sources, being neither versatile nor power-efficient, and impractical for scalable realizations of quantum technologies. Here, we introduce an actively-modulated, nested-cavity configuration that exploits the resonance pass-band characteristic of the micro-cavity to enable a mode-locked and energy-efficient excitation. We demonstrate that the scheme allows the generation of high-purity photons at large coincidence-to-accidental ratios (CAR). Furthermore, by increasing the repetition rate of the excitation field via harmonic mode-locking (i.e. driving the cavity modulation at harmonics of the fundamental repetition rate), we managed to increase the pair production rates (i.e. source efficiency), while maintaining a high CAR and photon purity. Our approach represents a significant step towards the realization of fully on-chip, stable, and versatile sources of pulsed quantum frequency combs, crucial for the development of accessible quantum technologies.

Generation of multi-photon entangled states with integrated optical frequency comb sources

We demonstrate that four-photon entangled qubit states can be generated from an optical frequency combs source. This state generation is evidenced by four-photon quantum interference, and quantum state tomography reveals a fidelity above 64%.

On-chip entangled D-level photon states – scalable generation and coherent processing

Exploiting a micro-cavity-based quantum frequency comb, we demonstrate the on-chip generation of high-dimensional entangled quantum states with a Hilbert-space dimensionality larger than 100, and introduce a coherent control approach relying on standard telecommunications components.

Integrated generation of complex optical quantum states and their coherent control

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.

Generation and coherent control of pulsed quantum frequency combs

We present a method for the generation and coherent manipulation of pulsed quantum frequency combs. Until now, methods of preparing high-dimensional states on-chip in a practical way have remained elusive due to the increasing complexity of the quantum circuitry needed to prepare and process such states. Here, we outline how high-dimensional, frequency-bin entangled, two-photon states can be generated at a stable, high generation rate by using a nested-cavity, actively mode-locked excitation of a nonlinear micro-cavity. This technique is used to produce pulsed quantum frequency combs. Moreover, we present how the quantum states can be coherently manipulated using standard telecommunications components such as programmable filters and electro-optic modulators. In particular, we show in detail how to accomplish state characterization measurements such as density matrix reconstruction, coincidence detection, and single photon spectrum determination. The presented methods form an accessible, reconfigurable, and scalable foundation for complex high-dimensional state preparation and manipulation protocols in the frequency domain.

On-chip quantum state generation by means of integrated frequency combs

Entangled photon-pair sources are key building blocks towards the realization of applications in quantum information processing [1], quantum communications [2], as well as imaging and sensing with resolutions exceeding the classical limit [3]. The generation of, e.g. polarization, time-energy and time-bin entangled photon-pairs has been demonstrated using spontaneous parametric down-conversion (SPDC) in nonlinear second-order media, as well as spontaneous four-wave mixing (SFWM) in third-order nonlinear media. Specifically, nonlinear (third-order) interactions in on-chip microring resonators have been widely used to achieve classical frequency combs [4], mode-lock lasers [5], signal processing [6], etc. Integrated photonics can also find applications for quantum state generation in compact, scalable and efficient devices, required for future optical quantum circuits. In particular, solutions focusing on an integrated (on-chip) approach have been recently investigated and developed, including integrated quantum circuits, sources and detectors [7]. In contrast to waveguides, microring resonators [8] with narrow resonances and high Q-factors, offer an improvement in photon-pair generation efficiency, as well as a narrow photon-pair bandwidth, making them compatible with quantum optical devices (e.g. high temporal-resolution single-photon detectors and quantum memories). Most importantly, in contrast to non-resonant waveguides, where individuals photon-pairs, featured by one signal/idler frequency pair, are generally produced, resonant nonlinear cavities (e.g., microring resonators) allow the generation of correlated photon-pairs on multiple signal/idler frequency channels [9], due to their periodic and equidistant resonance structure.