Integrated generation of complex optical quantum states and their coherent control
Piotr Roztocki, Michael Kues, Christian Reimer, Luis Romero Cortés, Stefania Sciara, Benjamin Wetzel, Yanbing Young Zhang, Alfonso Carmelo Cino, Sai Chu, Brent E. Little, David Moss, Lucia Caspani, José Azaña, Roberto Morandotti
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.
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.
Entangled optical quantum states are essential towards solving questions in fundamental physics and are at the heart of applications in quantum information science. For advancing the research and development of quantum technologies, practical access to the generation and manipulation of photon states carrying significant quantum resources is required. Recently, integrated photonics has become a leading platform for the compact and cost-efficient generation and processing of optical quantum states. Despite significant advances, most on-chip nonclassical light sources are still limited to basic bi-photon systems formed by two-dimensional states (i.e., qubits). An interesting approach bearing large potential is the use of the time or frequency domain to enabled the scalable onchip generation of complex states. In this manuscript, we review recent efforts in using on-chip optical frequency combs for quantum state generation and telecommunications components for their coherent control. In particular, the generation of bi- and multi-photon entangled qubit states has been demonstrated, based on a discrete time domain approach. Moreover, the on-chip generation of high-dimensional entangled states (quDits) has recently been realized, wherein the photons are created in a coherent superposition of multiple pure frequency modes. The time- and frequency-domain states formed with on-chip frequency comb sources were coherently manipulated via off-the-shelf telecommunications components. Our results suggest that microcavity-based entangled photon states and their coherent control using accessible telecommunication infrastructures can open up new venues for scalable quantum information science.
The investigation of integrated frequency comb sources characterized by equidistant spectral modes was initially driven by considerations toward classical applications, seeking a more practical and miniaturized way to generate stable broadband sources of light. Recently, in the context of scaling the complexity of optical quantum circuits, these on-chip approaches have provided a new framework to address the challenges associated with non-classical state generation and manipulation. For example, multi-photon and high-dimensional states were to date either inaccessible, lacked scalability, or were difficult to manipulate, requiring elaborate approaches. The emerging field of quantum frequency combs studying spectral multimode sources based on the judicious excitation of (typically) third-order nonlinear optical micro-cavities has begun to address these issues. Several quantum sources based on this concept have already been demonstrated, among them are combs of correlated photons, cross-polarized photon pairs, entangled photon pairs, multi-photon states, and high-dimensional entangled states. While sources have achieved increasing complexity, so have coherent state processing operations, demonstrated in a practical manner using standard telecommunications components. Here, we review our recent work in the development of this framework, with a focus on multi-photon and high-dimensional states. The integrated frequency comb platform thus demonstrates significant potential for the development of meaningful quantum optical 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.
The investigation and use of integrated frequency comb sources (i.e. featured by equally-spaced discrete spectral modes) have recently provided a unique framework to address the challenges of generation and coherent manipulation of complex quantum states in on-chip devices. We exploit integrated frequency combs for generating entangled photon pairs, as well as multi-photon states, and high-dimensional (D-level, i.e. quDit) entangled photons. In particular, we manage to coherently manipulate such complex quantum systems by using telecommunications components (standard fiber telecom).
The generation of optical quantum states on an integrated platform will enable low cost and accessible advances for quantum technologies such as secure communications and quantum computation. We demonstrate that integrated quantum frequency combs (based on high-Q microring resonators made from a CMOS-compatible, high refractive-index glass platform) can enable, among others, the generation of 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, constituting an important cornerstone for future practical implementations of photonic quantum information processing.
Abstract Integrated photonics has recently become a leading platform for the realization and processing of optical entangled quantum states in compact, robust, and scalable chip formats with applications in long-distance quantum-secured communication, quantum-accelerated information processing, and non-classical metrology. However, the quantum light sources developed so far have relied on external bulky excitation lasers making them impractical, not reproducible prototype devices, hindering scalability and the transfer out of the lab into real-world applications. Here we demonstrate a fully integrated quantum light source, which overcomes these challenges through the combined integration of a laser cavity, a highly efficient tunable noise suppression filter (> 55 dB) exploiting the Vernier effect, and a nonlinear microring for entangled photon pair generation through spontaneous four-wave mixing. The hybrid quantum source employs an electrically-pumped InP gain section and a Si₃N₄ low-loss microring filter system and demonstrates high-performance parameters, i.e., a pair emission over four resonant modes in the telecom band (bandwidth ∼ 1 THz), and a remarkable pair detection rate of ∼ 620 Hz at a high coincidence-to-accidental ratio of ∼80. The source directly creates high-dimensional frequency-bin entangled quantum states (qubits/qudits), verified by quantum interference measurements with visibilities up to 96% (violating Bell-inequality) and by density matrix reconstruction through state tomography showing fidelities of up to 99%. Our approach, leveraging a hybrid photonic platform, enables commercial-viable, low-cost, compact, light-weight, and field-deployable entangled quantum sources, quintessential for practical, out-of-lab applications, e.g., in quantum processors and quantum satellite communications systems.