Recent efforts have demonstrated the first prototypes for compact and programmable photonic quantum computers (PQCs). Utilization of time-bin encoding in loop-like architectures enabled programmable generation of quantum states and execution of different (programmable) logic gates on a single circuit. Actually, there is still space for better compactness and complexity of available quantum states and gate operations: a photonic circuit (PC) can operate at multiple frequencies. Here, we propose an electrically-programmable frequency comb that generates continuously tunable entanglement among different frequencies. The device is not directly integrated into fragile quantum processing components but is to be used as a fast (picoseconds) tunable auxiliary source provided into state-of-the-art measurement-induced loop-based photonic quantum computers employing programmable (50 MHz) beam-splitters (BSs) and phase-shifters. Multimode entanglement generation is controlled via Fano resonance in the nonlinear response. The generated entanglement can be tuned continuously via an applied voltage which can be delivered to the device via nm-thick wires. The proposed device is integrable, CMOS-compatible, and operates below ps but is limited with transistor clock speeds 5-100 GHz.
Recent efforts have demonstrated the first prototypes for compact and programmable photonic quantum computers (PQCs). Utilization of time-bin encoding in loop-like architectures enabled programmable generation of quantum states and execution of different (programmable) logic gates on a single circuit. Actually, there is still space for better compactness and complexity of available quantum states and gate operations: a photonic circuit (PC) can operate at multiple frequencies. Here, we propose an electrically-programmable frequency comb that generates continuously tunable entanglement among different frequencies. The device is not directly integrated into fragile quantum processing components but is to be used as a fast (picoseconds) tunable auxiliary source provided into state-of-the-art measurement-induced loop-based photonic quantum computers employing programmable (50 MHz) beam-splitters (BSs) and phase-shifters. Multimode entanglement generation is controlled via Fano resonance in the nonlinear response. The generated entanglement can be tuned continuously via an applied voltage which can be delivered to the device via nm-thick wires. The proposed device is integrable, CMOS-compatible, and operates below ps but is limited with transistor clock speeds 5-100 GHz.
Quantum optical microcombs in integrated ring resonators generate entangled photon pairs over many spectral modes, and allow the preparation of high dimensional qudit states. Ideally, those sources should be programmable and have a high generation rate, with comb lines tightly spaced for the implementation of efficient qudit gates based on electro-optic frequency mixing. While these requirements cannot all be satisfied by a single resonator device, for which there is a trade-off between high generation rate and tight bin spacing, a promising strategy is the use of multiple resonators, each generating photon pairs in specific frequency bins via spontaneous four-wave mixing. Based on this approach we present a programmable silicon photonics device for the generation of frequency bin entangled qudits, in which bin spacing, qudit dimension, and bipartite quantum state can be reconfigured on-chip. Using resonators with a radius of 22 microns, we achieve a high brightness (MHz/(mW)^2) per comb line with a bin spacing of 15 GHz, and fidelities above 85% with maximally entangled Bell states up to a Hilbert space dimension of sixteen. By individually addressing each spectral mode, we realize states that can not be generated on-chip using a single resonator. We measure the correlation matrices of maximally entangled two-qubit and two-qutrit states on a set of mutually unbiased bases, finding fidelities exceeding 98%, and indicating that the source can find application in high-dimensional secure communication protocols.
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.
Quantum optical microcombs in integrated ring resonators generate entangled photon pairs over many spectral modes, and allow the preparation of high-dimensional qudit states. Ideally, those sources should be programmable and have a high generation rate, with comb lines tightly spaced for the implementation of efficient qudit gates based on electro-optic frequency mixing. While these requirements cannot all be satisfied by a single resonator device, for which there is a trade-off between the high generation rate and tight bin spacing, a promising strategy is the use of multiple resonators, each generating photon pairs in specific frequency bins via spontaneous four-wave mixing. Based on this approach we present a programmable silicon photonics device for the generation of frequency-bin-entangled qudits, in which bin spacing, qudit dimension, and the bipartite quantum state can be reconfigured on chip. Using resonators with a radius of $22\phantom{\rule{0.2em}{0ex}}\text{\ensuremath{\mu}}\mathrm{m}$, we achieve a high brightness [about MHz/(mW)2] per comb line with a bin spacing of 15 GHz, and fidelities above 85% with maximally entangled Bell states up to a Hilbert space dimension of 16. By individually addressing each spectral mode, we realize states that cannot be generated on chip using a single resonator. We measure the correlation matrices of maximally entangled two-qubit and two-qutrit states on a set of mutually unbiased bases, finding fidelities exceeding 98%, and indicating that the source can find application in high-dimensional secure communication protocols.
Frequency-bin encoding has recently emerged as a powerful approach for photonic quantum information processing, offering high dimensionality, gate-parallelization, and compatibility with existing telecommunication infrastructure. However, its scalable deployment has so far been hindered by the lack of an integrated platform capable of unifying quantum state generation, coherent frequency mixing, and programmable spectral control.\\ Here, we report the first fully integrated quantum frequency processor, monolithically integrating on the same silicon photonic chip a microresonator-based biphoton quantum frequency comb source, a pump-rejection filter, high-speed phase modulators, and a four-channel, line-by-line pulse shaper. We demonstrate key functionalities, such as tunable frequency beamsplitters with success probabilities exceeding 94% and fidelities above 99.9%, as well as the ability to synthesize more general single-qubit gates. Finally, we generate and coherently manipulate high-dimensional frequency-bin entangled states entirely on chip, showcasing control over two-photon quantum walks and performing the first on-chip frequency-bin quantum state tomography of a Bell-state with a fidelity of 95.7(3)%. By integrating all key functional elements on the same 4×7mm2 chip, with the possibility of scaling to a larger number of modes, our work marks an important step toward large-scale frequency-domain photonic processors for both classical and quantum applications.
Frequency bin encoding is becoming of widespread use in photonic quantum applications due to its strong noise resilience, the large information content that can be encoded per photon, and the inherent parallelism through which gates can be applied over multiple frequency modes. Ideally, sources of frequency bin qubits and qudits should be programmable, have high brightness, and have tightly spaced spectral modes to be useful for quantum algorithms. Here we present an integrated photonic device that simultaneously met all these requirement. We exploit the coherent generation of photon pairs over multiple ring resonators to overcome the trade-offs that hurdle single resonators. Harnessing the dense integration and phase stability of silicon photonics, we demonstrate a photon pair source of frequency bin qudits and qudits with simultaneously high brightness, fidelity, and purity.
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.