This PDF file contains the front matter associated with SPIE Proceedings Volume 12015, including the Title Page, Copyright information, Table of Contents and Conference Committee lists.

On-chip generation, manipulation and detection quantum states of light with integrated photonic circuits opens the way to realizing complex quantum technologies for applications in the fields of computing, simulation and communication. In this talk we present recent progress in how to generate and control quantum entanglement with silicon quantum photonic devices for quantum information processing.

Photons are preferable carriers of quantum information having little interaction with environment. Most crucial elements of photonic quantum information processing are two qubit gates. Although usage of controlled-not gates for photonic two qubit operations is common, controlled-phase gates provide more versatile utilization by enabling desired phase difference to be generated between qubits. Ability to determine the phase after fabrication yields extra benefit. We demonstrate a design of integrated photonic controlled-phase gate with programmable phase difference. We used a unique and practical polarization rotator which is ring resonator with a single notch.

Quantum systems have highly variable requirements and tailored solutions depending on the technology. For instance, lasers are used for quantum control of particles or molecules and excitation for quantum systems. For the latter purpose, suitable applications are frequency comb generation, timing sources for picosecond pulses, and single photon emitters1,2,3. Key performance parameters for lasers used in this area are single-mode operation, narrow linewidth, and frequency stability. Wavelength of the lasers can vary from UV to NIR, as well as the optical output power. Narrow linewidth and frequency stability can be achieved with distributed feedback (DFB) or distributed Bragg reflector (DBR) lasers, when quarter-lambda phase shift is included, or with external cavities.4 For enhancing performance and flexibility, such laser diodes can be fabricated as arrays, or even individually addressable laser diode arrays (IABs).5 In this work, we report the state-of-the-art diode laser arrays scalable beyond 100 emitters per array, operating in single mode from 900 nm to 930 nm, enabling new possibilities for rising quantum system applications. Our array design enables high power levels per emitter and dense emitter pitch varying from 20 µm to 100 µm, offering opportunities for dense device design and flexibility for applications. The emitters show excellent uniformity and beam profiles throughout the array. Implementing individually addressable emitters to our design offers opportunities for improved control for more complex and demanding applications.

We study a quantum switch that creates shared end-to-end entangled quantum states to multiple sets of users that are connected to it. Each user is connected to the switch via an optical link across which bipartite Bell-state entangled states are generated in each time-slot with certain probabilities, and the switch merges entanglements of links to create end-to-end entanglements for users. One qubit of an entanglement of a link is stored at the switch and the other qubit of the entanglement is stored at the user corresponding to the link. Assuming that qubits of entanglements of links decipher after one time-slot, we characterize the capacity region, which is defined as the set of arrival rates of requests for end-to-end entanglements for which there exists a scheduling policy that stabilizes the switch. We propose a Max-Weight scheduling policy and show that it stabilizes the switch for all arrival rates that lie in the capacity region. We also provide numerical results to support our analysis.

We present a novel device for the efficient, low-noise down-conversion of low-power light from the red visible spectrum. It can be applied to interface electronic spin-qubits in nitrogen-vacancy centers in diamond to the telecommunication wavelengths and thus offers the possibility to implement large-scale quantum networks in existing fiber infrastructure. In comparison to state-of-the-art frequency converters for this application, the device presented here generates an overall noise rate, which is one order of magnitude better than current record values. Simultaneously, an internal conversion efficiency of 50 % was measured. Furthermore, the converter serves as a feasibility demonstration of a general concept for the down-conversion of light from the sub-micron spectral range to telecommunication wavelengths.

Piezoelectric optomechanical platforms represent one of the most promising routes towards achieving quantum transduction of photons between the microwave and optical frequency domains. However, there are significant challenges to achieving near-unity transduction efficiency in these transducers. In this talk, we will discuss some of these key limitations and engineering approaches to overcome them.

Microwave and optical photons are two principle carriers for quantum information. Microwave photons can be effectively manipulated by superconducting circuits at milli-Kelvin environments; optical photons transmit information over long distances in optical fibers. Therefore, microwave-to-optical (MO) quantum converters, which interface superconducting qubits and optical photons, represent an indispensable component in future quantum networks. Here, we present our recent efforts on developing integrated gigahertz piezo-optomechanics and electro-optics (EO) MO converters. We demonstrate efficient bi-directional conversion in both schemes at cryogenic temperatures. Moreover, we realized MO conversion with the device in the quantum ground state, while achieving efficient conversion efficiency. Our results represent a substantial step towards faithful microwave to optical quantum conversions.

High-performance quantum memories are a key component for quantum networking infrastructure. Here we demonstrate the first field-deployable, high-fidelity (<95%) memory for photonic polarization qubits based on electromagnetically induced transparency in a warm rubidium vapor. We discuss our work towards optimizing the memory for use in quantum networking applications, specifically to maximize the memory fidelity, efficiency and coherence time. We also describe our engineering efforts in scaling down the size of the memory into a turn-key 2U rackmount device. The module has been designed and tested for use in both academic and industrial settings.

Quantum key distribution (QKD) is one of the most mature among the quantum technologies that allows two remote users to generate secret keys with unconditional security. To increase its adoption, simple, low-cost, and robust systems are necessary, together with demonstrations in real environments. Here, we present a QKD field-trial over optical fibers deployed in the city center of Padua, Italy. Our system exploits two key technologies developed by our group: a low-error, self-stabilized polarization encoder, called iPOGNAC, and a novel synchronization technique, called Qubit4Sync, which allows us to minimize the experimental complexity of our system.

Stabilized cat qubits that possess biased noise channel with bit-flip errors exponentially smaller than phase-flip errors. Together with a set of bias-preserving (BP) gates, cat qubits are a promising candidate for realizing hardware efficient quantum error correction and fault-tolerant quantum computing. Compared to dissipatively stabilized cat qubits, the Kerr cat qubits can in principle support faster gate operations with higher gate fidelity, benefiting from the large energy gap that protects the code space. However, the leakage of the Kerr cats can increase the minor type of errors and compromise the noise bias. Both the fast implementation of gates and the interaction with environment can lead to such detrimental leakage if no sophisticated controls are applied. In this work, we introduce new fine-control techniques to overcome the above obstacles for Kerr cat qubits. To suppress the gate leakage, we use the derivative-based transition suppression technique to design derivative-based controls for the Kerr BP gates. We show that the fine-controlled gates can simultaneously have high gate fidelity and high noise bias and when applied to concatenated quantum error correction, can not only improve the logical error rate but also reduce resource overhead. To suppress the environment-induced leakage, we introduce colored single-photon dissipation, which can continuously cool the Kerr cats and suppress the minor errors while not enhancing the major errors.

Fiber-based entangled photon sources are a key resource in emerging quantum technologies due to compactness, long-term stability and alignment-free operation. Fast control of the quantum state properties is also necessary in practical scenarios, where a universal source can be employed with different temporal and spectral configurations. Here, we report on a flexible source design that produces high-quality entanglement from continuous-wave up to GHz-pulsed operation modes. Our approach uses off-the-shelf optical components in a Sagnac configuration to generate polarization-entangled photon pairs at telecom wavelength. This design further allows phase modulation above MHz speeds with high fidelity. Together with its 60 nm spectral bandwidth, the proposed system is entirely suitable for wavelength-multiplexed and reconfigurable quantum networks, where state modulation or active bandwidth allocation is required.

We discuss two novel entanglement sources utilizing spontaneous parametric downconversion in periodically poled waveguides. Using quasi-phase matched KTP crystals, we have demonstrated a post-selection-based polarization entangled degenerate source at 810 nm, as well as a post-selection-free non-degenerate collinear source producing entangled photons at 810 nm and 1550 nm. The sources exhibit high brightness and state quality – with the non-degenerate source achieving fidelities and purities up to 99% – with clear paths for further improvement. Furthermore, they are compact, stable, and need little alignment when set, critical for practical quantum communication and network applications. Lastly, their small size, weight, and power (SWaP) makes them an attractive option for mobile platforms, e.g., with drones or satellites.

Recent proposals suggest that distributed single photons serving as a ‘non-local oscillator’ can outperform coherent states as a phase reference for long-baseline interferometric imaging of weak sources [1,2]. Such nonlocal quantum states distributed between telescopes can, in-principle, surpass the limitations of conventional interferometric-based astronomical imaging approaches for very-long baselines such as: signal-to-noise, shot noise, signal loss, and faintness of the imaged objects. Here we demonstrate in a table-top experiment, interference between a nonlocal oscillator generated by equal-path splitting of an idler photon from a pulsed, separable, parametric down conversion process and a spectrally single-mode, quasi-thermal source. We compare the single-photon nonlocal oscillator to a more conventional local oscillator with uncertain photon number. Both methods enabled reconstruction of the source’s Gaussian spatial distribution by measurement of the interference visibility as a function of baseline separation and then applying the van Cittert-Zernike theorem [3,4]. In both cases, good qualitative agreement was found with the reconstructed source width and the known source width as measured using a camera. We also report an increase of signal-to-noise per ‘faux’ stellar photon detected when heralding the idler photon. 1593 heralded (non-local oscillator) detection events led to a maximum visibility of ~17% compared to the 10412 unheralded (classical local oscillator) detection events, which gave rise to a maximum visibility of ~10% – the first instance of quantum-enhanced sensing in this context.

The quantum Internet enables networking based on the fundamentals of quantum mechanics. Here, we define methods and procedures of resource prioritization and resource balancing for the quantum Internet. The aim of the proposed solutions is to optimize the resource allocation mechanisms and to reduce the resource consumptions of the network entities.