Environmental noise and disorder play a critical role in quantum particle and wave transport in complex media, including solid-state and biological systems. Recent work has predicted that coupling between noisy environments and disordered systems, in which coherent transport has been arrested due to localization effects, could actually enhance transport. Photonic integrated circuits are promising platforms for studying such effects, with a central goal being the development of large systems providing low-loss, high-fidelity control over all parameters of the transport problem. Here, we fully map out the role of static and dynamic disorder in quantum transport using a low-loss, phase-stable, nanophotonic processor consisting of a mesh of 56 generalized beamsplitters programmable on microsecond timescales. Over 85,600 transport experiments, we observe several distinct transport regimes, including environment-enhanced transport in strong, statically disordered systems. Low loss and programmability make this nanophotonic processor a promising platform for many-boson quantum simulation experiments.Quantum walks (QWs), the coherent analogy to classical random walks, have emerged as a useful model for experimental simulations of quantum transport (QT) phenomena in physical systems. QWs have been implemented in platforms including trapped ions 1,2 , ultra-cold atoms 3 , bulk optics 4-8 and integrated photonics 4,9-16 . Integrated photonic implementations are particularly attractive for relatively large coherence lengths, high interferometric visibilities, integration with single-photon sources 17,18 and detectors 19 , and the promise of scaling to many active and reconfigurable components. The role of static and dynamic disorder in the transport of quantum walkers has been of particular interest in the field of quantum simulation 20,21 .Control over static (time-invariant) and dynamic (timevarying) disorder enables studies of fundamentally interesting and potentially useful QT phenomena in discrete-time (DT) QWs. In systems with strong dynamic disorder, illustrated in Fig. 1(a), a quantum walker evolving over T time steps travels a distance proportional to √ T ; the coherent nature of the quantum walker is effectively erased, resulting in classical, diffusive transport characteristics 22,23 . In contrast, a quantum walker (or coherent wave) traversing an ordered system travels a distance proportional to T as a result of coherent interference between superposition amplitudes -a regime known as ballistic transport (see Fig. 1(b)). Perhaps most notably, a quantum walker propagating through a system with strong, static disorder becomes exponentially localized in space and time, inhibiting transport, as illustrated in Fig. 1(c). This QT phenomena is known as Anderson localization 24 and it has been observed in several systems, including optical media [9][10][11]25,26 . For systems in which transport has been arrested due to Anderson localization, it has recently been predicted that adding environmental noise (dynamic disord...

We report a technique for encoding both amplitude and phase variations onto a laser beam using a single digital micro-mirror device (DMD). Using this technique, we generate Laguerre-Gaussian and vortex orbital-angular-momentum (OAM) modes, along with modes in a set that is mutually unbiased with respect to the OAM basis. Additionally, we have demonstrated rapid switching among the generated modes at a speed of 4 kHz, which is much faster than the speed regularly achieved by phase-only spatial light modulators (SLMs). The dynamic control of both phase and amplitude of a laser beam is an enabling technology for classical communication and quantum key distribution (QKD) systems that employ spatial mode encoding.

Photonic integrated circuits (PICs) provide a compact and stable platform for quantum photonics.Here we demonstrate a silicon photonics quantum key distribution (QKD) transmitter in the first high-speed polarization-based QKD field tests. The systems reach composable secret key rates of 950 kbps in a local test (on a 103.6-m fiber with a total emulated loss of 9.2 dB) and 106 kbps in an intercity metropolitan test (on a 43-km fiber with 16.4 dB loss). Our results represent the highest secret key generation rate for polarization-based QKD experiments at a standard telecom wavelength and demonstrate PICs as a promising, scalable resource for future formation of metropolitan quantum-secure communications networks.Quantum key distribution (QKD) remains the only quantum-resistant method of sending secret information at a distance [1,2]. The first QKD system ever devised used polarization of photons to encode information [3,4]. QKD has since progressed rapidly to several deployed systems that can reach point-to-point secret key generation rates in the upwards of 100 kbps [5][6][7][8] and to other photonic degrees of freedom: time [9][10][11][12], frequency [13][14][15][16], phase [17], quadrature [18][19][20][21], and orbital angular momentum [22]. While polarization remains an attractive choice for free-space QKD due to its robustness against turbulence [23][24][25][26][27][28], polarization is commonly thought to be unstable for fiber-based QKD. For this reason, there has been a strong interest in translating the polarization QKD components into photonic integrated circuits (PICs), which provide a compact and phase-stable platform capable of correcting for polarization drifts in the channel. Recently, silicon-based polarization QKD transmitters were used for laboratory QKD demonstrations [29,30], but their performance advantage over standard telecommunication components has yet to be demonstrated. Here we report the first field tests using high-speed silicon photonics-based transmitter for polarization-encoded QKD.The silicon photonics platform allows for the integration of multiple high-speed photonic operations into a single compact circuit [31][32][33][34]. Operating at gigahertz bandwidth, a silicon photonics polarization QKD transmitter can correct for polarization drifts with typical millisecond time scales in a metropolitan-scale fiber link. Furthermore, silicon nanophotonic devices are compatible with the existing complementary metal-oxidesemiconductor (CMOS) processes that have enabled monolithic integration of photonics and electronics, possibly leading to future widespread utilization of QKD.The QKD transmitter demonstrated here is manufactured using a CMOS-compatible process. The trans-mitter combines a 10-Gbps Mach-Zehnder Modulator (MZM) with interleaved grating couplers, which convert the polarization of a photon in an optical fiber into the path the photon takes in the integrated circuit, and vice versa. The high-speed polarization control is enabled by electro-optic carrier depletion modulation withi...

Spectrally unentangled biphotons with high single-spatiotemporal-mode purity are highly desirable for many quantum information processing tasks. We generate biphotons with an inferred heralded-state spectral purity of 99%, the highest to date without any spectral filtering, by pulsed spontaneous parametric downconversion in a custom-fabricated periodically-poled KTiOPO₄ crystal under extended Gaussian phase-matching conditions. To efficiently characterize the joint spectral intensity of the generated biphotons at high spectral resolution, we employ a commercially available dispersion compensation module (DCM) with a dispersion equivalent to 100 km of standard optical fiber and with an insertion loss of only 2.8 dB. Compared with the typical method of using two temperature-stabilized equal-length fibers that incurs an insertion loss of 20 dB per fiber, the DCM approach achieves high spectral resolution in a much shorter measurement time. Because the dispersion amount and center wavelengths of DCMs can be easily customized, spectral characterization in a wide range of quantum photonic applications should benefit significantly from this technique.

Quantum algorithms for Noisy Intermediate-Scale Quantum (NISQ) machines have recently emerged as new promising routes towards demonstrating near-term quantum advantage (or supremacy) over classical systems. In these systems samples are typically drawn from probability distributions which -under plausible complexity-theoretic conjectures -cannot be efficiently generated classically. Rather than first define a physical system and then determine computational features of the output state, we ask the converse question: given direct access to the quantum state, what features of the generating system can we efficiently learn? In this work we introduce the Variational Quantum Unsampling (VQU) protocol, a nonlinear quantum neural network approach for verification and inference of near-term quantum circuits outputs. In our approach one can variationally train a quantum operation to unravel the action of an unknown unitary on a known input state; essentially learning the inverse of the black-box quantum dynamics. While the principle of our approach is platform independent, its implementation will depend on the unique architecture of a specific quantum processor. Here, we experimentally demonstrate the VQU protocol on a quantum photonic processor. Alongside quantum verification, our protocol has broad applications; including optimal quantum measurement and tomography, quantum sensing and imaging, and ansatz validation.