Showing papers on "Coherence (physics) published in 2021"

Demonstration of quantum volume 64 on a superconducting quantum computing system

Abstract: We improve the quality of quantum circuits on superconducting quantum computing systems, as measured by the quantum volume, with a combination of dynamical decoupling, compiler optimizations, shorter two-qubit gates, and excited state promoted readout. This result shows that the path to larger quantum volume systems requires the simultaneous increase of coherence, control gate fidelities, measurement fidelities, and smarter software which takes into account hardware details, thereby demonstrating the need to continue to co-design the software and hardware stack for the foreseeable future.

Single ion qubit with estimated coherence time exceeding one hour.

Abstract: Realizing a long coherence time quantum memory is a major challenge of current quantum technology. Until now, the longest coherence-time of a single qubit was reported as 660 s in a single 171Yb+ ion-qubit through the technical developments of sympathetic cooling and dynamical decoupling pulses, which addressed heating-induced detection inefficiency and magnetic field fluctuations. However, it was not clear what prohibited further enhancement. Here, we identify and suppress the limiting factors, which are the remaining magnetic-field fluctuations, frequency instability and leakage of the microwave reference-oscillator. Then, we observe the coherence time of around 5500 s for the 171Yb+ ion-qubit, which is the time constant of the exponential decay fit from the measurements up to 960 s. We also systematically study the decoherence process of the quantum memory by using quantum process tomography and analyze the results by applying recently developed resource theories of quantum memory and coherence. Our experimental demonstration will accelerate practical applications of quantum memories for various quantum information processing, especially in the noisy-intermediate-scale quantum regime. Extending qubit coherence times represent one of the key challenges for quantum technologies. Here, after properly suppressing magnetic-field fluctuations, frequency instability and leakage of the microwave reference-oscillator, the authors infer coherence times of 5500 s for an Yb ion qubit.

Ultrafast hole spin qubit with gate-tunable spin–orbit switch functionality

Abstract: Quantum computers promise to execute complex tasks exponentially faster than any possible classical computer, and thus spur breakthroughs in quantum chemistry, material science and machine learning. However, quantum computers require fast and selective control of large numbers of individual qubits while maintaining coherence. Qubits based on hole spins in one-dimensional germanium/silicon nanostructures are predicted to experience an exceptionally strong yet electrically tunable spin–orbit interaction, which allows us to optimize qubit performance by switching between distinct modes of ultrafast manipulation, long coherence and individual addressability. Here we used millivolt gate voltage changes to tune the Rabi frequency of a hole spin qubit in a germanium/silicon nanowire from 31 to 219 MHz, its driven coherence time between 7 and 59 ns, and its Lande g-factor from 0.83 to 1.27. We thus demonstrated spin–orbit switch functionality, with on/off ratios of roughly seven, which could be further increased through improved gate design. Finally, we used this control to optimize our qubit further and approach the strong driving regime, with spin-flipping times as short as ~1 ns. Quantum computing requires fast and selective control of a large number of individual qubits while maintaining coherence, which is hard to achieve concomitantly. All-electrical operation of a hole spin qubit in a Ge/Si nanowire demonstrates the principle of switching from a mode of selective and fast control to idling with increased coherence.

Optical coherence encryption with structured random light.

Abstract: Information encryption with optical technologies has become increasingly important due to remarkable multidimensional capabilities of light fields However, the optical encryption protocols proposed to date have been primarily based on the first-order field characteristics, which are strongly affected by interference effects and make the systems become quite unstable during light-matter interaction Here, we introduce an alternative optical encryption protocol whereby the information is encoded into the second-order spatial coherence distribution of a structured random light beam via a generalized van Cittert-Zernike theorem We show that the proposed approach has two key advantages over its conventional counterparts First, the complexity of measuring the spatial coherence distribution of light enhances the encryption protocol security Second, the relative insensitivity of the second-order statistical characteristics of light to environmental noise makes the protocol robust against the environmental fluctuations, eg, the atmospheric turbulence We carry out experiments to demonstrate the feasibility of the coherence-based encryption method with the aid of a fractional Fourier transform Our results open up a promising avenue for further research into optical encryption in complex environments

Polarization entanglement-enabled quantum holography

Abstract: Holography is a cornerstone characterization and imaging technique that can be applied to the full electromagnetic spectrum, from X-rays to radio waves or even particles such as neutrons. The key property in all these holographic approaches is coherence, which is required to extract the phase information through interference with a reference beam. Without this, holography is not possible. Here we introduce a holographic imaging approach that operates on first-order incoherent and unpolarized beams, so that no phase information can be extracted from a classical interference measurement. Instead, the holographic information is encoded in the second-order coherence of entangled states of light. Using spatial-polarization hyper-entangled photon pairs, we remotely reconstruct phase images of complex objects. Information is encoded into the polarization degree of the entangled state, allowing us to image through dynamic phase disorder and even in the presence of strong classical noise, with enhanced spatial resolution compared with classical coherent holographic systems. Beyond imaging, quantum holography quantifies hyper-entanglement distributed over 104 modes via a spatially resolved Clauser–Horne–Shimony–Holt inequality measurement, with applications in quantum state characterization. By exploiting polarization entanglement between photons, quantum holography can circumvent the need for first-order coherence that is vital to classical holography.

One-hour coherent optical storage in an atomic frequency comb memory.

Abstract: Photon loss in optical fibers prevents long-distance distribution of quantum information on the ground. Quantum repeater is proposed to overcome this problem, but the communication distance is still limited so far because of the system complexity of the quantum repeater scheme. Alternative solutions include transportable quantum memory and quantum-memory-equipped satellites, where long-lived optical quantum memories are the key components to realize global quantum communication. However, the longest storage time of the optical memories demonstrated so far is approximately 1 minute. Here, by employing a zero-first-order-Zeeman magnetic field and dynamical decoupling to protect the spin coherence in a solid, we demonstrate coherent storage of light in an atomic frequency comb memory over 1 hour, leading to a promising future for large-scale quantum communication based on long-lived solid-state quantum memories. Quantum memories are key components for quantum communication, but current storage times are still too short. Here, the authors use the atomic frequency comb protocol in a zero-first-order-Zeeman field to coherently store an optical pulse for an hour in a cryogenically cooled rare-earth doped crystal.

Distributed quantum phase estimation with entangled photons

Abstract: Distributed quantum metrology can enhance the sensitivity for sensing spatially distributed parameters beyond the classical limits. Here we demonstrate distributed quantum phase estimation with discrete variables to achieve Heisenberg limit phase measurements. Based on parallel entanglement in modes and particles, we demonstrate distributed quantum sensing for both individual phase shifts and an averaged phase shift, with an error reduction up to 1.4 dB and 2.7 dB below the shot-noise limit. Furthermore, we demonstrate a combined strategy with parallel mode entanglement and multiple passes of the phase shifter in each mode. In particular, our experiment uses six entangled photons with each photon passing the phase shifter up to six times, and achieves a total number of photon passes N=21 at an error reduction up to 4.7 dB below the shot-noise limit. Our research provides a faithful verification of the benefit of entanglement and coherence for distributed quantum sensing in general quantum networks.

Reducing the impact of radioactivity on quantum circuits in a deep-underground facility.

Abstract: As quantum coherence times of superconducting circuits have increased from nanoseconds to hundreds of microseconds, they are currently one of the leading platforms for quantum information processing. However, coherence needs to further improve by orders of magnitude to reduce the prohibitive hardware overhead of current error correction schemes. Reaching this goal hinges on reducing the density of broken Cooper pairs, so-called quasiparticles. Here, we show that environmental radioactivity is a significant source of nonequilibrium quasiparticles. Moreover, ionizing radiation introduces time-correlated quasiparticle bursts in resonators on the same chip, further complicating quantum error correction. Operating in a deep-underground lead-shielded cryostat decreases the quasiparticle burst rate by a factor thirty and reduces dissipation up to a factor four, showcasing the importance of radiation abatement in future solid-state quantum hardware.

Demonstration of the trapped-ion quantum CCD computer architecture

Abstract: The trapped-ion quantum charge-coupled device (QCCD) proposal1,2 lays out a blueprint for a universal quantum computer that uses mobile ions as qubits. Analogous to a charge-coupled device (CCD) camera, which stores and processes imaging information as movable electrical charges in coupled pixels, a QCCD computer stores quantum information in the internal state of electrically charged ions that are transported between different processing zones using dynamic electric fields. The promise of the QCCD architecture is to maintain the low error rates demonstrated in small trapped-ion experiments3–5 by limiting the quantum interactions to multiple small ion crystals, then physically splitting and rearranging the constituent ions of these crystals into new crystals, where further interactions occur. This approach leverages transport timescales that are fast relative to the coherence times of the qubits, the insensitivity of the qubit states of the ion to the electric fields used for transport, and the low crosstalk afforded by spatially separated crystals. However, engineering a machine capable of executing these operations across multiple interaction zones with low error introduces many difficulties, which have slowed progress in scaling this architecture to larger qubit numbers. Here we use a cryogenic surface trap to integrate all necessary elements of the QCCD architecture—a scalable trap design, parallel interaction zones and fast ion transport—into a programmable trapped-ion quantum computer that has a system performance consistent with the low error rates achieved in the individual ion crystals. We apply this approach to realize a teleported CNOT gate using mid-circuit measurement6, negligible crosstalk error and a quantum volume7 of 26 = 64. These results demonstrate that the QCCD architecture provides a viable path towards high-performance quantum computers. The quantum charge-coupled device architecture is demonstrated, with its various elements integrated into a programmable trapped-ion quantum computer and performing simple quantum operations with state-of-the-art levels of error.

Engineering high-coherence superconducting qubits

Abstract: Advances in materials science and engineering have played a central role in the development of classical computers and will undoubtedly be critical in propelling the maturation of quantum information technologies. In approaches to quantum computation based on superconducting circuits, as one goes from bulk materials to functional devices, amorphous films and non-equilibrium excitations — electronic and phononic — are introduced, leading to dissipation and fluctuations that limit the computational power of state-of-the-art qubits and processors. In this Review, the major sources of decoherence in superconducting qubits are identified through an exploration of seminal qubit and resonator experiments. The proposed microscopic mechanisms associated with these imperfections are summarized, and directions for future research are discussed. The trade-offs between simple qubit primitives based on a single Josephson tunnel junction and more complex designs that use additional circuit elements, or new junction modalities, to reduce sensitivity to local noise sources are discussed, particularly in the context of materials optimization strategies for each architecture. Superconducting qubits hold great promise for quantum computing, and recently there have been dramatic improvements in both coherence times and the power of quantum processors. This Review explores how the path forward involves balancing circuit complexity and materials perfection, eliminating defects while designing qubits with engineered noise resilience.

A bright and fast source of coherent single photons

Abstract: A single-photon source is an enabling technology in device-independent quantum communication1, quantum simulation2,3, and linear optics-based4 and measurement-based quantum computing5. These applications employ many photons and place stringent requirements on the efficiency of single-photon creation. The scaling on efficiency is typically an exponential function of the number of photons. Schemes taking full advantage of quantum superpositions also depend sensitively on the coherence of the photons, that is, their indistinguishability6. Here, we report a single-photon source with a high end-to-end efficiency. We employ gated quantum dots in an open, tunable microcavity7. The gating provides control of the charge and electrical tuning of the emission frequency; the high-quality material ensures low noise; and the tunability of the microcavity compensates for the lack of control in quantum dot position and emission frequency. Transmission through the top mirror is the dominant escape route for photons from the microcavity, and this output is well matched to a single-mode fibre. With this design, we can create a single photon at the output of the final optical fibre on-demand with a probability of up to 57% and with an average two-photon interference visibility of 97.5%. Coherence persists in trains of thousands of photons with single-photon creation at a repetition rate of 1 GHz. High efficiency, coherence and indistinguishability are key requirements for the application of single-photon sources for quantum technologies, but hard to achieve concurrently. A gated quantum dot in an open, tunable microcavity now can create single photons on-demand with an end-to-end efficiency of 57%, preserving coherence over microsecond-long trains of single photons.

Engineering long spin coherence times of spin-orbit qubits in silicon

Abstract: Electron-spin qubits have long coherence times suitable for quantum technologies. Spin–orbit coupling promises to greatly improve spin qubit scalability and functionality, allowing qubit coupling via photons, phonons or mutual capacitances, and enabling the realization of engineered hybrid and topological quantum systems. However, despite much recent interest, results to date have yielded short coherence times (from 0.1 to 1 μs). Here we demonstrate ultra-long coherence times of 10 ms for holes where spin–orbit coupling yields quantized total angular momentum. We focus on holes bound to boron acceptors in bulk silicon 28, whose wavefunction symmetry can be controlled through crystal strain, allowing direct control over the longitudinal electric dipole that causes decoherence. The results rival the best electron-spin qubits and are 104 to 105 longer than previous spin–orbit qubits. These results open a pathway to develop new artificial quantum systems and to improve the functionality and scalability of spin-based quantum technologies. Spin qubits in systems with strong spin–orbit coupling can be electrically controlled, but are usually affected by short coherence times. Here, coherence times up to 10 ms are obtained for strain-engineered hole states bound to boron acceptors in silicon 28.

Optical coherence transfer mediated by free electrons

Abstract: We theoretically investigate the quantum-coherence properties of the cathodoluminescence (CL) emission produced by a temporally modulated electron beam. Specifically, we consider the quantum-optical correlations of CL produced by electrons that are previously shaped by a laser field. Our main prediction is the presence of phase correlations between the emitted CL field and the electron-modulating laser, even though the emission intensity and spectral profile are independent of the electron state. In addition, the coherence of the CL field extends to harmonics of the laser frequency. Since electron beams can be focused to below 1 A, their ability to transfer optical coherence could enable the ultra-precise excitation, manipulation, and spectrally resolved probing of nanoscale quantum systems.

Minimizing Estimation Runtime on Noisy Quantum Computers

Abstract: The number of measurements demanded by hybrid quantum-classical algorithms such as the variational quantum eigensolver (VQE) is prohibitively high for many problems of practical value. For such problems, realizing quantum advantage will require methods which dramatically reduce this cost. Previous quantum algorithms that reduce the measurement cost (e.g. quantum amplitude and phase estimation) require error rates that are too low for near-term implementation. Here we propose methods that take advantage of the available quantum coherence to maximally enhance the power of sampling on noisy quantum devices, reducing measurement number and runtime compared to the standard sampling method of the variational quantum eigensolver (VQE). Our scheme derives inspiration from quantum metrology, phase estimation, and the more recent "alpha-VQE" proposal, arriving at a general formulation that is robust to error and does not require ancilla qubits. The central object of this method is what we call the "engineered likelihood function" (ELF), used for carrying out Bayesian inference. We show how the ELF formalism enhances the rate of information gain in sampling as the physical hardware transitions from the regime of noisy intermediate-scale quantum computers into that of quantum error corrected ones. This technique speeds up a central component of many quantum algorithms, with applications including chemistry, materials, finance, and beyond. Similar to VQE, we expect small-scale implementations to be realizable on today's quantum devices.

Quantum-coherent nanoscience.

Abstract: For the past three decades nanoscience has widely affected many areas in physics, chemistry and engineering, and has led to numerous fundamental discoveries, as well as applications and products. Concurrently, quantum science and technology has developed into a cross-disciplinary research endeavour connecting these same areas and holds burgeoning commercial promise. Although quantum physics dictates the behaviour of nanoscale objects, quantum coherence, which is central to quantum information, communication and sensing, has not played an explicit role in much of nanoscience. This Review describes fundamental principles and practical applications of quantum coherence in nanoscale systems, a research area we call quantum-coherent nanoscience. We structure this Review according to specific degrees of freedom that can be quantum-coherently controlled in a given nanoscale system, such as charge, spin, mechanical motion and photons. We review the current state of the art and focus on outstanding challenges and opportunities unlocked by the merging of nanoscience and coherent quantum operations. Although quantum physics underpins the behaviour of nanoscale objects, its role in nanoscience has been mostly limited to determining the static, equilibrium properties of small systems. This Review describes seminal developments and new directions for the explicit exploitation of quantum coherence in nanoscale systems, a research area termed quantum-coherent nanoscience.

Spooky action at a global distance: analysis of space-based entanglement distribution for the quantum internet

Abstract: Recent experimental breakthroughs in satellite quantum communications have opened up the possibility of creating a global quantum internet using satellite links. This approach appears to be particularly viable in the near term, due to the lower attenuation of optical signals from satellite to ground, and due to the currently short coherence times of quantum memories. The latter prevents ground-based entanglement distribution using atmospheric or optical-fiber links at high rates over long distances. In this work, we propose a global-scale quantum internet consisting of a constellation of orbiting satellites that provides a continuous, on-demand entanglement distribution service to ground stations. The satellites can also function as untrusted nodes for the purpose of long-distance quantum-key distribution. We develop a technique for determining optimal satellite configurations with continuous coverage that balances both the total number of satellites and entanglement-distribution rates. Using this technique, we determine various optimal satellite configurations for a polar-orbit constellation, and we analyze the resulting satellite-to-ground loss and achievable entanglement-distribution rates for multiple ground station configurations. We also provide a comparison between these entanglement-distribution rates and the rates of ground-based quantum repeater schemes. Overall, our work provides the theoretical tools and the experimental guidance needed to make a satellite-based global quantum internet a reality.

Bosonic condensation of exciton-polaritons in an atomically thin crystal.

Abstract: The emergence of two-dimensional crystals has revolutionized modern solid-state physics. From a fundamental point of view, the enhancement of charge carrier correlations has sparked much research activity in the transport and quantum optics communities. One of the most intriguing effects, in this regard, is the bosonic condensation and spontaneous coherence of many-particle complexes. Here we find compelling evidence of bosonic condensation of exciton–polaritons emerging from an atomically thin crystal of MoSe2 embedded in a dielectric microcavity under optical pumping at cryogenic temperatures. The formation of the condensate manifests itself in a sudden increase of luminescence intensity in a threshold-like manner, and a notable spin-polarizability in an externally applied magnetic field. Spatial coherence is mapped out via highly resolved real-space interferometry, revealing a spatially extended condensate. Our device represents a decisive step towards the implementation of coherent light-sources based on atomically thin crystals, as well as non-linear, valleytronic coherent devices. A coherent condensate of exciton–polaritons, extending spatially up to 4 µm and spin-polarizable with an external magnetic field, is observed at cryogenic temperatures in a MoSe2 monolayer embedded in a vertical microcavity.

Resolving catastrophic error bursts from cosmic rays in large arrays of superconducting qubits

Abstract: Scalable quantum computing can become a reality with error correction, provided coherent qubits can be constructed in large arrays. The key premise is that physical errors can remain both small and sufficiently uncorrelated as devices scale, so that logical error rates can be exponentially suppressed. However, energetic impacts from cosmic rays and latent radioactivity violate both of these assumptions. An impinging particle ionizes the substrate, radiating high energy phonons that induce a burst of quasiparticles, destroying qubit coherence throughout the device. High-energy radiation has been identified as a source of error in pilot superconducting quantum devices, but lacking a measurement technique able to resolve a single event in detail, the effect on large scale algorithms and error correction in particular remains an open question. Elucidating the physics involved requires operating large numbers of qubits at the same rapid timescales as in error correction, exposing the event's evolution in time and spread in space. Here, we directly observe high-energy rays impacting a large-scale quantum processor. We introduce a rapid space and time-multiplexed measurement method and identify large bursts of quasiparticles that simultaneously and severely limit the energy coherence of all qubits, causing chip-wide failure. We track the events from their initial localised impact to high error rates across the chip. Our results provide direct insights into the scale and dynamics of these damaging error bursts in large-scale devices, and highlight the necessity of mitigation to enable quantum computing to scale.

The coherence of light is fundamentally tied to the quantum coherence of the emitting particle

Abstract: Coherent emission of light by free charged particles is believed to be successfully captured by classical electromagnetism in all experimental settings. However, recent advances triggered fundamental questions regarding the role of the particle wave function in these processes. Here, we find that even in seemingly classical experimental regimes, light emission is fundamentally tied to the quantum coherence and correlations of the emitting particle. We use quantum electrodynamics to show how the particle’s momentum uncertainty determines the optical coherence of the emitted light. We find that the temporal duration of Cherenkov radiation, envisioned for almost a century as a shock wave of light, is limited by underlying entanglement between the particle and light. Our findings enable new capabilities in electron microscopy for measuring quantum correlations of shaped electrons. Last, we propose new Cherenkov detection schemes, whereby measuring spectral photon autocorrelations can unveil the wave function structure of any charged high-energy particle.

Partially coherent vortex beams: Fundamentals and applications

Abstract: It has been over 30 years since the concept of optical vortices was first proposed by Coullet et al. in 1989, and the field of structured beams has grown extremely. In the last two decades, partially coherent vortex beams (PCVBs) have received increasing interest in the fields of optical manipulation, optical communication, optical imaging, etc., and great progress has been made in the area of the coherence singularities, generation methods, topological charge measurements, and promising applications of PCVBs. In this review, we firstly outline the basic concepts of PCVBs. We explicate the relationship between the coherence vortices and optical vortices, and the evolution behavior of optical vortices to coherence vortices is summarized in detail. We discuss a special form of coherence singularity, ring dislocation, mathematically and physically. The ring dislocation in the correlation functions under low coherence is dependent on the mode indices, which provide a feasible approach to measure mode indices of PCVBs. Subsequently, we summarize the various methods for measuring the topological charge of PCVBs, highlight the measurement method based on the cross-correlation function, and a physical explanation on the relation between ring dislocation and topological charge is given. After that, we review the recent advances on experimental generation of several kinds of PCVBs. Lastly, we give an overview on the potential applications of PCVBs.

Higher-dimensional supersymmetric microlaser arrays

Abstract: The nonlinear scaling of complexity with the increased number of components in integrated photonics is a major obstacle impeding large-scale, phase-locked laser arrays. Here, we develop a higher-dimensional supersymmetry formalism for precise mode control and nonlinear power scaling. Our supersymmetric microlaser arrays feature phase-locked coherence and synchronization of all of the evanescently coupled microring lasers—collectively oscillating in the fundamental transverse supermode—which enables high-radiance, small-divergence, and single-frequency laser emission with a two-orders-of-magnitude enhancement in energy density. We also demonstrate the feasibility of structuring high-radiance vortex laser beams, which enhance the laser performance by taking full advantage of spatial degrees of freedom of light. Our approach provides a route for designing large-scale integrated photonic systems in both classical and quantum regimes.

Superconducting-like Heat Current: Effective Cancellation of Current-Dissipation Trade-Off by Quantum Coherence.

Abstract: Quantum coherence is a useful resource for increasing the speed and decreasing the irreversibility of quantum dynamics. Because of this feature, coherence is used to enhance the performance of various quantum information processing devices beyond the limitations set by classical mechanics. However, when we consider thermodynamic processes, such as energy conversion in nanoscale devices, it is still unclear whether coherence provides similar advantages. Here we establish a universal framework, clarifying how coherence affects the speed and irreversibility in thermodynamic processes described by the Lindblad master equation, and give general rules for when coherence enhances or reduces the performance of thermodynamic devices. Our results show that a proper use of coherence enhances the heat current without increasing dissipation; i.e., coherence can reduce friction. In particular, if the amount of coherence is large enough, this friction becomes virtually zero, realizing a superconducting-like "dissipation-less" heat current. Since our framework clarifies a general relation among coherence, energy flow, and dissipation, it can be applied to many branches of science from quantum information theory to biology. As an application to energy science, we construct a quantum heat engine cycle that exceeds the power-efficiency trade-off bound on classical engines and effectively attains the Carnot efficiency with finite power in fast cycles.

Coherences and the thermodynamic uncertainty relation: Insights from quantum absorption refrigerators.

Abstract: The thermodynamic uncertainty relation, originally derived for classical Markov-jump processes, provides a tradeoff relation between precision and dissipation, deepening our understanding of the performance of quantum thermal machines. Here, we examine the interplay of quantum system coherences and heat current fluctuations on the validity of the thermodynamics uncertainty relation in the quantum regime. To achieve the current statistics, we perform a full counting statistics simulation of the Redfield quantum master equation. We focus on steady-state quantum absorption refrigerators where nonzero coherence between eigenstates can either suppress or enhance the cooling power, compared with the incoherent limit. In either scenario, we find enhanced relative noise of the cooling power (standard deviation of the power over the mean) in the presence of system coherence, thereby corroborating the thermodynamic uncertainty relation. Our results indicate that fluctuations necessitate consideration when assessing the performance of quantum coherent thermal machines.

End-to-end nanophotonic inverse design for imaging and polarimetry

Abstract: By co-designing a meta-optical front end in conjunction with an image-processing back end, we demonstrate noise sensitivity and compactness substantially superior to either an optics-only or a computation-only approach, illustrated by two examples: subwavelength imaging and reconstruction of the full polarization coherence matrices of multiple light sources. Our end-to-end inverse designs couple the solution of the full Maxwell equations---exploiting all aspects of wave physics arising in subwavelength scatterers---with inverse-scattering algorithms in a single large-scale optimization involving $\gtrsim 10^4$ degrees of freedom. The resulting structures scatter light in a way that is radically different from either a conventional lens or a random microstructure, and suppress the noise sensitivity of the inverse-scattering computation by several orders of magnitude. Incorporating the full wave physics is especially crucial for detecting spectral and polarization information that is discarded by geometric optics and scalar diffraction theory.

Enhanced Sensing of Weak Anharmonicities through Coherences in Dissipatively Coupled Anti-PT Symmetric Systems

Abstract: In the last few years, the great utility of exceptional points in sensing linear perturbations has been recognized. However, physical systems are inherently anharmonic and macroscopic physics is most accurately described by nonlinear models. Considering the multitude of semiclassical and quantum effects ensuing from nonlinear interactions, the sensing of anharmonicities is a prerequisite to the primed control of these effects. Here, we propose an expedient sensing scheme relevant to dissipatively coupled anti parity-time (anti-PT) symmetric systems and customized for the fine-grained estimation of anharmonic perturbations. The sensitivity to anharmonicities is derived from the coherence between two modes induced by a common vacuum. Owing to this coherence, the linear response acquires a pole on the real axis. We demonstrate how this singularity can be exploited for the enhanced sensing of very weak anhamonicities at low pumping rates. Our results are applicable to a wide class of systems, and we specifically illustrate the remarkable sensing capabilities in the context of a weakly anharmonic yttrium iron garnet sphere interacting with a cavity via a tapered fiber waveguide. A small change in the anharmonicity leads to a substantial change in the induced spin current.

Engineering spatial coherence in lattices of polariton condensates

Abstract: Artificial lattices of coherently coupled macroscopic states are at the heart of applications ranging from solving hard combinatorial optimization problems to simulating complex many-body physical systems. The size and complexity of the problems scale with the extent of coherence across the lattice. Although the fundamental limit of spatial coherence depends on the nature of the couplings and lattice parameters, it is usually engineering constraints that define the size of the system. Here, we engineer polariton condensate lattices with active control on the spatial arrangement and condensate density that results in near-diffraction limited emission, and spatial coherence that exceeds by nearly two orders of magnitude the size of each individual condensate. We use these advancements to unravel the dependence of spatial correlations between polariton condensates on the lattice geometry.

Seamless High-Q Microwave Cavities for Multimode Circuit Quantum Electrodynamics.

Abstract: Multimode cavity quantum electrodynamics ---where a two-level system interacts simultaneously with many cavity modes---provides a versatile framework for quantum information processing and quantum optics. Due to the combination of long coherence times and large interaction strengths, one of the leading experimental platforms for cavity QED involves coupling a superconducting circuit to a 3D microwave cavity. In this work, we realize a 3D multimode circuit QED system with single photon lifetimes of $2$ ms and cooperativities of $0.5-1.5\times10^9$ across 9 modes of a novel seamless cavity. We demonstrate a variety of protocols for universal single-mode quantum control applicable across all cavity modes, using only a single drive line. We achieve this by developing a straightforward flute method for creating monolithic superconducting microwave cavities that reduces loss while simultaneously allowing control of the mode spectrum and mode-qubit interaction. We highlight the flexibility and ease of implementation of this technique by using it to fabricate a variety of 3D cavity geometries, providing a template for engineering multimode quantum systems with exceptionally low dissipation. This work is an important step towards realizing hardware efficient random access quantum memories and processors, and for exploring quantum many-body physics with photons.

Optical Coherence Transfer Mediated by Free Electrons

Abstract: We investigate the optical coherence properties carried by the quantum state of a single free-flying charged and massive p article – the electron. We propose feasible Mach-Zehnder-like linear and nonlinear optical interferometric detection, incorporating electron-microscope beams.

Ultra-low-loss on-chip zero-index materials.

Abstract: Light travels in a zero-index medium without accumulating a spatial phase, resulting in perfect spatial coherence. Such coherence brings several potential applications, including arbitrarily shaped waveguides, phase-mismatch-free nonlinear propagation, large-area single-mode lasers, and extended superradiance. A promising platform to achieve these applications is an integrated Dirac-cone material that features an impedance-matched zero index. Although an integrated Dirac-cone material eliminates ohmic losses via its purely dielectric structure, it still entails out-of-plane radiation loss, limiting its applications to a small scale. We design an ultra-low-loss integrated Dirac cone material by achieving destructive interference above and below the material. The material consists of a square array of low-aspect-ratio silicon pillars embedded in silicon dioxide, featuring easy fabrication using a standard planar process. This design paves the way for leveraging the perfect spatial coherence of large-area zero-index materials in linear, nonlinear, and quantum optics. Calculations suggest that a new design of engineered medium can simultaneously yield a refractive index of zero and low optical loss at the telecommunications wavelength of 1550 nm. The development could lead to applications in nonlinear and quantum optics benefiting from an infinite coherence length. The approach of Tian Dong and coworkers from China and the US is to embed an array of silicon pillars (about 180 nm in radius and 1100 nm high) into a matrix of silicon dioxide to create a Dirac-cone photonic-crystal slab. Importantly, if the pillar height is chosen correctly any upwards and downwards radiation out of the slab can be made to destructively interfere thus reducing propagation loss to a level of 0.15 dB/mm. The slab should be possible to fabricate using standard processes.