Jeremy O'Brien

Entanglement Enables Unambiguous Discrimination of Non-Orthogonal Quantum Processes

Researchers demonstrate unambiguous deterministic quantum process discrimination (QPD) for non-orthogonal processes using entanglement, auxiliary unitaries, or qudits. They experimentally discriminate single-qubit measurements/unitaries and non-separable multipartite unitaries on photons with ≥97% confidence. This extends quantum state discrimination to the process domain, achieving high-fidelity identification despite non-orthogonality.

Ultrafast Laser Direct-Writing Enables Maskless 3D Photonic Quantum Circuits Matching Lithographic Performance

Ultrafast laser processing fabricates photonic quantum circuits rapidly without lithographic masks, supporting 3D waveguide networks. Characterized directional couplers perform equivalently to lithographic devices. Demonstrates high-fidelity interferometers and first integrated multi-photon quantum interference, enabling scalable quantum optics.

On-Chip Waveguide Circuits Enable Precise Control of Multi-Photon Entanglement and Quantum Metrology

Researchers demonstrate on-chip manipulation of path-encoded qubit states using integrated resistive phase control, achieving 98.2±0.3% interference contrast for single photons. They generate and interfere 2- and 4-photon entangled states in waveguide circuits, attaining 97.2±0.4% and 92±4% contrasts respectively, surpassing the standard quantum limit. A reconfigurable circuit tunes quantum interference continuously up to 98.2±0.9% visibility, enabling adaptive photonic quantum technologies for arbitrary light states.

Measurement-Induced Nonlinearities Unlock Qutrit Manipulation in Biphotons

Biphotonic qutrits, formed by polarization states of two photons in the same spatio-temporal mode, enable higher-dimensional quantum information processing but are hard to manipulate. The technique uses measurement-induced nonlinearities to vastly expand the range of possible quantum transforms on these qutrits. It fully characterizes underlying biphoton-photon entanglement, achieving the first qubit-qutrit entanglement, with extensions to qutrit-qutrit entanglement and bosonic encodings.

First All-Fiber Optical CNOT Gate Achieves 90% Logical Fidelity

Researchers demonstrate the first experimental all-optical fiber-based controlled-NOT (CNOT) quantum gate using heralded single-photon sources. The gate exhibits 90% average logical fidelity and process fidelities between 0.83 and 0.91. A simple model attributes imperfections primarily to the photon sources, indicating the gate itself operates at very high fidelity.

Single Electron Spin in Quantum Dot Induces Giant Faraday Rotation for Photon-Mediated Spin Entanglement

A quantum non-demolition technique uses giant Faraday rotation to detect a single electron spin in a quantum dot coupled to a microcavity via a negatively-charged exciton. Circularly polarized light experiences distinct phase shifts from cavity QED and spin selection rules, enabling experimental detection. This detection facilitates scalable entanglement of multiple remote spins using a single photon.

Cavity QED Enables High-Fidelity NV Spin Readout with Low-Light Probing

Researchers propose measuring the ground-state spin of a single NV center in diamond via cavity-enhanced reflection of low-intensity laser light in the 1D atom/Purcell regime, avoiding high-Q cavity challenges. The scheme exploits a 7-10 μeV detuning between optical transitions from m=0 and m=±1 spin states to the excited ³E state, yielding distinct reflected intensities. This achieves ~5.5×10^{-3} error rate while minimizing fluorescence cycles to suppress non-spin-preserving shelving in the ¹A singlet state.

Entangled N-Qubit States Boost QKD Security Against Intercept-Resend Attacks

This protocol uses entangled N-qubit states transmitted individually in QKD, unlike BB84's single qubits. Eavesdropper access is restricted to one qubit at a time, enabling zero information gain if even one qubit evades interception. For N=2, Eve's information drops below 30% of BB84's level, with a general 1/(2N) upper bound, matching or exceeding BB84 security.

Photonic Module Enables Deterministic, Plug-and-Play Generation of Entangled Photon States

The photonic module is a versatile, application-independent device that deterministically prepares a broad class of entangled photon states for quantum computation and communication. It operates without number-discriminated photon detection, using either free-space optics or photonic bandgap structures to generate stabilizer states. Cavity-QED requirements are analyzed for experimental realization, offering a rapid, on-demand entanglement resource.

Higher-Dimensional Shortcuts Compress Quantum Gate Counts for Feasible Circuit Implementation

Researchers demonstrate a technique leveraging higher dimensions of quantum systems to drastically reduce the number of universal quantum logic gates needed for algorithms, overcoming a key scalability barrier. This enables the first experimental realization of critical circuits—the three-qubit Toffoli gate and two-qubit controlled-unitary—using linear optics, a platform previously deemed incapable. The approach constructs these circuits with existing technology by harnessing qudit subspaces.

High-Fidelity Silica-on-Silicon Waveguides Enable Scalable Quantum Photonic Circuits

Researchers demonstrate silica-on-silicon integrated optical waveguides realizing key quantum photonic circuits with high fidelities. Achievements include two-photon quantum interference at 94.8(5)% visibility, a controlled-NOT gate with 94.3(2)% logical basis fidelity, and a path-entangled two-photon state exceeding 92% fidelity. These results support integrated optics for scalable quantum technologies in processing, communication, metrology, and lithography.

Quantum-Dot Spin Microcavity Enables Perfect Fidelity Entanglement Beam Splitter

A quantum-dot spin in a double-sided optical microcavity acts as an entanglement beam splitter (EBS), splitting photon-spin product states into entangled transmission and reflection paths with up to 100% fidelity and efficiency. This leverages giant optical circular birefringence from cavity QED and trion spin selection rules via Pauli blocking. The EBS resists fine structure splitting in quantum dots and supports deterministic entanglement generation (photon-spin, photon-photon, spin-spin) plus single-shot spin QND measurement.

Weak Measurements of Photons Link Anomalous Weak Values to Leggett-Garg Inequality Violations

Researchers used weak polarization measurements on single photons, sandwiched between strong measurements, to probe correlations between anomalous quantum weak values and violations of measurement realism/non-intrusiveness. They violated the Leggett-Garg inequality (LGI) across multiple measurement strengths, quantifying non-invasiveness via sequential outcomes on individual systems. A direct one-to-one correspondence was observed between obtaining strange weak values and LGI violations.

Four-Photon Entanglement Surpasses Standard Quantum Limit in Optical Phase Measurement

Researchers demonstrate an optical phase measurement using four entangled photons with interference visibility exceeding the standard quantum limit (SQL), the precision boundary for non-entangled states. This entanglement-enhanced approach achieves higher precision for measuring distance, position, displacement, acceleration, and optical path length. The result establishes a pathway for entanglement-based high-precision metrology applications.

Benchmarking Photonic Quantum Gates Reveals Multi-Photon Emission as Primary Fault-Tolerance Barrier

Researchers introduce a technique to quantitatively benchmark quantum-logic gates against fault-tolerance thresholds using measurements, applicable to any architecture. Demonstrated on a photonic entangling gate with a comprehensive model, they identify multi-photon emission—previously underestimated—as the dominant logic error source over mode-mismatch. Mitigating this error positions photonic quantum computing near fault-tolerance thresholds.

Weak Measurements Enable Superior Feedback Control of Qubits Beyond Theoretical Limits of Strong Measurements

In stabilizing non-orthogonal qubit states against dephasing, weak measurements combined with feedback outperform both practical strong measurements and the theoretical optimum achievable without weak measurements. Despite implementation challenges, weak measurements provide partial information with minimal disturbance, enabling better robustness in noisy quantum environments. This demonstrates the critical role of general quantum measurements in advancing quantum control protocols.

Fock-State Filtration Enables Linear-Optics Entanglement Generation via Higher-Order Interference

Researchers demonstrate a Fock-state filter using linear optics, an ancilla photon, and measurement to preferentially block single photons over photon pairs through higher-order quantum interference. The filter coherently converts unentangled photon pairs into a path-entangled state. Quantum state tomography on the transformed polarization state yields a tangle of (20±9)%, quantifying the entanglement.

Linear Optical Quantum Computing Becomes Scalable with Reduced Resource Overhead

In 2001, scalable quantum computing using single photon sources, linear optical elements, and single photon detectors was shown feasible, though initial resource demands were prohibitive. Subsequent simplifications, proof-of-principle experiments, and advances in cluster states and error encoding have drastically cut overheads, positioning all-optical architectures as viable for large-scale quantum computers. Remaining challenges center on high-efficiency indistinguishable single-photon sources, low-loss scalable circuits, efficient detectors, and lossless component interfacing.

Robust Microcavity-Based Photon-Spin Entangling Gate Enables Key Quantum Network Primitives

Researchers propose a high-fidelity, efficient (up to 50%) photon-spin entangling gate using a charged quantum dot in a double-sided microcavity, leveraging giant circular birefringence from a single electron spin to act as a spin-dependent optical circular polarizer. The gate enables single-shot quantum non-demolition measurement of electron spin and functions as an entanglement filter for photon-spin, spin-spin, and photon-photon entanglement. This provides all essential building blocks for solid-state quantum networks using single photons and quantum-dot spins.

Cavity-QED Optical Network Enables Deterministic, Chip-Scale Quantum Computing via Cluster States

Researchers propose an optical network leveraging cavity quantum electrodynamics for deterministic operator measurements to generate arbitrarily large 2D cluster states. This architecture supports small-scale quantum information processing demonstrated experimentally and scales to a full quantum computer. The design is fully integrable on a photonic chip, advancing deterministic optical quantum computing.

Optical Entanglement Filter Demonstrated for Polarization-Correlated Photon Pairs

Researchers demonstrate an optical entanglement filter that selectively passes photon pairs exhibiting desired polarization correlations, extending single-qubit polarizers to multi-qubit systems with qubit-qubit interactions. This device requires photon-photon interactions to filter based on entanglement. The filter enables key applications in quantum information processing and technologies.

Four-Dimensional Photonic Qudits Enable Linear Optical One-Way Quantum Computation

The paper proposes linear optical one-way quantum computation using four-dimensional photonic qudits (quadbits), leveraging hyper-entanglement across polarization and two spatial modes. The core 2-quadbit cluster state is a hyper-entangled resource state. It details non-deterministic protocols to generate these states from single photons or Bell pairs, plus mechanisms for higher-dimensional fusion gates.

Photonic Crystal Fiber Enables Bright Heralded Single Photons and High-Fidelity Entangled Pairs via Four-Wave Mixing

Researchers demonstrate a photonic crystal fiber source generating heralded single photons and entangled photon pairs through χ^(3) four-wave mixing of pump photons, producing correlated pairs at 583 nm and 900 nm. Heralded photons from independent sources exhibit non-classical interference with 95% visibility. The entangled pair source achieves 89% fidelity to a Bell state, advancing optical quantum information processing.

Photonic Chip Executes Compiled Shor's Algorithm to Factor 15

Researchers demonstrated a compiled version of Shor's quantum factoring algorithm on an integrated silica-on-silicon waveguide chip using four single-photon qubits. The implementation successfully factors the number 15 by harnessing quantum superposition and entanglement. This marks an experimental photonic quantum computing milestone for integrated optical platforms.

N-Photon Interferometers Exceed SQL Phase Sensitivity at Non-Maximum Fringe Slopes

Theory reveals phase sensitivity in N-photon interferometers depends nonlinearly on detection efficiency η and fringe visibility V, with optimal sensitivity phase differing from maximum fringe slope phase. Quantum enhancement beyond SQL is achievable in specific η-V parameter ranges. Four-photon experiment with η=3/4 and V=82% demonstrates 1.3× SQL sensitivity at a distinct phase.

Optimal Success Probabilities for Post-Selected Photonic Controlled Phase Gates

In linear optical quantum computing with dual-rail encoding and vacuum ancillas, the paper identifies optimal success probabilities for post-selected controlled phase gates implementing arbitrary phases with one or two control qubits. All nonlinearities arise from measurements, making success probability a key metric. It provides explicit network constructions feasible with current experimental capabilities, particularly suited for integrated linear optical circuits.

Pryde et al. Defend Validity of Coincidence-Based Fidelity Measure for Photonic QND Measurement

Pryde et al. proposed and experimentally demonstrated a heralded nondeterministic QND measurement scheme for single-photon polarization qubits using linear optics and photodetection, with success signaled by single-photon detection in the meter. They introduced three fidelity metrics: measurement fidelity F_M, quantum state preparation fidelity F_QSP, and QND fidelity F_QND. In response to Kok and Munro's comment claiming F_M's inappropriateness due to reliance on coincidence measurements, the authors refute this from fundamental and operational perspectives.

Time-Reversal Symmetry Enables Entanglement-Free Phase Super-Resolution

Researchers exploit quantum mechanics' time-reversal symmetry to achieve phase super-resolution without preparing entangled states, instead measuring effective entangled states. The method uses classical interference from standard lasers and photon counters, proving robust. Experiments demonstrate high-visibility super-resolution with 3, 4, and 6 photons, with the 6-photon result setting the record for resolution and visibility at the time.

Physically-Constrained Process Tomography in Extended Hilbert Space Reduces Data Needs for Quantum Gate Characterization

Quantum process tomography for gate characterization models processes in an extended Hilbert space incorporating non-qubit degrees of freedom. Physical constraints ensure unphysical processes are excluded, unlike standard methods relying on generic mathematical constraints. This enables tomography with smaller experimental datasets and yields directly interpretable physical parameters. Demonstrated on mode-matching in an all-optical CNOT gate, with applicability to other optical and quantum architectures.

Universal Figures of Merit for QND Measurements on Qubits

Introduces figures of merit for characterizing quantum non-demolition (QND) measurements applicable to qubit systems of any Hilbert space dimension. Demonstrates the controlled-NOT gate and an optical implementation as practical QND devices for qubits. Extends QND concepts to the measurement of weak values.

Experimental Realization of Quantum Weak Values via Two-Photon Entanglement

Researchers experimentally measured weak values of a single photon's polarization using a weak measurement protocol involving two-photon entanglement and postselection. These weak values exceed the spectrum of the measured operator (S_1 Stokes parameter) by over an order of magnitude at weakest measurement strengths. Semiclassical wave theory fails to explain the results due to entanglement, confirming a fundamentally quantum effect.

Simplified Single-Mode Optical CZ Gate Enables Full Bell-State Analysis

Researchers demonstrate a novel optical entangling gate using partially-polarizing beamsplitters, requiring only one mode-matching condition versus multiple in prior designs. The controlled-Z (CZ) gate operates in both continuous-wave and pulsed regimes, fully characterized via quantum process tomography. This architecture supports a nondeterministic, fully-resolving optical Bell-state analyzer and suits guided-wave quantum optics implementations.

Leveraging Silicon Photonics for Million-Cubit Quantum Scaling

PsiQuantum is bypassing small-scale quantum demonstrations to build a million-qubit, fault-tolerant utility-scale computer. Their strategy relies on a silicon photonics platform designed to leverage existing semiconductor manufacturing infrastructure to overcome the scaling bottlenecks that hinder other modalities.

Quantum Computing as a First-Principles Simulator for Material and Chemical Discovery

Unlike conventional computers or AI, which rely on binary logic and data-driven approximations, quantum computers leverage superposition and entanglement to perform exact first-principles simulations of quantum physics. This capability is essential for solving computationally intractable problems in chemistry and materials science—such as enzyme metabolism and battery chemistry—where the required classical compute time exceeds the age of the universe. The path to utility requires million-qubit scale, fault-tolerant systems, achievable by leveraging existing semiconductor manufacturing and photonics.

Can Quantum Computing Solve Climate Change? (McKinsey & Company)

Scalable Silicon Photonics Platform Achieves Near-Perfect Photonic Qubit Fidelities

Researchers demonstrate a manufacturable silicon photonics platform for photonic quantum computing, benchmarking monolithically-integrated modules for qubit generation, manipulation, networking, and detection. Key metrics include 99.98% state preparation and measurement fidelity for dual-rail qubits, 99.50% Hong-Ou-Mandel interference visibility, 99.22% two-qubit fusion fidelity, and 99.72% chip-to-chip interconnect fidelity, excluding loss. The platform previews next-gen components like low-loss silicon nitride waveguides, fabrication-tolerant sources, high-efficiency detectors, low-loss fiber coupling, and barium titanate phase shifters, addressing scalability barriers.

UNSW Gerald Westheimer Lecture 2023 | Jeremy O'Brien (PsiQuantum)

Semidefinite Programming Corrects Finite Statistics Artifacts in Quantum Steering for Robust Entanglement Verification

A one-sided device-independent protocol uses semidefinite programming to derive optimal inequalities correcting apparent signaling from finite statistics and imperfections in quantum steering experiments, avoiding tomography. Applied to random, misaligned measurements, it eliminates false positives in entanglement tests. Results show significantly higher violation probabilities than state-of-the-art inequalities, advancing practical quantum networks.

Scaling Gboard to 900+ Global Language Varieties Through Deep Internationalization

Gboard supports over 900 language varieties across more than 70 writing systems via a deep internationalization program. The effort addresses rising everyday writing trends in numerous world languages, tackling technological and logistical challenges in scaling language technology products. Systems and processes were developed for efficient operation at this scale, validated by user studies with speakers of hundreds of languages.

Non-Classical Light Enables Sub-Shot-Noise Precision in Correlated Absorption and Phase Measurements

Non-classical optical techniques surpass the shot-noise limit in absorption measurements accompanied by phase shifts, for both known and unknown correlations between loss and phase. The work derives fundamental precision limits and practical strategies to achieve them using present-day lab methods. Absorption and lossy phase shift measurements exhibit equivalent potential for quantum-enhanced precision.

Automated Keyboard Layouts Enable Scalable Input for Low-Resource Latin-Script Languages

Researchers introduce an automated method to design and implement mobile keyboard layouts for low-resource Latin-script languages using minimal language data. This reduces human effort compared to manual configuration, which requires extensive files for key behaviors even for English-similar layouts. The approach scales input tool development, lowering barriers to online content creation and communication in native languages.

Scalable Silicon Photonics Enables Programmable Two-Qubit Quantum Processing

Researchers demonstrate a fully programmable two-qubit quantum processor using silicon photonics and linear combination of unitaries, fabricated via standard CMOS processes. The chip integrates four nonlinear photon sources, four filters, 82 beam splitters, and 58 individually addressable phase shifters. It achieves 93.2±4.5% average fidelity across 98 two-qubit unitaries, and executes a quantum approximate optimization algorithm and Szegedy quantum walks, advancing scalable photonic quantum computing.

Hybrid Cavity-MMI Chip Preserves Photon Coherence for Atomic Entanglement in Quantum Networks

Researchers integrate a cavity-enhanced light-matter interface using a single 87Rb atom with a photonic chip-based multimode interferometer (MMI) to generate and manipulate narrowband photon pairs. These deterministically produced photons maintain full two-photon coherence through MMI interference, matching Hong-Ou-Mandel visibility on a beam splitter. This hybrid platform enables entanglement distribution between remote atomic nodes via photonic qubits in quantum networks.

Large-Scale Silicon Photonics Enables Programmable 15x15-Dimensional Bipartite Quantum Entanglement

Researchers demonstrate a silicon-photonics integrated circuit generating, controlling, and analyzing high-dimensional entanglement up to 15x15 dimensions using over 550 photonic components, including 16 identical photon-pair sources. The programmable bipartite system verifies high precision and generality through quantum randomness expansion and self-testing of multidimensional states. This platform advances multidimensional quantum technologies for fundamental science and applications.

Silicon Photonics Poised to Drive Scalable Quantum Technology Revolution

Silicon photonics leverages its proven scalability, CMOS compatibility, and manufacturing prowess from microelectronics to enable large-scale integrated quantum photonic systems. These systems promise physically secure communications, sub-shot-noise measurements, and immense computational power. The review outlines component developments and identifies key challenges with potential solutions to realize quantum technologies on silicon.

Hybrid Quantum-Classical Bayesian Learning Achieves 10^{-5} Precision in Hamiltonian Parameter Estimation

Researchers interfaced a silicon-photonics quantum simulator with a diamond nitrogen-vacancy electron spin via a classical channel, employing Bayesian inference to learn the spin's Hamiltonian parameters with ~10^{-5} uncertainty. Saturation in the learning curve revealed model deficiencies, enabling iterative model refinement. An interactive protocol further demonstrated characterization of the photonic device's operations, establishing quantum-enhanced techniques for physical model validation and quantum technology assessment.

Bayesian Quantum Phase Estimation Proves Robust on Noisy Silicon Photonic Chips

Researchers demonstrate adaptive Bayesian quantum phase estimation on a silicon photonic chip, successfully simulating molecular energies. This method outperforms the standard iterative phase estimation algorithm in noise and decoherence resilience. The results indicate viability for pre-threshold, non-fault-tolerant quantum processors, challenging prior doubts on quantum phase estimation's near-term practicality.

Relative Multiplexing Cuts Active Switching Demands in Linear-Optical Quantum Computing by an Order of Magnitude

Relative multiplexing (RMUX) optimizes multiplexing in LOQC by reducing active switching requirements, lowering hardware complexity, energy use, and photonic component demands. Applied to probabilistic single-photon sources, RMUX enables an order-of-magnitude faster Bell state generation. For 3D cluster state percolation, it boosts loss tolerance by 2.4x.

High-Speed QKD Achieved via Silicon Photonics with 10 GHz Carrier-Depletion Modulation

Researchers demonstrate high-speed, low-error quantum key distribution using integrated silicon photonic devices. They combine slow thermo-optic DC biases with fast 10 GHz bandwidth carrier-depletion modulation to overcome prior limitations in standard silicon platforms. This approach leverages silicon's miniaturization, cost-effectiveness, and CMOS compatibility to enable scalable quantum communication technologies.

a-Si:H Microrings Enable CW-Pumped Photon Pair Generation with High Kerr Coefficient but Power-Dependent Loss Limits High-Q Performance

Researchers demonstrate photon pair generation in hydrogenated amorphous silicon (a-Si:H) microring resonators using a continuous-wave pump, measuring the Kerr coefficient at 3.73 ± 0.25 × 10^{-17} m²/W. Loss in these microrings scales linearly with pump power, as determined by coupled power-dependent Q factor measurements, ruling out two-photon absorption as the origin. Theoretical comparison with crystalline silicon (c-Si) shows a-Si:H's high Kerr suits low-Q devices (Q < 10^3), but first-order loss suppresses pair generation rates in higher-Q structures.