Discriminating enantiomer pairs is of central importance in the molecular sciences. The preferential interaction between chiral light and matter is commonly employed in this discrimination, despite fundamental challenges. Here we present quantitative error bounds for classifying the handedness of a chiral, randomly oriented, quantum optical emitter from the perspective of classical and quantum hypothesis testing. Most interestingly, we find that a collective measurement on $N$ photons can provide an orders-of-magnitude improvement in error rate relative to a corresponding separable measurement.
Discriminating enantiomer pairs is of central importance in the molecular sciences. The preferential interaction between chiral light and matter is commonly employed in this discrimination, despite fundamental challenges. Here we present quantitative error bounds for classifying the handedness of a chiral, randomly oriented, quantum optical emitter from the perspective of classical and quantum hypothesis testing. Most interestingly, we find that a collective measurement on N photons can provide an orders-of-magnitude improvement in error rate relative to a corresponding separable measurement.
Chiral discrimination of enantiomeric biomolecules is vital in chemistry, biology, and medicine. Conventional methods, relying on circularly polarized light, face weak chiroptical signals and potential photodamage. Despite extensive efforts to improve sensitivity under low-photon exposure, classical chiral probes remain fundamentally bounded by the shot-noise limit due to quantum fluctuations. To beat these limitations, we demonstrate quantum-elevated chiral discrimination using continuous-variable polarization-entangled states as moderate-photon-flux, high-sensitivity, quantum-noise-squeezed chiral probes. We achieve a 5 dB improvement beyond the SNL in distinguishing L- and D-amino acids in liquid phase. This non-destructive, biocompatible protocol enables high-sensitivity chiral analysis, with broad implications for drug development, biochemical research, environmental monitoring, and asymmetric synthesis.
We derive the form of the one-photon state of the electromagnetic field resulting from spontaneous optical emission of a randomly oriented chiral quantum source, including electric dipole, magnetic dipole, and electric quadrupole contributions to the radiation. We describe how spatial coherence in the emission can be exploited to reveal hidden information about the emitter. This work serves as a prelude and companion piece to our concurrently submitted work \cite{Backlund2026Letter} in which we present the classical and quantum error bounds associated with assigning the handedness of such an emitter.
Absorption spectroscopy identifies molecules by detecting frequency-dependent optical loss, and its fundamental limit is a quantum channel-discrimination problem. In this work, we develop a general framework for characterizing the ultimate quantum limit of molecular absorption spectroscopy without collective measurements: we formulate it as an asymmetric quantum hypothesis testing task between different absorption channels. Using a semidefinite-programming formulation, we construct optimal probe states and compare them with coherent-state photon counting, Fock states, and two-mode squeezed vacuum probes. We introduce a random-loss channel model for imperfectly known background transmission, reflecting the fact that a blank experiment need not return all incident photons due to scattering, solvent absorption, or collection losses. Our results show that optimized non-classical probes can substantially outperform conventional laser spectroscopy, and that the advantage is robust to realistic baseline random loss. % In multi-frequency spectroscopy, the optimization also determines how photons should be allocated across absorption lines, revealing an additional resource-allocation advantage beyond single-mode state engineering. We illustrate our framework using infrared detection of 4-nitrophenol against chemically relevant comparison molecules, showing how quantum channel-discrimination methods provide constructive performance limits for molecular spectroscopy under realistic photon constraints.
The phenomenon of molecular optical activity manifests itself as the rotation of the plane of linear polarization when light passes through chiral media. Measurements of optical activity and its wavelength dependence, that is, optical rotatory dispersion, can reveal information about intricate properties of molecules, such as the three-dimensional arrangement of atoms comprising a molecule. Given a limited probe power, quantum metrology offers the possibility of outperforming classical measurements. This has particular appeal when samples may be damaged by high power, which is a potential concern for chiroptical studies. We present the first experiment in which multiwavelength polarization-entangled photon pairs are used to measure the optical activity and optical rotatory dispersion exhibited by a solution of chiral molecules. Our work paves the way for quantum-enhanced measurements of chirality, with potential applications in chemistry, biology, materials science, and the pharmaceutical industry. The scheme that we use for probing wavelength dependence not only allows one to surpass the information extracted per photon in a classical measurement but also can be used for more general differential measurements.
In recent times there has been growing interest in Raman optical activity (ROA) for its label free detection of absolute configuration, conformation, and stereochemical structure in chiral biosamples and drug molecules. Since ROA signals are generally small, techniques such as stimulation by a probe beam can be used to enhance the signal strength. However, with a classical probe, the measurement precision is still fundamentally limited by its shot noise. To solve this problem we propose the use of two-mode squeezed vacuum and show that it can achieve sub-shot noise limited measurement sensitivity. Using quantum estimation theory, we derived the quantum Fisher information and the quantum Cramér-Rao bound (QCRB) for stimulated ROA measurement to quantify the precision enhancement. This improvement comes from photon-number correlations which suppress the intensity fluctuation common to both modes. We further show that balanced detection of the output intensity difference is a practical measurement scheme that approaches the QCRB and becomes optimal in the small-chirality limit. This opens a promising path toward more sensitive Raman chiroptical spectroscopy of weak and photosensitive samples.
Chiral discrimination that is efficient at detecting tiny amounts of chiral substances, especially at the single-molecule level, is in great demand. Here, we propose a single-shot nondestructive quantum sensing method addressing such an issue. Our scheme consists of two steps. In the first step, the two enantiomers are prepared in different rotational states via microwave enantio-specific state transfer. Then, chiral discrimination is transferred to quantum hypothesis testing. In the second step, we for the first time introduce a nondestructive quantum-state detection technique assisted with a microwave resonator for chiral discrimination, through which the molecular chirality is determined by the sign of the output signals. Using a typical chiral molecule, 1,2-propanediol, and an experimentally feasible model based on spherical Fabry–Pérot cavity, we show that the molecular chirality of slowly moving enantiopure gaseous samples with 10 2 –10 3 molecules can be highly credibly distinguished in a single-shot detection. By further trapping chiral molecules, it is promising to achieve chiral discrimination at the single-molecule level by using our approach.