Macroscopic quantumness underpins fundamental physics, serving as a bridge between the classical and quantum worlds. However, such states are fragile: interaction with the environment rapidly destroys them. It is therefore important to prepare them on timescales shorter than relaxation and dephasing. The development of strong-field quantum optics, in particular bright squeezed vacuum with photon numbers up to $10^{13}$ per mode, makes this approach realistic. The work sits alongside studies of quantum entanglement in collisions, 120-photon quantum cats, and quantum sensors for dark matter, but differs in that it focuses on ultrafast generation of macroscopic states of matter via a single light measurement.
The author builds a theory by combining the external-field approximation (valid for multiphoton light and weak electron-photon coupling) with the Yansky representation of the squeezed vacuum state as a superposition of coherent states. In this approach the field is treated as classical for each branch, while quantumness enters through the superposition. A measurement of the light quadrature after the interaction is then considered, described by Kraus operators. For the Tavis-Cummings model and the Dicke model, analytical expressions for the conditional state of matter are derived. The validity of the external-field approximation is checked numerically by comparison with a full simulation of the light-matter system.
The unconditional dynamics of matter under squeezed vacuum alone yields a classical mixture of laser-controlled states, with no quantum coherence. However, upon measuring the quadrature of the transmitted light, matter transitions into a Gaussian-weighted quantum superposition. For the Tavis-Cummings model this acts as a Gaussian filter on the collective electric polarization, selecting the Dicke state with zero eigenvalue. The quantum Fisher information (QFI) grows as $N^2$, reaching a density of N/2+1, which indicates macroscopic entanglement. At finite measurement resolution, the success-probability-weighted QFI scales as N^(3/2), surpassing the standard quantum limit. In the Dicke model, counter-rotating terms cause a stroboscopic transition to a z-polarized cat-like state, similar to a Greenberger-Horne-Zeilinger state. The preparation time is set by the inverse effective field strength $F_c$ = g exp(r), which allows attosecond timescales to be reached under realistic parameters.
The work differs from neighboring studies by proposing a single quadrature measurement of light for the ultrafast preparation of macroscopic quantum states of matter, whereas other approaches use continuous quantum nondemolition measurements or dissipative processes with timescales set by the measurement rate. Here the time is set by the field strength, which is fundamentally faster. This opens a path to quantum metrology beyond the standard quantum limit on ultrashort timescales.
The topic can be developed by taking into account a more realistic structure of the light modes, including multichannel scattering processes and mode matching with the local oscillator in homodyne detection. It is also important to study the effect of coupling inhomogeneity and frequency detuning on the filtering process. Another direction is optimizing the success probability by including outcomes with nonzero quadrature, which may restore Heisenberg scaling.
The method can be applied to atoms, molecules, and solid-state electronic systems for ultrafast engineering of quantum states. It will affect quantum metrology, quantum computing, and fundamental tests of quantum mechanics.
The next step will be a detailed analysis of the success probability taking into account all measurement outcomes, and optimization of the parameters to reach the Heisenberg limit. It is also necessary to quantify the effect of dephasing and relaxation in real systems.
The work is directly connected to the problem of preserving quantum coherence in macroscopic systems and achieving quantum advantage in metrology. It offers a solution that circumvents decoherence through ultrafast preparation, which is a key challenge in quantum physics.
Bright squeezed vacuum can contain up to $10^{13}$ photons in a single mode, comparable to the number of stars in a small galaxy. Yet its quantum nature manifests in a superposition of coherent states, not merely in noise.
The idea of a macroscopic quantum superposition echoes Schrödinger's cat, and the creation of cat-like states of matter brings to mind stories about quantum computers, where such states are used for computation.