Quantum Light Creates Superposition States in Matter at Record Speed

Shohei Imai, University of Tokyo, and colleagues have developed a new method for creating macroscopic quantum states in materials using bright squeezed vacuum light and single-shot measurement techniques. The approach overcomes limitations of weak-coupling regimes, enabling the preparation of a Gaussian-weighted quantum superposition of matter states. The team used heralding dynamics as a Gaussian filter on electric polarisation, accelerating the formation of a zero-eigenvalue Dicke state and subsequently driving a transition towards a cat-like state. These findings represent a key advance in the field, paving the way for ultrafast engineering of macroscopic quantum matter with strong-field quantum light.

Squeezed light and quadrature measurement circumvent light-matter entanglement limitations

Bright squeezed vacuum light, a non-classical state of light characterised by a reduction in quantum noise in one quadrature component at the expense of increased noise in the other, proved key to this advance. This unique property arises from manipulating the vacuum electromagnetic field, creating correlations between photons that deviate from those found in coherent light. A combination of this light source and single-shot quadrature measurement, a technique for simultaneously determining the amplitude and phase of a wave-like signal, allowed the generation of quantum states within an ensemble of 32 two-level systems. Traditional methods often struggle with maintaining quantum coherence due to the inherent entanglement between the probing light and the material system. This entanglement typically leads to decoherence and the collapse of the desired quantum state into a classical mixture. The team’s approach circumvents this issue by employing quadrature-based heralding, effectively filtering the quantum information and preparing the matter in a quantum superposition weighted by a Gaussian distribution. This Gaussian weighting is crucial for stabilising the fragile quantum state and enhancing its lifetime.

Calculations employed a weak-coupling regime with a coupling strength of g= 0.005 and r= 3, enabling the ultrafast generation of macroscopic quantum states in matter. The weak-coupling regime implies that the interaction between the light and the matter is sufficiently weak that perturbation theory can be used to describe the dynamics. The parameters g= 0.005 and r= 3 define the specific strength of this interaction and the ratio of the driving field to the atomic transition frequency, respectively. Containing 10 13 photons per mode, bright squeezed vacuum light and single-shot quadrature measurement prepare matter in a Gaussian-weighted quantum superposition, unlike multiphoton quantum light which yields classical mixtures due to light-matter entanglement. The high photon number in the squeezed vacuum light is essential for achieving a significant reduction in quantum noise and enhancing the signal-to-noise ratio in the measurement. An ensemble of resonantly electric-dipole-coupled two-level systems is utilised, where brighter squeezed-vacuum light accelerates preparation of the zero-eigenvalue Dicke state. The Dicke state represents a highly correlated collective state of N atoms, where all atoms are in a superposition of being either excited or in the ground state, exhibiting strong quantum correlations.

The success-probability-weighted quantum Fisher information (QFI) scales as N 3/2 with particle number N, demonstrating metrological performance beyond the standard quantum limit. The quantum Fisher information is a measure of the precision with which a parameter can be estimated in a quantum system. The N 3/2 scaling indicates a significant enhancement in precision compared to classical methods, which typically exhibit a linear scaling with N. This improvement is a direct consequence of the quantum correlations induced by the squeezed light and the heralding dynamics. The success-probability-weighted quantum Fisher information scaling as N 3/2 with particle number N exceeds the standard quantum limit for precision measurements. Quadrature-based heralding prepares matter in a Gaussian-weighted quantum superposition, acting as a Gaussian filter with respect to electric polarization and accelerating the formation of a zero-eigenvalue Dicke state, a highly ordered collective state of atoms. Counter-rotating terms then drive a transition towards a cat-like state, a superposition of distinct quantum states. These cat states are particularly interesting as they exhibit macroscopic quantum coherence, meaning that the superposition extends to a macroscopic number of atoms. While these results show major advancements in manipulating matter at a quantum level, ideal measurement conditions are currently required and scalability to complex systems or practical quantum technologies remains to be demonstrated. Maintaining these ideal conditions, such as precise temperature control and minimal environmental noise, is crucial for preserving the fragile quantum states.

Squeezed light and single-shot measurements enable macroscopic quantum states in a two-level atomic system

Generating macroscopic quantum states, where the bizarre rules governing the subatomic world extend to visible matter, promises revolutionary technologies, from ultra-precise sensors to powerful quantum computers. The ability to create and control these states could lead to the development of sensors with unprecedented sensitivity, capable of detecting extremely weak signals, and quantum computers that can solve problems intractable for classical computers. Achieving this feat has long been hampered by the delicate nature of quantum entanglement, as interactions between light and matter often collapse these states into mundane, classical behaviours. This technique bypasses that fragility by utilising bright squeezed vacuum light and single-shot measurements, but currently relies on a specific arrangement of two-level atoms. The use of two-level atoms simplifies the theoretical analysis and experimental implementation, allowing for a clearer demonstration of the underlying principles.

Restricting this technique to two-level atoms is not a fundamental limitation, but a necessary initial step. Extending this approach to more complex systems, such as multi-level atoms or molecules, will require more sophisticated theoretical models and experimental techniques. This type of light, combined with a single-shot measurement of the light’s quadrature, allows for more precise control over quantum systems. The precise control arises from the ability to selectively prepare and manipulate the quantum state of the matter ensemble based on the measurement outcome. The approach enables the ultrafast generation of macroscopic quantum states in matter. This ultrafast generation is a significant advantage, as it allows for the creation of these states before they are destroyed by decoherence. Measurement of the light’s amplitude and phase prepares matter in a Gaussian-weighted quantum superposition, accelerating the preparation of a zero-eigenvalue Dicke state, a highly ordered collective atomic state. Brighter squeezed-vacuum light drives a stroboscopic transition from this Dicke state to a cat-like state, circumventing limitations caused by light-matter entanglement. The stroboscopic transition refers to the repeated application of the squeezed light pulses, effectively “flashing” the system into the desired quantum state.

The research demonstrated the ultrafast generation of macroscopic quantum states in matter using bright squeezed vacuum light and single-shot measurements. This technique overcomes the typical fragility of quantum entanglement between light and matter, preparing an ensemble of two-level atoms in a quantum superposition. By utilising this method, researchers accelerated the preparation of a zero-eigenvalue Dicke state and induced a transition to a cat-like state. The authors suggest extending this approach to more complex systems will require further theoretical modelling and experimental refinement.