How ‘Bright Squeezed Vacuum’ Light Achieves a 24-Fold Boost in Laser Interactions Without Extra Power

Those rare, extreme spikes are the key. When one coincides with an atom, it can surpass the threshold for a nonlinear process like tunneling ionization without any increase in average power.

The experiment: 300 nJ does the work of 7.1 µJ

In the critical experiment, Jian Wu’s group at East China Normal University directed a BSV pulse with an average energy of just 300 nanojoules at a single sodium atom . The resulting tunneling ionization yield matched what the team could only otherwise achieve using a classical coherent laser pulse with an energy of 7.1 microjoules .

That is an effective ~24-fold enhancement in nonlinear efficiency. The researchers did not increase the laser power; they manipulated the quantum statistics of the light. Furthermore, by adjusting the degree of phase squeezing, they could precisely control the effective intensity of the BSV—like turning a dial—while keeping the average pulse energy fixed .

Key experimental result

This is the first experimental observation of a nonlinear quantum resource outperforming classical light in a strong-field process .

Why this matters for attosecond science

Tunneling ionization is the crucial first step in high-harmonic generation (HHG), the standard table-top method for producing attosecond pulses of extreme ultraviolet (XUV) light . These pulses are the strobe lights of the atomic world, used to film electron motion. The BSV technique could change the game in several ways.

First, it offers a path to brighter attosecond pulses without building larger and more damaging pump lasers. By boosting the ionization yield with quantum statistics, researchers can potentially generate more intense high-harmonic light from the same or even lower average pump energy .

Second, the quantum properties of the BSV driver can be transferred to the attosecond pulses. Recent work has shown that when BSV combines with a strong laser field to drive HHG, the resulting XUV pulses can inherit the squeezing characteristics of the driver, producing nonclassical light in a new spectral range .

Third, and perhaps most practically, the technique could drastically reduce sample damage. In many attosecond pump-probe experiments, the very bright pulses needed to trigger a response also risk destroying the sample. BSV delivers high peak fields while keeping the total energy deposited low, making it a potentially gentler probe .

A critical supporting advance has come from the Technion – Israel Institute of Technology, where researchers recently demonstrated single-shot temporal characterization of femtosecond BSV pulses . Being able to measure the precise temporal profile of individual BSV shots is essential for deploying these inherently fluctuating pulses in real experimental sequences.

A new toolkit for nonlinear optics

The principle extends far beyond gas-phase atoms. BSV has been shown to drive strong-field photoemission from metal needle tips, producing the telltale high-energy electron plateaus and cutoffs that are signatures of extreme nonlinear physics . Theoretical and early experimental work points to potential quantum enhancement in high-harmonic generation itself, above-threshold ionization, and even nonlinear tunneling in solid-state dielectrics .

There is, however, a serious challenge. BSV is fragile. Propagating these quantum states through any medium introduces losses that degrade the squeezing. Atomic ground-state depletion and ionization of the medium can act as decoherence channels, with one study finding that such effects can reduce harmonic yield by more than two orders of magnitude compared to coherent laser light . Engineering materials and interaction geometries that preserve the quantum statistics during propagation is now a central research target.

The bigger picture: quantum noise as a resource

This work sits at the center of a paradigm shift in quantum optics. For most of its history, quantum noise was the enemy—a fundamental limit on measurement precision that engineers fought to suppress. The BSV result is the latest and most dramatic demonstration that quantum fluctuations can be reframed as a controllable, functional resource .

Squeezing effectively converts quantum statistics into a new kind of nonlinear driving power. This idea is crystallizing across multiple research frontiers:

• Attosecond-resolved control: The degree and type of squeezing can now be modulated on sub-cycle, attosecond timescales, opening the door to real-time quantum state engineering .
• Quantum-optimized microscopy: Bright squeezed pulses have been generated at power levels compatible with biological imaging, offering a route to quantum-enhanced precision microscopy that doesn't fry live cells .
• Strong-field quantum light sources: The strong-field process itself can become a source of exotic quantum states. HHG driven by intense squeezed light can produce optical Schrödinger cat states and multimode squeezed fields, turning a classical nonlinear process into a quantum light engine .

A 24-fold enhancement achieved by switching the light's statistics rather than turning up the power is not just a clever experimental trick. It resets the conversation about how we drive nonlinear processes at the quantum limit and marks a step toward a future where the boundary between quantum optics and strong-field physics disappears entirely.