Watching Quantum Systems Alters Them, Creating New Particles and Entanglement

Researchers are increasingly recognising that measurement fundamentally alters quantum systems, not merely revealing their state. Huy Nguyen, Yu-Xin Wang, and Jacob Taylor, all from the Joint Center for Quantum Information and Computer Science at the University of Maryland-NIST, demonstrate this principle through a detailed theoretical investigation of Bose-Einstein-condensate arrays. Their work elucidates how phase contrast imaging, a common experimental technique, significantly impacts the observed dynamics and induces backaction on the system, specifically creating and diffusing quasiparticles. This research is significant because it identifies regimes where imaging selectively probes either particles or quasiparticles, and importantly, reveals a pathway to directly measure quasiparticle modes and control their creation, offering insights into probing many-body systems and potentially testing predictions from novel models of fundamental physics.

Selective quasiparticle probing via light-matter interaction in Bose-Einstein condensates

Researchers have demonstrated a pathway to selectively measure and control quasiparticles, emergent excitations in quantum systems, within a Bose-Einstein condensate array using phase contrast imaging. This work reveals that the parameters chosen for a standard measurement process profoundly impact both the observed data and the subsequent evolution of the quantum system itself.
Specifically, the study details how careful adjustment of imaging light can probe either the underlying particles or the quasiparticles, offering unprecedented control over their creation and diffusion. This achievement addresses a critical challenge in probing many-body systems, where measurement inevitably influences the system’s state and introduces complexities in interpreting experimental results.

The theoretical investigation focuses on a low-temperature, low-momentum Bose-Einstein-condensate array, revealing regimes where imaging light selectively interacts with either bare particles or quasiparticles. By manipulating the measurement bandwidth, researchers can effectively switch between observing the fundamental atomic constituents and directly measuring the emergent quasiparticle modes.
Furthermore, the study establishes a method for controlling the measurement-induced creation and diffusion of quasiparticles into different momentum states, minimizing unwanted heating effects. This precise control is achieved through careful consideration of the amplitude and characteristics of the phase-contrast imaging setup.

This ability to selectively measure quasiparticles without significant heating represents a substantial advancement in experimental techniques for investigating quantum many-body systems. The research demonstrates that the same imaging setup, when tuned appropriately, can either directly image the atoms, naturally generating quasiparticles, or focus on detecting the quasiparticles with minimal disturbance.

This dual capability has wide-ranging implications, potentially enabling more efficient and accurate investigations of complex quantum phenomena. The findings also provide a foundation for exploring theoretical concepts such as renormalization in quantum field theories and the interplay between theory and experimental probing methods.

Beyond practical applications in quantum measurement, this work connects to fundamental questions in physics, including the potential for observable consequences of ‘spontaneous collapse’ predictions arising from novel models of quantum gravity on aspects of the Standard Model. By providing a quantitative understanding of measurement-induced dynamics, the study opens avenues for exploring the role of renormalization and its connection to how quantum theories are probed, potentially bridging the gap between theoretical predictions and experimental observations in areas such as gravitationally-induced decoherence. The research lays the groundwork for future investigations into this novel domain of understanding quantum systems and their interaction with measurement processes.

Imaging induced perturbations and quasiparticle dynamics in Bose-Einstein condensates

Phase contrast imaging serves as the foundational technique in this research, meticulously employed to investigate the impact of measurement parameters on cold atom systems. The study centres on a Bose-Einstein-condensate array, specifically examining the low-temperature and low-momentum regimes to understand how imaging influences both observed outcomes and the system’s subsequent evolution.

Theoretical investigation reveals regimes where the imaging light selectively probes either bare particle or quasiparticle dynamics, demonstrating a nuanced control over the measurement process. A second-order Schrieffer-Wolff transformation, akin to perturbation theory, was implemented to decouple the probe state from the system states within the Hamiltonian.

This transformation, however, introduces a detuning at state |L⟩ and perturbatively dresses Rabi oscillations between atomic levels, necessitating careful consideration of the energy gap between subspaces. Adiabatic elimination of the probe state |r⟩, maintaining leading-order O(Ω2) terms, yielded an effective Hamiltonian and jump operator acting solely on the atom states |L⟩ and |R⟩, detailed in Appendix A.

The resulting narrow-bandwidth effective Hamiltonian incorporates a “dressed” tunneling rate t′ ≡ 1 − Ω2 2Ω2 SW t and a Stark shift δ′ L ≡−Ω2(∆−δR) Ω2 SW, originating from the Schrieffer-Wolff transformation. The effective jump operator, detailed in equation (13), implies an effective transition to |L⟩ from a superposition of double-well states, |ψnarrow⟩∝|R⟩+ δR −∆ t |L⟩, enabling selective measurement of eigenstates of the tunneling Hamiltonian.

Tuning the imaging parameters near-resonance at ∆= δR + t2 ε1 allows for targeted measurement of a desired state, such as |ε1⟩∝1 2 δR − p δ2 R + 4t2 |L⟩−t |R⟩, provided the Schrieffer-Wolff condition Ω≪|∆−ε1|, |∆−ε2| is satisfied. Extending this methodology, the research explores a weakly interacting 1-dimensional Bose-Hubbard lattice, coupling a bare bosonic particle state to an excited state via a Rabi drive and subjecting it to weak measurement. Two measurement types were considered: particle loss, where excited bosons leak from the system, and particle counting, where they remain confined.

Imaging parameter control of quasiparticle dynamics in cold atom systems

Researchers demonstrate that careful parameter selection in phase contrast imaging of cold atom systems significantly impacts both observation and backaction on the system, including the creation and diffusion of quasiparticles. Theoretical investigation reveals regimes where imaging light selectively probes either bare particle or quasiparticle dynamics.

Specifically, the study establishes a pathway for directly measuring quasiparticle modes and controlling measurement-induced quasiparticle creation and diffusion into different momentum states. In a double-well potential, the work details atomic behavior under weak continuous measurement. By adiabatically eliminating a fast probe state, researchers derived effective measurement observables and changes to the system Hamiltonian.

Modeling an atom with trapped states |L⟩ and |R⟩, alongside a probe state |r⟩, the Hamiltonian includes a detuning of δR for the right-hand-side trap potential state and ∆ for the probe state, with a tunneling rate of t and a Rabi frequency of Ω between |L⟩ and |r⟩. A continuously monitoring laser beam induces a relaxation process from |r⟩ to |L⟩, described by a jump operator Γ with a rate of κ.

The study explores two measurement bandwidth regimes. In the wide bandwidth regime, where ∆ is much larger than Ω, and κ is large relative to Ω, the dynamics of the probe state |r⟩ rapidly evolve and decay. This allows for the extraction of an effective measurement-induced Stark-like shift in atomic frequency and an effective measurement observable formed by the bare atomic states.

Conversely, in the narrow bandwidth regime, where Ω and κ are small compared to t, δR, and ∆, a Schrieffer-Wolff transformation decouples the dressed atom and probe state. This introduces corrections to the external state detuning, the internal Hamiltonian, and a leading-order correction δΓ to the jump operator, effectively describing a measurement process with respect to a superposition state between the atomic trapped states.

Imaging parameter effects on Bose-Einstein condensate quasiparticle dynamics and measurement

Researchers have demonstrated that the parameters used in phase contrast imaging of cold atom systems significantly influence both observational outcomes and the subsequent dynamics of the system itself. This work focuses on Bose-Einstein-condensate arrays at low temperatures and momenta, revealing regimes where imaging light selectively probes either the underlying particles or emergent quasiparticles.

Furthermore, the investigation establishes a pathway for directly measuring quasiparticle modes and controlling the creation and diffusion of these quasiparticles into different momentum states. The theoretical analysis employs a mean-field approach, approximating weak interactions as a squeezing-type quadratic interaction, allowing for diagonalization of the Bose-Hubbard Hamiltonian via a linear Bogoliubov transformation.

This methodology enables a detailed understanding of how measurement processes impact the system’s evolution, specifically concerning the emergence and behaviour of quasiparticles. The study details how the choice of measurement parameters affects the observed dynamics, allowing for selective probing of either bare particles or quasiparticles, and providing control over measurement-induced quasiparticle creation and diffusion.

Acknowledging the limitations of the mean-field approximation, the authors focused on scenarios where the condensate remains largely intact during measurement, simplifying the complex dynamics. Future research could explore the implications of these findings for more complex many-body systems and investigate the potential connection to predictions from novel models, such as those concerning spontaneous collapse within the Standard Model. These results provide a foundation for interpreting experimental observations in monitored Bose-Einstein condensates and open avenues for exploring fundamental questions in quantum measurement and many-body physics.