[Paper Review] Quantum measurement induces a many-body transition

Michael S. Ferguson, Leon C. Camenzind+6 more|arXiv (Cornell University)|Oct 9, 2020

Quantum and electron transport phenomena29 references4 citations

TL;DR

This paper demonstrates that quantum measurement backaction—typically considered a minor disturbance—can induce a many-body transition in a double quantum dot system by altering the system's ground-state charge configuration. Using a charge sensor dot to measure electron population, the authors observe a transition to a higher-energy charge state due to detector-induced backaction, which is explained by a theoretical model incorporating non-ideal detector dynamics and spin degeneracy effects.

ABSTRACT

The current revolution in quantum technologies relies on the ability to isolate, coherently control, and measure the state of quantum systems. The act of measurement in quantum mechanics, however, is naturally invasive as the measurement apparatus becomes entangled with the system that it observes. Even for ideal detectors, the measurement outcome always leads to a disturbance in the observed system, a phenomenon called quantum measurement backaction. Here we report a profound change in the many-body properties of the measured system due to quantum measurements. We observe this backaction-induced transition in a mesoscopic double quantum-dot in the Coulomb-blockade regime, where we switch the electron population through measurement with a charge sensor dot. Our finding showcases the important changes in behaviour that can arise due to quantum detectors, which are ubiquitous in quantum technologies.

Motivation & Objective

• To investigate how non-ideal quantum measurement backaction—beyond the ideal detector paradigm—affects the many-body state of a quantum system.
• To identify and characterize a transition in the ground-state charge configuration of a double quantum dot induced by measurement backaction.
• To develop a theoretical framework that explains the observed distortion in the charge stability diagram due to detector-induced backaction.
• To probe the role of detector dynamics, including broadening mechanisms and spin degeneracy, in shaping the observed transition.
• To establish that quantum detectors can act as out-of-equilibrium environments that drive phase transitions in many-body systems.

Proposed method

• Employ a mesoscopic double quantum dot (DD) in the Coulomb blockade regime, with tunnel coupling to leads and a nearby charge sensor dot (CSD) for real-time charge state monitoring.
• Apply a measurement bias voltage $V_{ m M}$ to the CSD, inducing backaction on the DD through capacitive coupling and electron tunneling processes.
• Measure the charge stability diagram of the DD under varying $V_{ m M}$ and temperature $T$, observing distortions in the boundaries between charge states.
• Model the backaction using rate equations with broadened transition rates, incorporating both Lorentzian and Gaussian forms for the detector's response function.
• Include spin degeneracy in the rate equations to assess its impact on particle-hole symmetry breaking in the observed 'S'-shaped features.
• Fit the model to experimental data using parameters $\chi^\mathrm{G} = 0.099$ and $\xi^\mathrm{G} = 0.084U$, adjusting for detector-induced broadening and asymmetry.

Experimental results

Research questions

• RQ1 Can quantum measurement backaction induce a transition between distinct many-body phases in a double quantum dot system?
• RQ2 How does the non-ideal nature of the detector—specifically its backaction—alter the observed charge configuration in the double quantum dot?
• RQ3 What role does spin degeneracy play in breaking particle-hole symmetry in the measured 'S'-shaped features of the stability diagram?
• RQ4 How do different broadening mechanisms (Lorentzian vs. Gaussian) affect the shape and asymmetry of the transition region?
• RQ5 Can the observed backaction-induced transition be quantitatively described by a theoretical model that accounts for detector dynamics and non-equilibrium effects?

Key findings

• Quantum measurement backaction induces a transition from a low-energy charge configuration to a higher-energy one, as evidenced by a distortion in the charge stability diagram of the double quantum dot.
• The transition is observed when measurement bias $V_{ m M} = 75\,\mu\mathrm{V}$, where interdot tunneling is suppressed, revealing backaction effects that are absent at higher $V_{ m M}$.
• The system preferentially populates a state that is energetically unfavorable in the absence of backaction, consistent with theoretical predictions for population switching.
• Spin degeneracy is identified as a key factor in the observed particle-hole asymmetry of the 'S'-shaped feature, with a stronger effect in Lorentzian broadening than in Gaussian broadening.
• The data suggest the detector's response function has a sharper tail than a Lorentzian, as indicated by the asymmetry dependence on broadening type.
• The model fits the experimental data using $\chi^\mathrm{G} = 0.099$ and $\xi^\mathrm{G} = 0.084U$, with the latter value significantly larger than expected ($\sim 1\,\mu\mathrm{eV}$), indicating strong non-ideal backaction effects.