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Oxford University Press eBooks

Can Quantum Measurements Control Temperature?

Gershon Kurizki, Goren Gordon

Published Apr 16, 2020• 0 citations
Publisher

Abstract

Henry is trapped in a burning building because of Eve’s mischief. Henry’s entanglement with the hot environment is ominous: his states receive excessive energy from the environment, threatening his physiology. Can quantum effects rescue Henry from the fire? Unexpectedly, Eve comes to his rescue. She frequently measures his energy in a “quantum non-demolition” (QND) fashion which is expected to keep his state intact. Surprisingly, Henry grows hotter or colder depending on Eve’s measurement rate! These quantum effects seem to violate the laws of thermodynamics. Engraved in stone as these laws and the ensuing time directionality (time arrow) may be, they fail if the system is examined too frequently, whence time arrow loses its meaning. One may speculate over the role of such anomalies in the Big Bang. These anomalies support Parmenides’ view that time flow is determined by the observer’s choices. The appendix to this chapter elaborates on the dynamics induced by system–environment interaction changes.

DOI
10.1093/oso/9780198787464.003.0011

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Henry is trapped in a burning building because of Eve’s mischief. Henry’s entanglement with the hot environment is ominous: his states receive excessive energy from the environment, threatening his physiology. Can quantum effects rescue Henry from the fire? Unexpectedly, Eve comes to his rescue. She frequently measures his energy in a “quantum non-demolition” (QND) fashion which is expected to keep his state intact. Surprisingly, Henry grows hotter or colder depending on Eve’s measurement rate! These quantum effects seem to violate the laws of thermodynamics. Engraved in stone as these laws and the ensuing time directionality (time arrow) may be, they fail if the system is examined too frequently, whence time arrow loses its meaning. One may speculate over the role of such anomalies in the Big Bang. These anomalies support Parmenides’ view that time flow is determined by the observer’s choices. The appendix to this chapter elaborates on the dynamics induced by system–environment interaction changes.
Henry and Eve try to outrace each other, so as to get first to the unique quantum crystals deposited in an underground mine. In his descent into the mine, Henry resorts to coherent transfer. When he tries to return the same way, he gets stuck halfway up, because his entanglement to Eve’s sensors decoheres him. This scenario represents any environment where the interaction of each constituent with the quantum system is weak. Yet, together these many constituents and the system become completely entangled, causing environment-induced decoherence. This consensual narrative on decoherence conceals conceptual hurdles. Where does one draw the line between the system and the environment? How can the unitarity of QM be reconciled, whereby our past and future are interchangeable, with the irreversible evolution of quantum systems under decoherence, culminating in death? The appendix to this chapter describes coherent oscillations in a quantum system and their decay by decoherence.
In a strange dream, Henry is coherently transported towards his bride down the aisle. But just as a small portion of him arrives next to her, that portion disappears in a flash of light caused by a snapshot! Henry keeps trying to be united with his bride, but repeated snapshots cause Henry’s collapse to being far away from her. This dream illustrates the quantum Zeno effect (QZE): if a measurement collapses the quantum state with high probability to the initial state, then frequent repeated measurements can almost stop the change of the quantum state. Yet less frequent measurements cause the opposite, anti-Zeno effect (AZE), whereby change or decay increases. Thus, decay is controllable. These effects confirm Zeno’s argument that change is an illusion, as it is up to the observer to prevent or induce it by appropriate observation. The appendix to this chapter explains the QZE for coherent and decay processes.
The crucial role of ambient correlations in determining thermodynamic behavior is established. A class of entangled states of two macroscopic systems is constructed such that each component is in a state of thermal equilibrium at a given temperature, and when the two are allowed to interact heat can flow from the colder to the hotter system. A dilute gas model exhibiting this behavior is presented. This reversal of the thermodynamic arrow is a consequence of the entanglement between the two systems, a condition that is opposite to molecular chaos and shown to be unlikely in a low-entropy environment. By contrast, the second law is established by proving Clausius' inequality in a low-entropy environment. These general results strongly support the expectation, first expressed by Boltzmann and subsequently elaborated by others, that the second law is an emergent phenomenon which requires a low-entropy cosmological environment, one that can effectively function as an ideal information sink.
Chapter 4 introduces a great QM mystery: the notion of quantum measurements. Henry is in a superposition of versions localized in several places, but when Eve measures Henry’s position she (as a classical observer) either sees Henry or she does not. Physical reality is made of such measurements. Eve’s measurement projects or collapses Henry’s superposition state to a single location. The meaning of quantum-state or wavefunction “collapse” and the role of the observer have been at the heart of the historical debate concerning the interpretation of QM. Whereas Von Neumann and Wigner stressed the inseparability of the observed (measured) world from the human mind, alternative “observer-free” views were suggested, such as Everett’s many-world interpretation or Zurek’s quantum Darwinism that replaces the observer by the environment. In the appendix to this chapter the notion of probability amplitudes is elucidated, new notations for operators are introduced and projection operators are presented.
Eve attempts to eavesdrop on Henry, who discovers that her eavesdropping strategy is susceptible to hindrances imposed by the QM uncertainty principle. Eve can detect Henry’s whereabouts to any precision she chooses. However, a precise position measurement induces a large spread of Henry’s velocities and renders his position extremely uncertain, defeating Eve’s attempt to further track him. Such restriction on the maximal accuracy of measuring “complementary” observables was viewed by Bohr as a fundamental limitation on human perception of the world. A deeper explanation of complementarity may assume that the observer’s cognition and the observed object are inseparable, in the spirit of Spinoza, who considered nature as one substance with both mental and physical modalities. By studying these modalities, human knowledge may some day be freed from complementarity constraints. The appendix to this chapter introduces operators and their commutation relation and clarifies the position–momentum uncertainty relation.
We study deviations from thermal equilibrium between two-level systems (TLS) and a bath by frequent and brief quantum measurements of the TLS energy-states. The resulting entropy and temperature of both the system and the bath are found to be completely determined by the measurement rate, and unrelated to what is expected by standard thermodynamical rules that hold for Markovian baths. These anomalies allow for very fast control heating, cooling and state-purification (entropy reduction) of quantum systems much sooner than their thermal equilibration time.