Abstract
Quantum technologies are built on the power of coherent superposition. Atomic coherence is typically generated from optical coherence, most often via Rabi oscillations. However, canonical coherent states of light create imperfect resources; a fully-quantized description of "$\tfrac{\pi}{2}$ pulses" shows that the atomic superpositions generated remain entangled with the light. We show that there are quantum states of light that generate coherent atomic states perfectly, with no residual atom-field entanglement. These states can be found for arbitrarily short times and approach slightly-number-squeezed $\tfrac{\pi}{2}$ pulses in the limit of large intensities; similar ideal states can be found for any $(2k+1)\tfrac{\pi}{2}$ pulses, requiring more number squeezing with increasing $k$. Moreover, these states can be repeatedly used as "quantum catalysts" to successfully generate coherent atomic states with high probability. From this perspective we have identified states that are "more coherent" than coherent states.
Highlights
Coherence is the quintessential property of quantum systems
Quantum coherence can be used to drive energy transfer between two systems at thermal equilibrium [7,8], a task forbidden by classical thermodynamics, helping launch the nascent field of quantum thermodynamics [9,10,11,12]
We find quantum states that can produce perfect atomic coherence with no residual atom-field entanglement, but that these states are markedly different from the canonical coherent states— they share some important properties
Summary
Coherence is the quintessential property of quantum systems. Underlying interference, coherence plays a role in optics [1,2], atomic physics [3,4], and beyond, and in multipartite systems is the defining feature of entanglement. Field in units of single-photon intensity, such that a “π/2 pulse” perfectly transfers the atom i√nto a coherent superposition of |g and |e after a time 0 nt = π/2 According to this semiclassical description, coherence between |g and |e is perfect, and the oscillations continue indefinitely in the absence of spontaneous emission or other broadening. Since the pulse area is proportional to n, an uncertainty in photon number amounts to an uncertain pulse area, and atomic coherence that never attains its theoretical maximum; on the other hand, the n → 0 limit leads to perfect coherence between |g, n and |e, n − 1 , the fact that the field has complete welcher Weg information about the state of the atom means that the atom is in a completely classical, incoherent, mixture of |g and |e. The transcoherent states in this optimal family generalize π/2 pulses to quantum descriptions of light and can be used as precursors, or maybe even catalysts [8,46,47], for creating optimally coherent atomic states
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