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arXiv:1803.06599 · Physical Review Applied

Single-Photon-Triggered Quantum Phase Transition

Xin-You Lü, Li-Li Zheng, Gui-Lei Zhu, Ying Wu

Published Jun 7, 2018• 66 citations
arXivHTMLPDF

Abstract

The superradiant quantum phase transition (QPT) is of fundamental interest, and has potential applications in modern quantum technology. The existence of an equilibrium superradiant QPT in cavity and circuit QED systems is still debated, though, considering the no-go theorem. Combining cavity QED and optomechanics, the authors predict at thermal equilibrium a single-photon-triggered superradiant QPT that is immune to the no-go theorem. Being able to manipulate a QPT in this manner is expected to inspire both the creation of innovative photonic quantum devices and the exploration of single-photon quantum-criticality physics.

DOI
10.1103/physrevapplied.9.064006

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The superradiant quantum phase transition (QPT) is of fundamental interest, and has potential applications in modern quantum technology. The existence of an equilibrium superradiant QPT in cavity and circuit QED systems is still debated, though, considering the no-go theorem. Combining cavity QED and optomechanics, the authors predict at thermal equilibrium a single-photon-triggered superradiant QPT that is immune to the no-go theorem. Being able to manipulate a QPT in this manner is expected to inspire both the creation of innovative photonic quantum devices and the exploration of single-photon quantum-criticality physics.
We investigate the equilibrium behavior of a superconducting circuit QED system containing a large number of artificial atoms. It is shown that the currently accepted standard description of circuit QED via an effective model fails in an important aspect: it predicts the possibility of a superradiant phase transition, even though a full microscopic treatment reveals that a no-go theorem for such phase transitions known from cavity QED applies to circuit QED systems as well. We generalize the no-go theorem to the case of (artificial) atoms with many energy levels and thus make it more applicable for realistic cavity or circuit QED systems.
Equilibrium phase transitions between a normal and a photon condensate state (also known as superradiant phase transitions) are a highly debated research topic, where proposals for their occurrence and no-go theorems have chased each other for the past four decades. Previous no-go theorems have demonstrated that gauge invariance forbids second-order phase transitions to a photon condensate state when the cavity-photon mode is assumed to be spatially uniform. First-order phase transitions were previously considered as possible paths to bypass the no-go theorem. In particular, it has been theoretically predicted that a collection of three-level systems coupled to light can display a first-order phase transition to a photon condensate state. Here, we lay down a general no-go theorem, which forbids first-order as well as second-order superradiant phase transitions in a spatially-uniform cavity field. By using the tools of lattice gauge theory, we then derive a fully gauge-invariant model for three-level systems coupled to light. In agreement with the general theorem, our model does not display photon condensation.
The superradiant phase transition (SPT) has been widely studied in cavity quantum electrodynamics (CQED). However, this SPT is still subject to ongoing debates due to the no-go theorem induced by the so-called A2{\mathbf{A}}^{2}A2 term (AT). We propose a hybrid quantum system, consisting of a single-mode cavity simultaneously coupled to both a two-level system and yttrium-iron-garnet sphere supporting magnons with Kerr nonlinearity, to restore the SPT against the AT. The Kerr magnons here can effectively introduce an additional AT tunable and strong enough to counteract the intrinsic AT, via adiabatically eliminating the degrees of freedom of the magnons. We show that the Kerr-magnon-induced SPT can exist in both cases of ignoring and including the intrinsic AT. Without the intrinsic AT, the critical coupling strength can be dramatically reduced by introducing the Kerr magnons, which greatly relaxes the experimental conditions for observing the SPT. With the intrinsic AT, the forbidden SPT can be recovered with the Kerr magnons in a reversed way. Our work paves a potential way to manipulate the SPT against the AT in hybrid systems combining CQED and nonlinear magnonics.
Abstract The superradiant phase transition in thermal equilibrium is a fundamental concept bridging statistical physics and electrodynamics, which has never been observed in real physical systems since the first proposal in the 1970s. The existence of this phase transition in cavity quantum electrodynamics systems is still subject of ongoing debates due to the no-go theorem induced by the so-called A2 term. Moreover, experimental conditions to study this phase transition are hard to achieve with current accessible technology. Based on the platform of nuclear magnetic resonance, here we experimentally simulate the occurrence of an equilibrium superradiant phase transition beyond no-go theorem by introducing the antisqueezing effect. The mechanism relies on that the antisqueezing effect recovers the singularity of the ground state via exponentially enhancing the zero point fluctuation of system. The strongly entangled and squeezed Schrödinger cat states of spins are achieved experimentally in the superradiant phase, which may play an important role in fundamental tests of quantum theory and implementations of quantum metrology.
We predict the occurrence of single-photon-induced entanglement and quantum superposition in a hybrid quantum model, introducing an optomechanical coupling into the Rabi model. Originally, it comes from the photon-dependent quantum property of the ground state featured by the proposed hybrid model. It is associated with a single-photon-induced quantum phase transition, and is immune to the A2{A}^{2}A2 term of the spin-field interaction. Moreover, the obtained quantum superposition state is actually a squeezed cat state, which can significantly enhance precision in quantum metrology. This work offers an approach to manipulate entanglement and quantum superposition with a single photon, which might have potential applications in the engineering of new single-photon quantum devices, and also fundamentally broaden the regime of cavity QED.