Chiral control of quantum states in non-Hermitian spin-orbit-coupled fermions

Abstract

While spin-orbit coupling (SOC), an essential mechanism underlying quantum phenomena from the spin Hall effect to topological insulators [1, 2], has been widely studied in well-isolated Hermitian systems, much less is known when the dissipation plays a major role in spin-orbit-coupled quantum systems [3]. Here, we realize dissipative spin-orbit-coupled bands filled with ultracold fermions, and observe a parity-time (PT ) symmetrybreaking transition as a result of the competition between SOC and dissipation. Tunable dissipation, introduced by state-selective atom loss, enables the energy gap, opened by SOC, to be engineered and closed at the critical dissipation value, the so-called exceptional point (EP) [4]. The realized EP of the non-Hermitian band structure exhibits chiral response when the quantum state changes near the EP. This topological feature enables us to tune SOC and dissipation dynamically in the parameter space, and observe the state evolution is direction-dependent near the EP, revealing topologically robust spin transfer between different quantum states when the quantum state encircles the EP. This topological control of quantum states for non-Hermitian fermions provides new methods of quantum control, and also sets the stage for exploring non-Hermitian topological states with SOC.

Gain/loss effects on spin-orbit coupled ultracold atoms in two-dimensional optical lattices

“…1, the second term represents a spin-orbit coupling with the inter-spin hopping strength t and the phase η σ φ m between nearest-neighbor sites denoted by nm , where η σ=↑,↓ = ±, and φ m = 0, −π/2, π, π/2 for m = (n x + 1, n y ), (n x , n y + 1), (n x − 1, n y ), (n x , n y − 1), respectively. h (∆) is a complex number and can be regarded as a complex Zeeman field (staggered potential), resulting from the state-dependent atom loss [17,18]. In the following, we set the intra-spin hopping strength t = 1 as the energy unit and a = 1 as the length unit.…”

“…In realistic experiments, the loss cannot be completely avoided due to the coupling of systems to the environment or measurement [11]; for cold atoms, few-body losses play inevitable roles in the preparation of degenerate quantum gases [2] and in the simulation of quantum many-body physics [3]. On the other hand, the non-Hermitian physics attracts increasing attention of almost all branches of physics in recent years [12], where abundant exotic phenomena, such as the spontaneous breaking of the parity-time (PT ) symmetry [13][14][15][16][17][18], the breakdown of the conventional bulk-boundary correspondence [19][20][21][22][23][24][25][26][27][28][29], the exceptional topology [30], and the interplay with Anderson localization [31][32][33][34][35][36][37], have been widely exploited both in theory and experiment. As for cold atoms, the experimental techniques are mature to engineer state-dependent atom losses [17,18] and the effective nonreciprocal hoppings [38] of non-Hermitian systems, which are fundamental operations for the construction of a non-Hermitian model.…”

“…Since the non-Hermiticity can be experimentally engineered in cold atoms, we're wondering about gain/loss effects on spin-orbit coupled ultracold atoms in optical lattices. To this aim, by incorporating the gain/loss techniques [17,18] into the spin-orbit coupled ultracold atoms in 2D optical lattices experimentally realized by Ref. [7,8], we investigate the corresponding four-band tight-binding model with both the spin-dependent and the sublattice-staggered gains/losses, and analytically illustrate a gain/loss induced topological phase transition by the method of block diagonalization.…”

Non-Hermitian Absorption Spectroscopy

“…Such systems usually exhibit complex band structures with exceptional points or rings . In cold atom systems, non-Hermitian Hamiltonians have been experimentally realized by introducing atom loss [35][36][37][38][39][40][41]. The paritytime (PT ) symmetry breaking has been observed by measuring the population of an evolving state of a system through quench dynamics [35,36,42].…”

“…In cold atom systems, non-Hermitian Hamiltonians have been experimentally realized by introducing atom loss [35][36][37][38][39][40][41]. The paritytime (PT ) symmetry breaking has been observed by measuring the population of an evolving state of a system through quench dynamics [35,36,42]. However, such a method is very hard to generalize to a generic case without PT symmetry.…”