Quantized Conductance through a Spin-Selective Atomic Point Contact

We implement a microscopic spin filter for cold fermionic atoms in a quantum point contact (QPC) and create fully spin-polarized currents while retaining conductance quantization. Key to our scheme is a near-resonant optical tweezer inducing a large effective Zeeman shift inside the QPC while its local character limits dissipation. We observe a renormalization of this shift due to interactions of only a few atoms in the QPC. Our work represents the analog of an actual spintronic device and paves the way to studying the interplay between spin splitting and interactions far from equilibrium.

Figure 1

Creating spin-polarized currents through an atomic QPC. (a) A near-resonant optical tweezer (red) with waist $w_s=2.0(1)\mu \mathrm{m}$, which introduces an effective Zeeman shift $V_s$ inside an optically defined QPC (green). This tweezer allows ${}^6\mathrm{Li}$ atoms in the lowest hyperfine state ($|\downarrow ⟩$, orange) to flow between two reservoirs while blocking atoms in the third-lowest state ($|\uparrow ⟩$, blue), thereby acting as a spin filter with losses determined by the photon scattering rate ${\mathrm{\Gamma }}_s$. A far-detuned attractive gate beam with negative potential $-V_g$ (dashed circle) locally increases the chemical potential ${\mu }_{\text{res}}$ imposed by the reservoirs. (b) Time evolution of the relative atom number difference between the left and right reservoirs $\mathrm{\Delta }N/N=(N_L-N_R)/(N_L+N_R)$, obtained for a mean chemical potential $V_g+{\mu }_{\text{res}}=k_B\times 0.61(2)\mu \mathrm{K}$ and a near-resonant beam power $P_s=20(6)\mathrm{pW}$, and at a scattering length $a=0(7)a_0$. It is constant for $|\uparrow ⟩$ and has a decay time of 25(1) s for $|\downarrow ⟩$. The associated currents $I_{\uparrow }=-19\pm 85$ atoms/s and $I_{\downarrow }=833\pm 98\mathrm{atoms}/\mathrm{s}$ indicate fully polarized transport within the fit error. Error bars show the standard error of the mean of five measurements here and in Fig. 2. Inset: Optical density (OD) after 6 s of left and right reservoirs over $970\times 320\mu \mathrm{m}$, averaged over five absorption images.

Figure 2

Evolution of the reservoir thermodynamical quantities. (a) Losses in atom number $N=N_L+N_R$, leading to a small variation of (b) mean reservoir chemical potential ${\mu }_{\text{res}}=({\mu }_L+{\mu }_R)/2$. (c) Mean temperature $T=(T_L+T_R)/2$, which is constant over experimental timescales.

Figure 3

Lifting the spin degeneracy of the QPC ground state. (a) Conductance $G$ of each spin state at scattering length $a=91(7)a_0$ versus local chemical potential $V_g+{\mu }_{\text{res}}$ without a near-resonant beam; (b) with peak intensity $I_s=0.13(4)I_{\text{sat}}$ (as in Figs. 1 and 2), where $I_{\text{sat}}$ is the $D_2$-line saturation intensity; (c) with $I_s=0.17(5)I_{\text{sat}}$. Here and in the following, error bars correspond to the standard error of the mean of three measurements. Fits by a Landauer model are shown as solid curves and indicate an increase of the spin-dependent potential up to $V_s=k_B\times 0.44(2)\mu \mathrm{K}$. (d)–(f) Quasi-1D potentials along the transport direction $y$. The chemical potential of Figs. 1 and 2 is indicated by dashed lines in (b) and (e).

Figure 4

Effect of 1D interactions on transport. (a) Conductance $G$ at scattering lengths $a=-800.0(7)a_0$ and $a=+800(16)a_0$, near-resonant beam intensity $I_s=0.13(4)I_{\text{sat}}$, and equal detuning from both states, $\overline{\delta }=0$. Solid curves indicate fits with a logistic function to extract the energy separation $V_{s,\mathrm{eff}}$. (b) Fitted separation $V_{s,\mathrm{eff}}$ versus $I_s$ normalized by the $D_2$-line saturation intensity $I_{\text{sat}}$. The vertical dotted line indicates the intensity used in (a). (c) Scattering length $a$ versus magnetic field, tuned to the left of a broad Feshbach resonance (dash-dotted line). The magnetic field is calibrated with a relative uncertainty of 0.1%. (d) Ratio $V_{s,\mathrm{eff}}/(I/I_{\text{sat}})$ versus scattering length $a$, obtained by linear regression displayed as solid lines in (b). A Hartree mean-field model including a finite temperature of 66 nK is indicated as dashed lines in (b) and (d).