Coherence in Microchip Traps

We report the coherent manipulation of internal states of neutral atoms in a magnetic microchip trap. Coherence lifetimes exceeding 1 s are observed with atoms at distances of $5–130\mu \mathrm{m}$ from the microchip surface. The coherence lifetime in the chip trap is independent of atom-surface distance within our measurement accuracy and agrees well with the results of similar measurements in macroscopic magnetic traps. Because of the absence of surface-induced decoherence, a miniaturized atomic clock with a relative stability in the ${10}^{-13}$ range can be realized. For applications in quantum information processing, we propose to use microwave near fields in the proximity of chip wires to create potentials that depend on the internal state of the atoms.

Figure 1

Ramsey spectroscopy of the $|0⟩\leftrightarrow |1⟩$ transition with atoms held at a distance $d=9\mu \mathrm{m}$ from the chip surface. An exponentially damped sine fit to the Ramsey fringes yields a $1/e$ coherence lifetime of ${\tau }_c=2.8\pm 1.6\mathrm{s}$. Each data point corresponds to a single shot of the experiment.

Figure 2

(a) Layout of the relevant wires on the chip substrate. C1: position of the initial magnetic trap. C2: position of the measurement trap used in the experiments. (b) Layer structure of the substrate. Current is carried by gold wires $h_e\sim 27\mu \mathrm{m}$ below the silver surface. $d$ denotes the atom-surface distance, while $d_w$ refers to the atom-wire distance.

Figure 3

Contrast $C(T_R)$ of the Ramsey fringes as a function of atom-surface distance $d$ for two values of the time delay $T_R$ between the $\pi /2$ pulses. For each data point, $C(T_R)=(N_{\max }-N_{\min })/(N_{\max }+N_{\min })$ was obtained from a sinusoidal fit to frequency-domain Ramsey fringes. $N_{\max }$ ($N_{\min }$) is the maximum (minimum) of the oscillation in $N_1$. The data points for $d=(5,9,22,54,132)\mu \mathrm{m}$ were measured with atomic ensembles of temperatures $T=(0.2,0.6,0.7,0.6,0.3)\mu \mathrm{K}$ and peak densities $n_0=(4,3,1,1,5)\times {10}^{12}{\mathrm{cm}}^{-3}$.

Figure 4

(a) Ramsey resonance for $T_R=1\mathrm{s}$ in the microtrap at $d=54\mu \mathrm{m}$ from the surface (solid circles). The line is a sinusoidal fit to the data. The arrow indicates the operating point on the slope of the resonance used in the measurement of $\sigma (\tau )$. Frequency fluctuations $\delta \nu$ lead to fluctuations $\delta N$ in the detected number of atoms. (b) Measured Allan standard deviation $\sigma (\tau )$ of the atomic clock in the microtrap compared to the quartz reference oscillator (open circles). The solid line is a fit with $\sigma (\tau )\propto {\tau }^{-1/2}$, representing the performance of the atomic clock. For $\tau >6\times {10}^2\mathrm{s}$, the drift of the quartz reference becomes apparent. The manufacturers’ specification of the quartz stability is shown as solid squares.