Long coherence times for Rydberg qubits on a superconducting atom chip

Superconducting atom chips and Rydberg atoms are promising tools for quantum information processing operations based on the dipole blockade effect. Nevertheless, one has to face the severe problem of stray electric fields in the vicinity of the chip. We demonstrate a simple method circumventing this problem. Microwave spectroscopy reveals extremely long coherence lifetimes (in the millisecond range) for a qubit stored in a Rydberg level superposition close to the chip surface. This is an essential step for the development of quantum simulations with Rydberg atoms and of a hybrid quantum information architecture based on atomic ensembles and superconducting circuits.

Figure 1

(a) Scheme of the experimental setup. (b) Scheme of the superconducting atom chip. The letters $G$ to $M$ label the current input pads on the chip. (c) Scheme of the field-ionization detection system. The field-ionization electrode is $I$. The electrode $D$ deflects the ions towards the channeltron counter. Note the axes' definitions in the three panels. The origin $O$ is taken at the center of the horizontal segment of the $Z$-wire connecting pads $G$ and $L$ (the axes are shown offset with respect to this origin for clarity).

Figure 2

Laser spectroscopy of the $5S\to 60S$ two-photon transition. The ion signal (arbitrary units) is shown as a function of the blue laser detuning with regard to the expected line position in zero field (dashed vertical line). The black and red/gray points are taken at a 40 min time interval in a MOT at $y=550$ $\mu \text{m}$ from the chip with a gold front mirror. The respective linewidths are 30 and 40 MHz. The asymmetric Lorentzian line shapes are guides to the eye. The blue triangles show the laser line after the rubidium metallic layer deposit, in a magnetic trap at $y=670$ $\mu \text{m}$, $z=0$. The solid line is a Gaussian fit, with a 1.7 MHz full width at half maximum (FWHM).

Figure 3

(a) Spectroscopy of the $60S_{1/2},m_J=1/2\to 60P_{3/2},m_J=-1/2$ (left) and $m_J=3/2$ (right) transitions in the magnetic trap at $y=455\mu \text{m}$ from the chip. The transfer probability is shown as a function of the microwave detuning with regard to the zero-field line position. The points are experimental, with statistical standard deviation error bars. The red lines are Lorentzian fits. (b) Parallel electric field modulus $|E_{\parallel }|$ with respect to the atom-chip distance $y$.

Figure 4

(a) Spectroscopy of the $60S\to 61S$ two-photon transition. The transfer probability is shown as a function of twice the applied microwave frequency. The points are experimental with statistical standard deviation error bars. The line is a Lorentzian fit. (b) Ramsey spectroscopy of the $60S\to 61S$ transition at $y=455\mu \text{m}$ and $z=-350\mu \text{m}$. The transfer probability is shown as a function of the delay $T$ between the two $\pi /2$ pulses. The dots are experimental, and the red line is a fit. The error bars are the statistical standard deviation. (c) Contrast of the spin-echo experiment as a function of the total duration $T$ of the sequence. The line is a Gaussian fit.