M. J. Madsen

Ion trap in a semiconductor chip

The electromagnetic manipulation of isolated atoms has led to many advances in physics, from laser cooling1 and Bose–Einstein condensation of cold gases2 to the precise quantum control of individual atomic ions3. Work on miniaturizing electromagnetic traps to the micrometre scale promises even higher levels of control and reliability4. Compared with ‘chip traps’ for confining neutral atoms5,6,7, ion traps with similar dimensions and power dissipation offer much higher confinement forces and allow unparalleled control at the single-atom level. Moreover, ion microtraps are of great interest in the development of miniature mass-spectrometer arrays8, compact atomic clocks9 and, most notably, large-scale quantum information processors10,11. Here we report the operation of a micrometre-scale ion trap, fabricated on a monolithic chip using semiconductor micro-electromechanical systems (MEMS) technology. We confine, laser cool and measure heating of a single 111Cd+ ion in an integrated radiofrequency trap etched from a doped gallium-arsenide heterostructure.

Planar Ion Trap Geometry for Microfabrication

We describe a novel high aspect ratio radiofrequency linear ion trap geometry that is amenable to modern microfabrication techniques. The ion trap electrode structure consists of a pair of stacked conducting cantilevers resulting in confining fields that take the form of fringe fields from parallel plate capacitors. The confining potentials are modeled both analytically and numerically. This ion trap geometry may form the basis for large scale quantum computers or parallel quadrupole mass spectrometers.

Experimental Bell Inequality Violation with an Atom and a Photon

We report the measurement of a Bell inequality violation with a single atom and a single photon prepared in a probabilistic entangled state. This is the first demonstration of such a violation with particles of different species. The entanglement characterization of this hybrid system may also be useful in quantum information applications.

Sympathetic cooling of trapped Cd ¿ isotopes

A collection of cold trapped ions offers one of the most promising avenues towards realizing a quantum computer @1‐4 #. Quantum information is stored in the internal states of individual trapped ions, while entangling quantum logic gates are implemented via a collective quantized mode of motion of the ion crystal. The internal qubit states can have extremely long coherence times @5#, but decoherence of the motion of the ion crystal may limit quantum logic gate fidelity @6#. Furthermore, when ions are nonadiabatically shuttled between different trapping regions for large-scale quantum computer schemes @4,7#, their motion must be recooled for subsequent logic operations. Direct laser cooling of the qubit ions is not generally possible without disturbing coherence of the internal qubit states. Instead, additional ‘‘refrigerator’’ ions in the crystal can be directly laser cooled, with the qubit ions cooled in sympathy by virtue of their Coulomb-coupled motion @8#. The laser cooling of the refrigerator ions can quench unwanted motion of the ion crystal, while not affecting the internal states of the qubit ions @4,9,10#. Sympathetic cooling has been observed in large ensembles of ions in Penning traps @8,11#, impurities in small collections of ion crystals, and in small ion crystals consisting of a single species, where strong laser focusing was required to access a particular ion without affecting the others @12#. Here, we report the first demonstration of sympathetic cooling in a small ion crystal with two different species where both species are independently optically addressed. We study sympathetic cooling of individual Cd 1 isotopes confined in a rf trap. One ion isotope~the refrigerator ion! is continuously Doppler-cooled by a laser beam red detuned from its D2 line ( S1/2-P3/2), while the other isotope ~the probe ion! is either Doppler cooled or Doppler heated by another beam, whose frequency is scanned around its D2 resonance line. The effect of the sympathetic cooling is to enable measuring fluorescence on the blue side of the probe ion’s resonance. Ordinarily, when the probe laser beam is tuned to the blue of the probe ion’s resonance, the ion ceases fluorescing due to Doppler heating, but the sympathetic cooling from the refrigerator ion keeps the probe ion cold and fluorescing regardless of the probe tuning. In the experiment, the probe ion is 112 Cd 1 , while the refrigerator ion is 114 Cd 1 ~both isotopes have zero nuclear spin!. The respective D2 lines of these two neighboring isotopes are separated by about 680 MHz, with the heavier ion at lower frequency, and the natural linewidth of each ion’s excited P3/2 state is g/2p.47 MHz. The experimental apparatus is schematically shown in Fig. 1. The Cd 1 D2 line resonant light near 214.5 nm is generated by quadrupling a Ti:sapphire laser. The laser is stabilized to a molecular tellurium feature near 429 nm to better than 1 MHz. The quadrupled UV output is split into two parts; one part is upshifted by ;420 MHz, while the

Probabilistic quantum gates between remote atoms through interference of optical frequency qubits

Entangled quantum states, at the heart of quantuminformation processing, are notoriously difficult to generate and control. Generating entangled states becomes dramatically simpler when the entanglement operations are allowed to succeed with only a finite perhaps small probability, as long as it is known when the operations succeed 1‐5 .I f entangling gates can be implemented in such a probabilistic fashion, it has recently been shown that scalable quantum computation is still possible, no matter how small the gate success probability 6,7. Compared with deterministic gates, the additional overhead in resources such as the number of qubit manipulations for probabilistic quantum computation scales only polynomially with both the size of the computation and the inverse of the gate success probability 6. There have been recent proposals for implementation of probabilistic gates 7‐9, using atomic qubits inside optical cavities. In this paper, we propose a scheme for probabilistic quantum gate operations that act on trapped atoms or ions in free space with or without cavities. Compared with previous methods, this scheme has two outstanding features. First, optical frequency qubits are used to connect and entangle matter qubits at distant locations. The two states comprising this optical qubit have the same polarization, but differ in

Quantum networking with photons and trapped atoms (Invited)

Distributed quantum information processing requires a reliable quantum memory and a faithful carrier of quantum information. Atomic qubits have very long coherence times and are thus excellent candidates for quantum information storage, whereas photons are ideal for the transport of quantum information as they can travel long distances with a minimum of decoherence. We discuss the theoretical and experimental combination of these two systems and their use for not only quantum information transfer but also scalable quantum computation architectures.

Zero-point cooling and low heating of trapped {sup 111}Cd{sup +} ions

We report on ground-state laser cooling of single {sup 111}Cd{sup +} ions confined in radio-frequency (Paul) traps. Heating rates of trapped ion motion are measured for two different trapping geometries and electrode materials, where no effort was made to shield the electrodes from the atomic Cd source. The low measured heating rates suggest that trapped {sup 111}Cd{sup +} ions may be well suited for experiments involving quantum control of atomic motion, including applications in quantum information science.

Precision lifetime measurements of a single trapped ion with ultrafast laser pulses

We report precision measurements of the excited state lifetime of the

Atomic qubit manipulations with an electro-optic modulator

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