K. Kim

Carrier dynamics and high-speed modulation properties of tunnel injection InGaAs-GaAs quantum-dot lasers

We have performed pump-probe differential transmission spectroscopy (DTS) measurements on In/sub 0.4/Ga/sub 0.6/As-GaAs-AlGaAs heterostructures, which show that at room temperature, injected electrons preferentially occupy the excited states in the dots and states in the barriers layers. The relaxation time of these carriers to the dot ground state is >100 ps. This leads to large gain compression in quantum-dot (QD) lasers and limits the attainable small-signal modulation bandwidth to /spl sim/ 5-7 GHz. The problem can be alleviated by tunneling cold electrons into the lasing states of the dots from an adjoining injector layer. The design, growth, and steady-state and small-signal modulation characteristics of tunnel injection In/sub 0.4/Ga/sub 0.6/As-GaAs QD lasers are described and discussed. The tunneling times, directly measured by three-pulse DTS measurements, are /spl sim/ 1.7 ps and independent of temperature. The measured small-signal modulation bandwidth for I/I/sub th/ /spl sim/ 7 is f/sub -3 dB/ = 23 GHz and the gain compression factor for this frequency response is /spl epsiv/ = 8.2 /spl times/ 10/sup -16/ cm/sup 3/. The differential gain obtained from the modulation data is dg/dn /spl cong/ 2.7 /spl times/ 10/sup -14/ cm/sup 2/ at room temperature. The value of the K-factor is 0.205 ns and the maximum intrinsic modulation bandwidth is 43.3 GHz. Analysis of the transient characteristics with appropriate carrier and photon rate equations yield modulation response characteristics identical to the measured ones. The Auger coefficients are in the range /spl sim/ 3.3 /spl times/ 10/sup -29/ cm/sup 6//s to 3.8 /spl times/ 10/sup -29/ cm/sup 6//s in the temperature range 15/spl deg/C<T<85/spl deg/C, determined from large-signal modulation measurements, and these values are smaller than those measured in separate confinement heterostructure QD lasers. The measured high-speed data are comparable to, or better than, equivalent quantum-well lasers for the first time.

Realization of the Quantum Toffoli Gate with Trapped Ions

Gates acting on more than two qubits are appealing as they can substitute complex sequences of two-qubit gates, thus promising faster execution and higher fidelity. One important multiqubit operation is the quantum Toffoli gate that performs a controlled NOT operation on a target qubit depending on the state of two control qubits. Here we present the first experimental realization of the quantum Toffoli gate in an ion trap quantum computer, achieving a mean gate fidelity of 71(3)%. Our implementation is particularly efficient as the relevant logic information is directly encoded in the motion of the ion string.

Entanglement and Tunable Spin-Spin Couplings between Trapped Ions Using Multiple Transverse Modes

We demonstrate tunable spin-spin couplings between trapped atomic ions, mediated by laser forces on multiple transverse collective modes of motion. A sigma_{x}sigma_{x}-type Ising interaction is realized between quantum bits stored in the ground hyperfine clock states of ;{171}Yb;{+} ions. We demonstrate entangling gates and tailor the spin-spin couplings with two and three trapped ions. The use of closely spaced transverse modes provides a new class of interactions relevant to quantum computing and simulation with large collections of ions in a single crystal.

Onset of a quantum phase transition with a trapped ion quantum simulator

A quantum simulator is a well-controlled quantum system that can follow the evolution of a prescribed model whose behaviour may be difficult to determine. A good example is the simulation of a set of interacting spins, where phase transitions between various spin orders can underlie poorly understood concepts such as spin liquids. Here we simulate the emergence of magnetism by implementing a fully connected non-uniform ferromagnetic quantum Ising model using up to 9 trapped (171)Yb(+) ions. By increasing the Ising coupling strengths compared with the transverse field, the crossover from paramagnetism to ferromagnetic order sharpens as the system is scaled up, prefacing the expected quantum phase transition in the thermodynamic limit. We measure scalable order parameters appropriate for large systems, such as various moments of the magnetization. As the results are theoretically tractable, this work provides a critical benchmark for the simulation of intractable arbitrary fully connected Ising models in larger systems.

Process Tomography of Ion Trap Quantum Gates

A crucial building block for quantum information processing with trapped ions is a controlled-NOT quantum gate. In this Letter, two different sequences of laser pulses implementing such a gate operation are analyzed using quantum process tomography. Fidelities of up to 92.6(6)% are achieved for single-gate operations and up to 83.4(8)% for two concatenated gate operations. By process tomography we assess the performance of the gates for different experimental realizations and demonstrate the advantage of amplitude-shaped laser pulses over simple square pulses. We also investigate whether the performance of concatenated gates can be inferred from the analysis of the single gates.

Large-scale quantum computation in an anharmonic linear ion trap

We propose a large-scale quantum computer architecture by more easily stabilizing a single large linear ion chain in a very simple trap geometry. By confining ions in an anharmonic linear trap with nearly uniform spacing between ions, we show that high-fidelity quantum gates can be realized in large linear ion crystals under the Doppler temperature based on coupling to a near-continuum of transverse motional modes with simple shaped laser pulses.

Frequency Uncertainty for Optically Referenced Femtosecond Laser Frequency Combs

We present measurements and analysis of the currently known relative frequency uncertainty of femtosecond laser frequency combs (FLFCs) based on Kerr-lens mode-locked Ti:sapphire lasers. Broadband frequency combs generated directly from the laser oscillator, as well as octave-spanning combs generated with nonlinear optical fiber are compared. The relative frequency uncertainty introduced by an optically referenced FLFC is measured for both its optical and microwave outputs. We find that the relative frequency uncertainty of the optical and microwave outputs of the FLFC can be as low as 8times10-20 and 1.7times10-18, with a confidence level of 95%, respectively. Photo-detection of the optical pulse train introduces a small amount of excess noise, which degrades the stability and subsequent relative frequency uncertainty limit of the microwave output to 2.6times10-17

Gain dynamics and ultrafast spectral hole burning in In(Ga)As self-organized quantum dots

Using a femtosecond three-pulse pump-probe technique, we investigated spectral hole-burning and gain recovery dynamics in self-organized In(Ga)As quantum dots. The spectral hole dynamics are qualitatively different from those observed in quantum wells, and allow us to distinguish unambiguously the gain recovery due to intradot relaxation and that due to carrier capture. The gain recovery due to carrier–carrier scattering-dominated intradot relaxation is very fast (∼130u2009fs), indicating that this is not the factor limiting the bandwidth of directly modulated quantum dot lasers.

Stabilized frequency comb with a self-referenced femtosecond Cr:forsterite laser

The frequency comb of a Cr:forsterite femtosecond laser is stabilized using the f-to-2f self-referencing technique. The frequency noise of the comb components at 1064, 1314, and 1550 nm differs significantly from the noise of f/sub 0/.

Absolute frequency measurement of the 40Ca+ 4s(2)S_(1/2)-3d(2)D_(5/2) clock transition.

We report on the first absolute transition frequency measurement at the 10 -15 level with a single, laser-cooled 40 Ca + ion in a linear Paul trap. For this measurement, a frequency comb is referenced to the transportable Cs atomic fountain clock of LNE-SYRTE and is used to measure the 40 Ca + 4s 2 S 1/2 -3d 2 D 5/2 electric-quadrupole transition frequency. After the correction of systematic shifts, the clock transition frequency v Ca + = 411 042 129 776 393.2 (1.0) Hz is obtained, which corresponds to a fractional uncertainty within a factor of 3 of the Cs standard. In addition, we determine the Lande g factor of the 3d 2 D 5/2 level to be g 5/2 = 1.2003340(3).