Any quantum system, such as those used in quantum information or magnetic resonance, is subject to random phase errors that can dramatically affect the fidelity of a desired quantum operation or measurement. In the context of quantum information, quantum error correction techniques have been developed to correct these errors, but resource requirements are extraordinary. The realization of a physically tractable quantum information system will therefore be facilitated if qubit (quantum bit) error rates are far below the so-called fault-tolerance error threshold, predicted to be of the order of 10(-3)-10(-6). The need to realize such low error rates motivates a search for alternative strategies to suppress dephasing in quantum systems. Here we experimentally demonstrate massive suppression of qubit error rates by the application of optimized dynamical decoupling pulse sequences, using a model quantum system capable of simulating a variety of qubit technologies. We demonstrate an analytically derived pulse sequence, UDD, and find novel sequences through active, real-time experimental feedback. The latter sequences are tailored to maximize error suppression without the need for a priori knowledge of the ambient noise environment, and are capable of suppressing errors by orders of magnitude compared to other existing sequences (including the benchmark multi-pulse spin echo). Our work includes the extension of a treatment to predict qubit decoherence under realistic conditions, yielding strong agreement between experimental data and theory for arbitrary pulse sequences incorporating nonidealized control pulses. These results demonstrate the robustness of qubit memory error suppression through dynamical decoupling techniques across a variety of qubit technologies.

We report reliable transport of (9)Be(+) ions through an "X junction" in a 2D trap array that includes a separate loading and reservoir zone. During transport the ion's kinetic energy in its local well increases by only a few motional quanta and internal-state coherences are preserved. We also examine two sources of energy gain during transport: a particular radio-frequency noise heating mechanism and digital sampling noise. Such studies are important to achieve scaling in a trapped-ion quantum information processor.

We report on the design, fabrication, and preliminary testing of a 150 zone array built in a 'surface-electrode' geometry microfabricated on a single substrate. We demonstrate transport of atomic ions between legs of a 'Y'-type junction and measure the in-situ heating rates for the ions. The trap design demonstrates use of a basic component design library that can be quickly assembled to form structures optimized for a particular experiment.

We present a detailed experimental study of the Uhrig Dynamical Decoupling (UDD) sequence in a variety of noise environments. Our qubit system consists of a crystalline array of 9 Be + ions confined in a Penning trap. We use an electron-spin-flip transition as our qubit manifold and drive qubit rotations using a 124 GHz microwave system. We study the effect of the UDD sequence in mitigating phase errors and compare against the well known CPMG-style multipulse spin echo as a function of pulse number, rotation axis, noise spectrum, and noise strength. Our results agree well with theoretical predictions for qubit decoherence in the presence of classical phase noise, accounting for the effect of finite-duration π pulses. Finally, we demonstrate that the Uhrig sequence is more robust against systematic over/underrotation and detuning errors than is multipulse spin echo, despite the precise prescription for pulse-timing in UDD.

The ability to detect extremely small forces and nanoscale displacements is vital for disciplines such as precision spin-resonance imaging [1], microscopy [2], and tests of fundamental physical phenomena [3][4][5]. Current force-detection sensitivity limits have surpassed 1 aN/ √ Hz [6,7] (atto = 10 −18 ) through coupling of nanomechanical resonators to a variety of physical readout systems [1,[7][8][9][10]]. Here we demonstrate that crystals of trapped atomic ions [11,12] behave as nanoscale mechanical oscillators and may form the core of exquisitely sensitive force and displacement detectors. We report the detection of forces with a sensitivity 390±150 yN/ √ Hz (more than three orders of magnitude better than existing reports using nanofabricated devices [7]), and discriminate ion displacements ∼18 nm. Our technique is based on the excitation of tunable normal motional modes in an ion trap [13] and detection via phase-coherent Doppler velocimetry [14,15], and should ultimately permit force detection with sensitivity better than 1 yN/ √ Hz [16]. Trappedion-based sensors could permit scientists to explore new regimes in materials science where augmented force, field, and displacement sensitivity may be traded against reduced spatial resolution. Trapped atomic ions exhibit well characterized and broadly tunable (kHz to MHz) normal motional modes in their confining potential [16,17]. The presence of these modes, the light mass of atomic ions, and the strong coupling of charged particles to external fields makes trapped ions excellent detectors of small forces with tunable spectral response [13]. Another advantage is that readout is achieved through resonant-fluorescence detection using only a single laser. Previous studies have suggested that by using ions it is possible to measure forces approaching the yoctonewton scale, for instance, through experiments on motional heating in Paul traps due to fluctuating electric fields [18][19][20], or resonant excitation techniques [17,21].In particular, small forces applied to ions in weak trapping potentials (trapping frequencies ∼0.1 MHz or lower) can excite micron-scale motional excursions resolvable using real-space imaging [21,22].While the intrinsic sensitivity of trapped ions to external forces and fields is well supported, it remains an experimental challenge to determine the maximum achievable sensitivity to a given external excitation as set by systematic limitations including the efficiency of a measurement procedure. Establishing ions as components in ultrasensitive detectors requires two primary issues to be addressed: a known excitation must be applied to allow precise calibration of the system's response; and it must be possible to compare the results of these experiments with the existing literature on detectors based on integrated nanostructures. Our aims are to unify the seemingly disparate fields of nanotechnology and atomic devices, through use of comparable experimental conditions and a demonstration of the potential utility of ion-based sensors...

We have demonstrated transport of 9 Be + ions through a two-dimensional Paul-trap array that incorporates an X junction, while maintaining the ions near the motional ground state of the confining potential well. We expand on the first report of the experiment in Blakestad et al. [Phys. Rev. Lett. 102, 153002 (2009)], including a detailed discussion of how the transport potentials were calculated. Two main mechanisms that caused motional excitation during transport are explained, along with the methods used to mitigate such excitation. We reduced the motional excitation below the results in the above reference by a factor of approximately 50. The effect of a mu-metal shield on qubit coherence is also reported. Finally, we examined a method for exchanging energy between multiple motional modes on the few-quanta level, which could be useful for cooling motional modes without directly accessing the modes with lasers. These results establish how trapped ions can be transported in a large-scale quantum processor with high fidelity.

We present theoretical and experimental studies of the decoherence of hyperfine ground-state superpositions due to elastic Rayleigh scattering of light off-resonant with higher lying excited states. We demonstrate that under appropriate conditions, elastic Rayleigh scattering can be the dominant source of decoherence, contrary to previous discussions in the literature. We show that the elastic-scattering decoherence rate of a two-level system is given by the square of the difference between the elastic-scattering amplitudes for the two levels, and that for certain detunings of the light, the amplitudes can interfere constructively even when the elastic scattering rates from the two levels are equal. We confirm this prediction through calculations and measurements of the total decoherence rate for a superposition of the valence electron spin levels in the ground state of 9 Be + in a 4.5 T magnetic field.Off-resonant light scattering (spontaneous emission) is an important source of decoherence in many coherentcontrol experiments with atoms and molecules. Examples include the use of optical-dipole forces for gates in quantum computing [1], the generation of spin squeezed states through laser-mediated interactions [2][3][4][5][6], and the trapping and manipulation of neutral atoms in optical lattices [7,8]. These experiments frequently involve superpositions of two-level atomic systems (qubits) and use laser beams off-resonant with higher lying excited states to control and measure the atomic states.In general, decoherence of an atomic superposition state due to off-resonant light scattering occurs if the scattered photon carries information about the qubit state. During Raman scattering the initial and final qubit states differ. The state of the scattered photon is entangled with the atomic state, providing "welcherweg" (which-way) information and leading to decoherence [9,10]. By contrast the role of elastic Rayleigh scattering for decoherence is not as clear. Two very different regimes have been discussed and are supported by experiment. On the one hand it has been found that in some experiments Rayleigh scattering gives rise to negligible decoherence provided that the elastic scattering rates from both qubit levels are approximately equal [10]. On the other hand, decoherence due to Rayleigh scattering of photons on a cycling transition is used for strong projective state measurement [11].In this letter we develop a microscopic theory for the decoherence of a qubit due to elastic Rayleigh scattering that gives a unified treatment of these different regimes. Our key finding is that the decoherence induced by Rayleigh scattering is proportional to the square of the * Electronic address: huys@csir.co.za † Electronic address: john.bollinger@nist.gov difference of the probability amplitudes for elastic scattering from the two levels [Eq. (7)]. When the two amplitudes are approximately equal the resulting decoherence rate can be small (first case above) and when one amplitude dominates the other, Rayleigh decoherence c...

We propose a method of single photon detection of infrared (IR) photons at potentially higher efficiencies and lower noise than allowed by traditional IR band avalanche photodiodes (APDs). By up-converting the photon from the IR, e.g. 1550nm, to a visible wavelength in a nonlinear crystal, we can utilize the much higher efficiency of silicon APDs at these wavelengths. We have used a periodically poled lithium niobate (PPLN) crystal and a pulsed 1064nm Nd:YAG laser to perform the up-conversion to a 631 nm photon. We observed conversion efficiencies as high as -80%, and demonstrated scaling down to the single photon level while maintaining a background of 3 x lop4 dark counts per count. We also propose a 2-crystal extension of this scheme, whereby orthogonal polarizations may be up-converted coherently, thus enabling complete quantum state transduction of arbitrary states. Journal of Modern Optics