The electronic spin degrees of freedom in semiconductors typically have decoherence times that are several orders of magnitude longer than other relevant timescales. A solid-state quantum computer based on localized electron spins as qubits is therefore of potential interest. Here, a scheme that realizes controlled interactions between two distant quantum dot spins is proposed. The effective long-range interaction is mediated by the vacuum field of a high finesse microcavity. By using conduction-band-hole Raman transitions induced by classical laser fields and the cavity-mode, parallel controlled-not operations and arbitrary single qubit rotations can be realized. Optical techniques can also be used to measure the spin-state of each quantum dot. 03.67.Lx, 42.50.Dv, 03.65.Bz Within the last few years, quantum computation (QC) has developed into a truly interdisciplinary field involving the contributions of physicists, engineers, and computer scientists [1]. The seminal discoveries of Shor and others, both in developing quantum algorithms for important problems like prime factorization [2], and in developing protocols for quantum error correction (QEC) [3] and fault-tolerant quantum computation [4], have indicated the desirability and the ultimate feasibility of the experimental realization of QC in various quantum systems.The elementary unit in most QC schemes is a twostate system referred to as a quantum bit (qubit). Since QEC can only work if the decoherence rate is small, it is crucial to identify schemes where the qubits are well isolated from their environment. Ingenious schemes based on Raman-coupled low-energy states of trapped ions [5] and nuclear spins in chemical solutions [6] satisfy this criterion, in addition to providing methods of fast quantum manipulation of qubits that do not introduce significant decoherence. Even though these schemes are likely to provide the first examples of quantum information processing at 5-10 qubit level, they do not appear to be scalable to larger systems containing more than 100 qubits.Here, we propose a new scheme for quantum information processing based on quantum dot (QD) electron spins coupled through a microcavity mode. The motivation for this scheme is threefold: (1) a QC scheme based on semiconductor quantum dot arrays should be scalable to ≥ 100 coupled qubits; (2) recent experiments demonstrated very long spin decoherence times for conduction band electrons in III-V and II-VI semiconductors [7], making electron spin a likely candidate for a qubit; and (3) cavity-QED techniques can provide longdistance, fast interactions between qubits [8]. The QC scheme detailed below relies on the use of a single cavity mode and laser fields to mediate coherent interactions between distant QD spins. As we will show shortly, the proposed scheme does not require that QDs be identical and can be used to carry out parallel quantum logic operations [9].We note that a QC scheme based on electron spins in QDs have been previously proposed [10]: this scheme is based on local exchan...

We experimentally demonstrate that the decoherence of a spin by a spin bath can be completely eliminated by fully polarizing the spin bath. We use electron paramagnetic resonance at 240 GHz and 8 T to study the electron-spin coherence time T 2 of nitrogen-vacancy centers and nitrogen impurities in diamond from room temperature down to 1.3 K. A sharp increase of T 2 is observed below the Zeeman energy (11.5 K). The data are well described by a suppression of the flip-flop induced spin bath fluctuations due to thermal electron-spin polarization. T 2 saturates at 250 s below 2 K, where the polarization of the electron-spin bath exceeds 99%. DOI: 10.1103/PhysRevLett.101.047601 PACS numbers: 76.30.Mi, 03.65.Yz Overcoming spin decoherence is critical to spintronics and spin-based quantum information processing devices [1,2]. For spins in the solid state, a coupling to a fluctuating spin bath is a major source of the decoherence. Therefore, several recent theoretical and experimental efforts have aimed at suppressing spin bath fluctuations [3][4][5][6][7][8][9]. One approach is to bring the spin bath into a well-known quantum state that exhibits little or no fluctuations [10,11]. A prime example is the case of a fully polarized spin bath. The spin bath fluctuations are fully eliminated when all spins are in the ground state. In quantum dots, nuclear spin bath polarizations of up to 60% have been achieved [12,13]. However, a polarization above 90% is needed to significantly increase the spin coherence time [14]. Moreover, thermal polarization of the nuclear spin bath is experimentally challenging due to the small nuclear magnetic moment. Electron-spin baths, however, may be fully polarized thermally at a few degrees of Kelvin under an applied magnetic field of 8 T.Here we investigate the relationship between the spin coherence of nitrogen-vacancy (N-V) centers in diamond and the polarization of the surrounding spin bath consisting of nitrogen (N) electron spins. N-V centers consist of a substitutional nitrogen atom adjoining to a vacancy in the diamond lattice. The N-V center, which has long spin coherence times at room temperature [15,16], is an excellent candidate for quantum information processing applications as well as conducting fundamental studies of interactions with nearby electronic spins [16 -18] and nuclear spins [19,20]. In the case of type-Ib diamond, as studied here, the coupling to a bath of N electron spins is the main source of decoherence for an N-V center spin [15,21]. We have measured the spin coherence time (T 2 ) and spin-lattice relaxation time (T 1 ) in spin ensembles of N-V centers and single N impurity centers (P1 centers) using pulsed electron paramagnetic resonance (EPR) spectroscopy at 240 GHz. By comparing the values of T 1 and T 2 at different temperatures, we verify that the mechanism determining T 2 is different from that of T 1 . Next, we investigate the temperature dependence of T 2 .At 240 GHz and 8.6 T where the Zeeman energy of the N centers corresponds to 11.5 K, the polariza...

No abstract

An intense laser field can remove an electron from an atom or molecule and pull the electron into a large-amplitude oscillation in which it repeatedly collides with the charged core it left behind. Such recollisions result in the emission of very energetic photons by means of high-order-harmonic generation, which has been observed in atomic and molecular gases as well as in a bulk crystal. An exciton is an atom-like excitation of a solid in which an electron that is excited from the valence band is bound by the Coulomb interaction to the hole it left behind. It has been predicted that recollisions between electrons and holes in excitons will result in a new phenomenon: high-order-sideband generation. In this process, excitons are created by a weak near-infrared laser of frequency f(NIR). An intense laser field at a much lower frequency, f(THz), then removes the electron from the exciton and causes it to recollide with the resulting hole. New emission is predicted to occur as sidebands of frequency f(NIR) + 2nf(THz), where n is an integer that can be much greater than one. Here we report the observation of high-order-sideband generation in semiconductor quantum wells. Sidebands are observed up to eighteenth order (+18f(THz), or n = 9). The intensity of the high-order sidebands decays only weakly with increasing sideband order, confirming the non-perturbative nature of the effect. Sidebands are strongest for linearly polarized terahertz radiation and vanish when the terahertz radiation is circularly polarized. Beyond their fundamental scientific significance, our results suggest a new mechanism for the ultrafast modulation of light, which has potential applications in terabit-rate optical communications.

Ever since Ernest Rutherford first scattered α-particles from gold foils1, collision experiments have revealed unique insights into atoms, nuclei, and elementary particles2. In solids, many-body correlations also lead to characteristic resonances3, called quasiparticles, such as excitons, dropletons4, polarons, or Cooper pairs. Their structure and dynamics define spectacular macroscopic phenomena, ranging from Mott insulating states via spontaneous spin and charge order to high-temperature superconductivity5. Fundamental research would immensely benefit from quasiparticle colliders, but the notoriously short lifetimes of quasiparticles6 have challenged practical solutions. Here we exploit lightwave-driven charge transport7–24, the backbone of attosecond science9–13, to explore ultrafast quasiparticle collisions directly in the time domain: A femtosecond optical pulse creates excitonic electron–hole pairs in the layered dichalcogenide tungsten diselenide while a strong terahertz field accelerates and collides the electrons with the holes. The underlying wave packet dynamics, including collision, pair annihilation, quantum interference and dephasing, are detected as light emission in high-order spectral sidebands17–19 of the optical excitation. A full quantum theory explains our observations microscopically. This approach opens the door to collision experiments with a broad variety of complex quasiparticles and suggests a promising new way of sub-femtosecond pulse generation.