Quantum computation offers the possibility of new and highly-efficient methods of solving computational problems. At NIST, we have demonstrated the first controllable-input two-bit quantum logic gate, using a single trapped \(^9Be^+\) ion(C. Monroe, \emet. al.), \emPhys. Rev. Lett. \bf75, 4714 (1995).. We are currently investigating ways to implement a simplified version of this gate (one which does not require the use of an auxiliary level) and ways to generalize the gate to one which can operate on two or more ions. We are also examining some of the difficulties which may arise in scaling the system up to large numbers of ions, and investigating simple error-correction schemes which may mitigate these difficulties.
[B13.02] Different Schemes of Quantum Computation with Cold Trapped Ions
Daniel James (Theoretical Division T-4, Los Alamos National Laboratory), Raymond Laflamme (Theoretical Division T-6, Los Alamos National Laboratory), Richard Hughes (Physics Division P-23, Los Alamos National Laboratory)
We will discuss various alternative methods that have been proposed for constructing simple quantum logic elements using cold trapped ions, an area of research which may lead to important technological applications of precision measurements using ion traps. In particular we will compare the use of one and two photon optical transitions, and suggest a new method of engineering standing waves, which offers the possibility of improving the ultimate speed of quantum computations using this technology.
[B13.03] Persistence Probability for Qubit Devices
Howard Brandt (Army Research Laboratory)
The persistence probability for a qubit device may be formulated as the probability of measuring the qubit device in the unperturbed state without the decoherence arising from environmental interactions. The decoherence time can be obtained from the persistence probability. Drawing on recent work of Garg, and also Palma, Suomine, and Ekert, I apply the persistence probability formalism to a single-qubit device coupled to a thermal field environment, and also apply it to a trapped-ion quantum register coupled to the ion-vibrational modes.
[B13.04] Quantum Cryptography Over 24 km of Underground Optical Fibers
Richard Hughes, Gabriel Luther, George Morgan, Charles Peterson, Charles Simmons (University of California)
The secure distribution of the secret random bit sequences known as ''key'' material, is an essential precursor to their use for the encryption and decryption of confidential communications. Quantum cryptography is an emerging technology for secure key distribution with single-photon transmissions: Heisenberg's uncertainty principle ensures that an adversary can neither successfully tap the key transmissions, nor evade detection (eavesdropping raises the key error rate above a threshold value). We are performing quantum cryptography over 24-km of underground optical fiber using non-orthogonal single-photon interference states. Key material is built up by transmitting a single-photon per bit of an initial secret random sequence. A quantum-mechanically random subset of this sequence is identified, becoming the key material after a data reconciliation stage with the sender. Our experiment demonstrates that secure, real-time key generation over "open" multi-km node-to-node optical fiber communications links is feasible.
[B13.05] Correlation functions in an optical Cavity QED system
S. L. Mielke, G. T. Foster, L. A. Orozco (Dept. of Physics SUNY Stony Brook, NY)
We study the photon correlation function g^ of the transmitted light from a collection of N two-level atoms coupled to a single mode of a driven optical cavity. We investigate changes in g^(2) for different driving intensities while keeping the system on resonance and in the lower branch of optical bistability. The regime explored has a small saturation photon number n_0. For small values of n_0, g^(2) does not show antibunching. This behavior is related to the large change in the atomic polarization whenever a photon escapes out of the cavity( H. J. Carmichael, R. J. Brecha P. R. Rice Opt. Comm. 82, 73 (1991).). The correlation function shows features related to the dynamical structure of the system. This structure is responsible for the Vacuum Rabi sidebands in the transmission spectrum and their evolution as the intensity increases. Our experimental realization consists of a high finesse optical cavity traversed by a beam of optically prepumped ^85Rb atoms. It is characterized by a single atom coupling rate g three times larger than either the cavity or the atomic decay rates and by n_0 = 0.08. This work is supported in part by NSF.
[B13.06] Phase dependence in the spectrum of spontaneous emission of a coherently driven three level atom
M.A.G. Martinez (ECE Department, Drexel University), L.M. Narducci, C. Samuels (Department of Physics, Drexel University), P.R. Herczfeld (ECE Department, Drexel University), C.H. Keitel (Blackett Laboratory, Imperial College, UK)
We investigate quantum interference effects in the resonance fluorescence spectrum of a Lambda three-level atom when the lower level doublet is driven by a coherent field. The interfering pathways that lead to the same final state involve both spontaneous and stimulated transitions, and differ form one another by an odd number of stimulated processes induced by the driving field. As a consequence, the interference structures depend upon the phase of the coherent field, an effect which is absent in other resonance fluorescence phenomena. The phase dependence of the quantum interference is especially significant when the level splitting of the driven doublet is comparable to the spontaneous decay rates of the competing optical paths.
[B13.07] Electromagnetic Wave Propagation Below Cutoff.
P. Sprangle (Plasma Physics Divsison, Naval Research Laboratory), B. Hafizi (ICARUS Research, Inc.), E. Esarey (Plasma Physics Division, Naval Research Laboratory), S. E. Harris (Stanford University)
A small amplitude electromagnetic wave can propagate in a plasma below cutoff in the presence of a high frequency large amplitude wave. This is referred to as electromagnetically induced transparency (EIT)(S. E. Harris, Phys. Rev. Lett. 77), 5357 (1996). We extend the analysis of EIT to include relativistic and collisional effects as well as density gradients. We show that the small amplitude wave is unstable in certain parameter regimes. Well above cutoff the instability goes over to the usual backward Raman instability. In addition, the propagation of a large amplitude wave in an inhomogeneous plasma and its penetration beyond the critical surface is analyzed.
[B13.08] Theoretical treatment of lineshapes observed in two-color resonant, four-wave mixing spectra of vibrationally autoionizing Rydberg states of nitric oxide.
F. Di Teodoro, E.F. McCormack (Bryn Mawr College)
A theoretical treatment of the resonant four-wave mixing technique known as two-color laser-induced grating spectroscopy (TC-LIGS) is performed to analyze experimental lineshapes obtained by using TC-LIGS to probe the vibrationally autoionizing ns, np, and nf, v = 1 Rydberg states of NO. In the Liouville-space framework, a perturbative calculation using double-sided Feynman diagrams is combined with the Wigner-Weisskopf scattering model to derive an expression for the third-order electric susceptibility that takes into account the internal coupling between the bound states and the ionization continuum. The ability of the developed model to reproduce the experimental results is discussed.
[B13.09] Spectral modification of an ultrashort pulse propagating through a two-level system
J.K. Ranka, R.W. Schirmer, A.L. Gaeta (Cornell University)
We describe a theoretical and experimental study of the spectral modification of a near-resonant femtosecond pulse propagating through a two-level system in the limit in which only a small amount of temporal reshaping occurs. Through the use of a high-finesse scanning Fabry-Perot micro-cavity, we perform measurements of the pulse spectrum with sub-GHz resolution over an 8~THz bandwidth. In the optically thin regime, our results show that the shape of the resulting spectral feature at the transition frequency depends on the pulse area and the detuning. This behavior can be understood as the interference of the the incident pulse with the radiation emitted by the excited atoms undergoing free-induction decay. At higher atomic densities, propagation effects play an important role and the resulting spectral feature develops a complicated oscillatory structure. Our experimental results are well described by analytical and numerical solutions of the Maxwell-Bloch equations. These observations demonstrate the potential for using femtosecond pulses to perform transmission absorption spectroscopy of atomic and molecular systems.