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Biomedical subjects

C F Roos

Publications and source records attributed to C F Roos.

13 recordsLinked to original sources
PubMed2006

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.

Journal Article↗
PubMed2006

'Designer atoms' for quantum metrology.

Entanglement is recognized as a key resource for quantum computation and quantum cryptography. For quantum metrology, the use of entangled states has been discussed and demonstrated as a means of improving the signal-to-noise ratio. In addition, entangled states have been used in experiments for efficient quantum state detection and for the measurement of scattering lengths. In quantum information processing, manipulation of individual quantum bits allows for the tailored design of specific states that are insensitive to the detrimental influences of an environment. Such 'decoherence-free subspaces' (ref. 10) protect quantum information and yield significantly enhanced coherence times. Here we use a decoherence-free subspace with specifically designed entangled states to demonstrate precision spectroscopy of a pair of trapped Ca+ ions; we obtain the electric quadrupole moment, which is of use for frequency standard applications. We find that entangled states are not only useful for enhancing the signal-to-noise ratio in frequency measurements--a suitably designed pair of atoms also allows clock measurements in the presence of strong technical noise. Our technique makes explicit use of non-locality as an entanglement property and provides an approach for 'designed' quantum metrology.

Journal Article↗
PubMed2005

Scalable multiparticle entanglement of trapped ions.

The generation, manipulation and fundamental understanding of entanglement lies at the very heart of quantum mechanics. Entangled particles are non-interacting but are described by a common wavefunction; consequently, individual particles are not independent of each other and their quantum properties are inextricably interwoven. The intriguing features of entanglement become particularly evident if the particles can be individually controlled and physically separated. However, both the experimental realization and characterization of entanglement become exceedingly difficult for systems with many particles. The main difficulty is to manipulate and detect the quantum state of individual particles as well as to control the interaction between them. So far, entanglement of four ions or five photons has been demonstrated experimentally. The creation of scalable multiparticle entanglement demands a non-exponential scaling of resources with particle number. Among the various kinds of entangled states, the 'W state' plays an important role as its entanglement is maximally persistent and robust even under particle loss. Such states are central as a resource in quantum information processing and multiparty quantum communication. Here we report the scalable and deterministic generation of four-, five-, six-, seven- and eight-particle entangled states of the W type with trapped ions. We obtain the maximum possible information on these states by performing full characterization via state tomography, using individual control and detection of the ions. A detailed analysis proves that the entanglement is genuine. The availability of such multiparticle entangled states, together with full information in the form of their density matrices, creates a test-bed for theoretical studies of multiparticle entanglement. Independently, 'Greenberger-Horne-Zeilinger' entangled states with up to six ions have been created and analysed in Boulder.

Journal Article↗
PubMed2004

Deterministic quantum teleportation with atoms.

Teleportation of a quantum state encompasses the complete transfer of information from one particle to another. The complete specification of the quantum state of a system generally requires an infinite amount of information, even for simple two-level systems (qubits). Moreover, the principles of quantum mechanics dictate that any measurement on a system immediately alters its state, while yielding at most one bit of information. The transfer of a state from one system to another (by performing measurements on the first and operations on the second) might therefore appear impossible. However, it has been shown that the entangling properties of quantum mechanics, in combination with classical communication, allow quantum-state teleportation to be performed. Teleportation using pairs of entangled photons has been demonstrated, but such techniques are probabilistic, requiring post-selection of measured photons. Here, we report deterministic quantum-state teleportation between a pair of trapped calcium ions. Following closely the original proposal, we create a highly entangled pair of ions and perform a complete Bell-state measurement involving one ion from this pair and a third source ion. State reconstruction conditioned on this measurement is then performed on the other half of the entangled pair. The measured fidelity is 75%, demonstrating unequivocally the quantum nature of the process.

Journal Article↗
PubMed2004

Bell states of atoms with ultralong lifetimes and their tomographic state analysis.

Arbitrary atomic Bell states with two trapped ions are generated in a deterministic and preprogrammed way. The resulting entanglement is quantitatively analyzed using various measures of entanglement. For this, we reconstruct the density matrix using single qubit rotations and subsequent measurements with near-unity detection efficiency. This procedure represents the basic building block for future process tomography of quantum computations. As a first application, the temporal decay of entanglement is investigated in detail. We observe ultralong lifetimes for the Bell states Psi(+/-), close to the fundamental limit set by the spontaneous emission from the metastable upper qubit level and longer than all reported values by 3 orders of magnitude.

Journal Article↗
PubMed2000

Experimental demonstration of ground state laser cooling with electromagnetically induced transparency.

Ground state laser cooling of a single trapped Ca(+)on is achieved with a technique which tailors the absorption profile for the cooling laser by exploiting electromagnetically induced transparency. Using the Zeeman structure of the S(1/2) to P(1/2) dipole transition we achieve up to 90% ground state probability. The new method is robust, easy to implement, and proves particularly useful for cooling several motional degrees of freedom simultaneously, which is of great practical importance for the implementation of quantum logic schemes with trapped ions.

Journal Article↗
PubMed1990

Near-monochromatic X-ray beams produced by the free electron laser and Compton backscatter.

The intense photon output of a free electron laser may be made to collide with its own high energy electron beam to create nearly monochromatic x-rays using Compton backscatter techniques. These x-rays can be used for imaging and non-imaging diagnostic and therapeutic experiments. The initial configuration of the Vanderbilt Medical Free Electron Laser (Sierra Laser Systems, Sunnyvale, CA) produces intense x-rays up to 17.9 keV, although higher energies are easily attainable through the use of frequency doubling methods, alteration of the energy of the electron beam and coupling to conventional laser inputs.

Electrons↗
PubMed1990

Inexpensive, air-driven ventricular assist or replacement device for use in MR research.

A simple, inexpensive, non-magnetizable, pulsatile air-driven pump has been devised for use in magnetic resonance environments. The pump has a variable stroke volume and stroke rate, is easily cleaned and sterilized, requires relatively small priming volumes and causes no observable hemolysis when blood is used as the perfusate. This device is usable for phantoms, isolated organs or in situ preparations.

Animals↗
PubMed1988

Doisynolic-type acids--uterotropically potent estrogens which compete poorly with estradiol for cytosolic estradiol receptors.

Doisynolic acids, a class of seco-steroid acids some of which exhibit greater uterotropic estrogenicity than estradiol-17 beta, are D-ring cleavage products of steroidal estrogens formed by fusion with KOH above 200 degrees C. We have found that electron-transfer reactions between estrone or estradiol and CCl4 or CBrCl3 in KOH-t-BuOH at 25 degrees C rapidly provide 16,16-dichloro- or -dibromodoisynolic acid, respectively, the former approaching estradiol in uterotropic potency. Simple esters from these highly hindered tertiary carboxylic acids, easily prepared via phase-transfer-catalyzed alkylations, also rival estradiol in uterotropic activity. Unlike natural steroidal estrogens or their commonly used artificial equivalents (DES, hexoestrol, ethynylestradiol, etc.) whose uterotropic activity is accompanied by substantial binding affinity for cytosolic estradiol receptors, these highly uterotropic doisynolic-type acids and esters exhibit binding affinities for this receptor of only about 1% that of estradiol-17 beta as determined by the usual competitive binding-inhibition studies with [3H]estradiol. Other highly uterotropic carboxylic acids may exhibit similar characteristics. These unusual results leave open the possibilities that uterotropic seco-steroid and related carboxylic acids undergo some unknown metabolic activation, are exceptionally persistent estrogens, bind to a cytosolic receptor site other than the conventional (type I) estradiol site, or bind directly to type I or type II nuclear receptor sites. At dosages of 1000 times those required for a uterotropic effect, the doisynolic-type acids (24 doses over an 8-week period) were neither toxic nor carcinogenic.

Animals↗
PubMed1985

Fiber-optic pressure transducer for use near MR magnetic fields.

Measurement of physiologic pressures in the vicinity of magnetic fields has been difficult because present pressure transducers contain conductive elements. A fiber-optic pressure transducer was developed that contains no conductive parts and is useful for making continuous pressure measurements within magnetic resonance magnetic fields.

Animals↗