Papers by Christof Wunderlich
Physical Review Letters, 1996
A mesoscopic superposition of quantum states involving radiation fields with classically distinct... more A mesoscopic superposition of quantum states involving radiation fields with classically distinct phases was created and its progressive decoherence observed. The experiment involved Rydberg atoms interacting one at a time with a few photon coherent field trapped in a high Q microwave cavity. The mesoscopic superposition was the equivalent of an "atom 1 measuring apparatus" system in which the "meter" was pointing simultaneously towards two different directions -a "Schrödinger cat." The decoherence phenomenon transforming this superposition into a statistical mixture was observed while it unfolded, providing a direct insight into a process at the heart of quantum measurement. [S0031-9007(96)01848-0] The transition between the microscopic and macroscopic worlds is a fundamental issue in quantum measurement theory . In an ideal model of measurement, the coupling between a macroscopic apparatus ("meter") and a microscopic system ("atom") results in their entanglement and produces a quantum superposition state of the "meter 1 atom" system. Such a superposition is however never observed. Schrödinger has illustrated vividly this problem, replacing the meter by a "cat" [2] and considering the dramatic superposition of dead and alive animal "states." Although such a striking image can only be a metaphor, quantum superpositions involving "meter states" are often called "Schrödinger cats." Following von Neumann [3], it is postulated that an irreversible reduction process takes the quantum superposition into a statistical mixture in a "preferred" basis, corresponding to the eigenvalues of the observable measured by the meter. From then on, the information contents in the system can be described classically. The nature of this reduction has been much debated, with recent theories stressing the role of quantum decoherence . According to these approaches, the meter coordinate is always coupled to a large reservoir of microscopic variables inducing a fast dissipation of macroscopic coherences.
Journal of Modern Optics, 1997
ABSTRACT
Journal of Physics B: Atomic, Molecular and Optical Physics, 2015
In order to faithfully detect the state of an individual two-state quantum system (qubit) realize... more In order to faithfully detect the state of an individual two-state quantum system (qubit) realized using, for example, a trapped ion or atom, state selective scattering of resonance fluorescence is well established. The simplest way to read out this measurement and assign a state is the threshold method. The detection error can be decreased by using more advanced detection methods like the time-resolved method [1] or the π-pulse detection method . These methods were introduced to qubits with a single possible state change during the measurement process. However, there exist many qubits like the hyperfine qubit of 171 Y b + where several state change are possible. To decrease the detection error for such qubits, we develope generalizations of the time-resolved method and the π-pulse detection method for such qubits. We show the advantages of these generalized detection methods in numerical simulations and experiments using the hyperfine qubit of 171 Y b + . The generalized detection methods developed here can be implemented in an efficient way such that experimental real time state discrimination with improved fidelity is possible.
Lectures from the Les Houches Winter School, 2014
Physical Review A, 2015
For experiments with ions confined in a Paul trap, minimization of micromotion is often essential... more For experiments with ions confined in a Paul trap, minimization of micromotion is often essential. This is the case, for example, for experiments in quantum information science using trapped ions, in combined traps for neutral atoms and ions, and for precision measurements using trapped ions. In order to diagnose and compensate micromotion we have implemented a method that allows for finding the position of the radio-frequency (RF) null reliably and efficiently, in principle, without any variation of direct current (DC) voltages. We apply a trap modulation technique and tomographic imaging to extract 3d ion positions for various RF drive powers and analyze the power dependence of the equilibrium position of the trapped ion. In contrast to commonly used methods, the search algorithm directly makes use of a physical effect as opposed to efficient numerical minimization in a high-dimensional parameter space. The precise position determination of an harmonically trapped ion employed here can also be utilized for the detection of small forces. This is demonstrated by determining light pressure forces with an accuracy of 135 yN. As the method is based on imaging only, it can be applied to several ions simultaneously and is independent of laser direction and thus well-suited to be used with surface-electrode traps.
A quantum system being observed evolves more slowly. This "quantum Zeno effect" is reviewed with ... more A quantum system being observed evolves more slowly. This "quantum Zeno effect" is reviewed with respect to a previous attempt of demonstration, and to subsequent criticism of the significance of the findings. A recent experiment on an individual cold trapped ion has been capable of revealing the micro-state of this quantum system, such that the effect of measurement is indeed discriminated from dephasing of the quantum state by either the meter or the environment.
We present a laser cooling scheme for trapped ions and atoms using a combination of laser couplin... more We present a laser cooling scheme for trapped ions and atoms using a combination of laser couplings and a magnetic gradient field. In a Schrieffer-Wolff transformed picture, this setup cancels the carrier and blue sideband terms completely resulting in an improved cooling behaviour compared to standard cooling schemes (e.g. sideband cooling) and allowing cooling to the vibrational ground state. A condition for optimal cooling rates is presented and the cooling behaviour for different Lamb-Dicke parameters and spontaneous decay rates is discussed. Cooling rates of one order of magnitude less than the trapping frequency are achieved using the new cooling method. Furthermore the scheme turns out to be robust under deviations from the optimal parameters and moreover provides good cooling rates also in the multi particle case.
The sixteenth international conference on atomic physics, 1999
The interaction of circular Rydberg atoms with a high-quality microwave cavity makes it possible ... more The interaction of circular Rydberg atoms with a high-quality microwave cavity makes it possible to realize complex quantum state manipulations. The state of an atom can be ``copied'' onto the cavity. Reversing this operation at a later time with a second atom, we realize an elementary ``quantum memory'' holding an atomic quantum coherence for a while in a cavity mode.
The uv spectra of isotopic variants of the neutral triatomic hydrogen molecule were measured as t... more The uv spectra of isotopic variants of the neutral triatomic hydrogen molecule were measured as they are emitted by a fast molecular beam. The spectra are continuous. They are interpreted as being emitted by the n=3 levels and to end on the upper and lower sheet of the dissociating ground state surface, by which the two maxima of the spectrum
Quantum Electronics International Conference, 1998
Summary form only given. The preparation of simple entangled quantum systems and the demonstratio... more Summary form only given. The preparation of simple entangled quantum systems and the demonstration of their nonclassical properties has been a challenging goal since the early days of quantum mechanics and the famous Einstein Poldolsky Rosen (ERR) paper. Using “circular Rydberg atoms” strongly interacting with a zero or one-photon field stored in a microwave superconducting cavity, we have been able
Facial Plastic Surgery Clinics of North America, 1997
ABSTRACT
We demonstrate the possibility of realizing a neural network in a chain of trapped ions with indu... more We demonstrate the possibility of realizing a neural network in a chain of trapped ions with induced long range interactions. Such models permit to store information distributed over the whole system. The storage capacity of such network, which depends on the phonon spectrum of the system, can be controlled by changing the external trapping potential and/or by applying longitudinal local
Laser Physics at the Limits, 2002
Digital information processing builds upon elementary physical elements ("bits") that may occupy ... more Digital information processing builds upon elementary physical elements ("bits") that may occupy either one of two possible states labeled 0 and 1, respectively. If a quantum system, for example, an individual atom having discrete energy eigenstates, is chosen as elementary switch ("qubit" ), then the general state of this system will be a superposition of the two computational basis states, i.e. the states chosen to represent the logic 0 and 1. When applying the superposition principle to a register comprising N qubits, one immediately sees that such a register can exist in a superposition of 2 N states thus representing 2 N binary encoded numbers simultaneously. Any operation on this register will act on all states at once, effecting parallel processing on an exponentially growing (with N ) number of states. The outcome of a measurement on this register after such an operation will, of course, yield just one out of 2 N possible results with a certain probability.
Laser Physics at the Limits, 2002
Quantum information processing (QIP) requires thorough assessment of decoherence. Atoms or ions p... more Quantum information processing (QIP) requires thorough assessment of decoherence. Atoms or ions prepared for QIP often become addressed by radiation within schemes of alternating microwaveoptical double resonance. A well-defined amount of decoherence may be applied to the system when spurious resonance light is admitted simultaneously with the driving radiation. This decoherence is quantified in terms of longitudinal and transversal relaxation. It may serve for calibrating observed decoherence as well as for testing error-correcting quantum codes.
Advances In Atomic, Molecular, and Optical Physics, 2003
... APS full text | Full Text via CrossRef | View Record in Scopus | Cited By in Scopus (18). O. ... more ... APS full text | Full Text via CrossRef | View Record in Scopus | Cited By in Scopus (18). O. Alter and Y. Yamamoto, Quantum Measurement of a Single System, John Wiley and Sons Inc, New York (2001). B. Appasamy, Y. Stalgies and PE Toschek, Phys. Rev. Lett. ...
Physical Review Letters, 1997
The quantum information carried by a two-level atom was transferred to a high-Q cavity and, after... more The quantum information carried by a two-level atom was transferred to a high-Q cavity and, after a delay, to another atom. We realized in this way a quantum memory made of a field in a superposition of 0 and 1 photon Fock states. We measured the "holding time" of this memory corresponding to the decay of the field intensity or amplitude at the single photon level. This experiment implements a step essential for quantum information processing operations. [S0031-9007(97)03701-0] PACS numbers: 89.70. + c, 03.65. -w, 32.80. -t, 42.50. -p The manipulation of simple quantum systems interacting in a well-controlled environment is a very active field in quantum optics, with strong connections to the theory of quantum information . Atoms and photons can be viewed as carriers of "quantum bits" (or qubits) storing and processing information in a nonclassical way. The interaction between two qubit carriers can model the operation of a quantum gate in which the evolution of one qubit is conditioned by the state of the other [2,3]. Combining a few qubits and gates could lead to the realization of simple quantum networks in which an "engineered entanglement" between the interacting qubits carriers could be achieved. Even if practical applications to large scale quantum computing are likely to remain inaccessible [4], fundamental tests of quantum theory could be performed, such as demonstrations of new quantum nonlocal effects , decoherence studies, etc.
Journal of Physics B: Atomic, Molecular and Optical Physics, 2009
The control of internal and motional quantum degrees of freedom of laser cooled trapped ions has ... more The control of internal and motional quantum degrees of freedom of laser cooled trapped ions has been subject to intense theoretical and experimental research for about three decades. In the realm of quantum information science the ability to deterministically prepare and measure quantum states of trapped ions is unprecedented. This expertise may be employed to investigate physical models conceived to describe systems that are not directly accessible for experimental investigations. Here, we give an overview of current theoretical proposals and experiments for such quantum simulations with trapped ions. This includes various spin models (e.g., the quantum transverse Ising model, or a neural network), the Bose-Hubbard Hamiltonian, the Frenkel-Kontorova model, and quantum fields and relativistic effects.
Physical Review Letters - PHYS REV LETT, 2003
A quantum information processor is proposed that combines experimental techniques and technology ... more A quantum information processor is proposed that combines experimental techniques and technology successfully demonstrated either in nuclear magnetic resonance experiments or with trapped ions. An additional inhomogenenous magnetic field applied to an ion trap i) shifts individual ionic resonances (qubits), making them distinguishable by frequency, and, ii) mediates the coupling between internal and external degrees of freedom of trapped ions. This scheme permits one to individually address and coherently manipulate ions confined in an electrodynamic trap using radiation in the radiofrequency or microwave regime. 03.67.Lx, 42.50.Vk Quantum information processing (QIP) holds the promise of extending today's computing capabilities to problems that, with increasing complexity, require exponentially growing resources in time and/or the number of physical elements . The computation of properties of quantum systems themselves is particularly suited to be performed on a quantum computer, even on a device where logic operations can only be carried out with limited precision . Elements of quantum logic operations have been successfully demonstrated in experiments using ion traps , cavity quantum electrodynamics and in the case of nuclear magnetic resonance (NMR) even algorithms have been performed . Whereas quantum computation with nuclear spins in macroscopic ensembles can most likely not be extended beyond about 10 qubits (quantum mechanical two-state systems) [8], ion traps do not suffer from limited scalability in principle and represent a promising system to explore QIP experimentally. They can be employed to also investigate fundamental questions of quantum physics, for example related to decoherence [9] or multiparticle entanglement . However, they still pose considerable experimental challenges.
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Papers by Christof Wunderlich