426 research outputs found
Conditional quantum state engineering in repeated 2-photon down conversion
The U(1,1) and U(2) transformations realized by three-mode interaction in the
respective parametric approximations are studied in conditional measurement,
and the corresponding non-unitary transformation operators are derived. As an
application, the preparation of single-mode quantum states using an optical
feedback loop is discussed, with special emphasis of Fock state preparation.
For that example, the influence of non-perfect detection and feedback is also
considered.Comment: 17 pages, 4 figures, using a4.st
Exploring Interacting Quantum Many-Body Systems by Experimentally Creating Continuous Matrix Product States in Superconducting Circuits
Improving the understanding of strongly correlated quantum many body systems
such as gases of interacting atoms or electrons is one of the most important
challenges in modern condensed matter physics, materials research and
chemistry. Enormous progress has been made in the past decades in developing
both classical and quantum approaches to calculate, simulate and experimentally
probe the properties of such systems. In this work we use a combination of
classical and quantum methods to experimentally explore the properties of an
interacting quantum gas by creating experimental realizations of continuous
matrix product states - a class of states which has proven extremely powerful
as a variational ansatz for numerical simulations. By systematically preparing
and probing these states using a circuit quantum electrodynamics (cQED) system
we experimentally determine a good approximation to the ground-state wave
function of the Lieb-Liniger Hamiltonian, which describes an interacting Bose
gas in one dimension. Since the simulated Hamiltonian is encoded in the
measurement observable rather than the controlled quantum system, this approach
has the potential to apply to exotic models involving multicomponent
interacting fields. Our findings also hint at the possibility of experimentally
exploring general properties of matrix product states and entanglement theory.
The scheme presented here is applicable to a broad range of systems exploiting
strong and tunable light-matter interactions.Comment: 11 pages, 9 figure
Geometric Phase and Non-Adiabatic Effects in an Electronic Harmonic Oscillator
Steering a quantum harmonic oscillator state along cyclic trajectories leads
to a path-dependent geometric phase. Here we describe an experiment observing
this geometric phase in an electronic harmonic oscillator. We use a
superconducting qubit as a non-linear probe of the phase, otherwise
unobservable due to the linearity of the oscillator. Our results demonstrate
that the geometric phase is, for a variety of cyclic trajectories, proportional
to the area enclosed in the quadrature plane. At the transition to the
non-adiabatic regime, we study corrections to the phase and dephasing of the
qubit caused by qubit-resonator entanglement. The demonstrated controllability
makes our system a versatile tool to study adiabatic and non-adiabatic
geometric phases in open quantum systems and to investigate the potential of
geometric gates for quantum information processing
Information/disturbance trade-off in continuous variable Gaussian systems
We address the information/disturbance trade-off for state-measurements on
continuous variable Gaussian systems and suggest minimal schemes for
implementations. In our schemes, the symbols from a given alphabet are encoded
in a set of Gaussian signals which are coupled to a probe excited in a known
state. After the interaction the probe is measured, in order to infer the
transmitted state, while the conditional state of the signal is left for the
subsequent user. The schemes are minimal, {\em i.e.} involve a single
additional probe, and allow for the nondemolitive transmission of a continuous
real alphabet over a quantum channel. The trade-off between information gain
and state disturbance is quantified by fidelities and, after optimization with
respect to the measurement, analyzed in terms of the energy carried by the
signal and the probe. We found that transmission fidelity only depends on the
energy of the signal and the probe, whereas estimation fidelity also depends on
the alphabet size and the measurement gain. Increasing the probe energy does
not necessarily lead to a better trade-off, the most relevant parameter being
the ratio between the alphabet size and the signal width, which in turn
determine the allocation of the signal energy.Comment: 9 pages, 6 figures, revised version, title changed, accepted PR
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