97 research outputs found
Cavity-QED based on collective magnetic dipole coupling: spin ensembles as hybrid two-level systems
We analyze the magnetic dipole coupling of an ensemble of spins to a
superconducting microwave stripline structure, incorporating a Josephson
junction based transmon qubit. We show that this system is described by an
embedded Jaynes-Cummings model: in the strong coupling regime, collective
spin-wave excitations of the ensemble of electrons pick up the nonlinearity of
the cavity mode, such that the two lowest eigenstates of the coupled spin-wave
+ microwave-cavity + Josephson-junction system define a hybrid two-level
system. The proposal described here enables the use of spin ensembles as qubits
which can be coherently manipulated and coupled using the same
nonlinear-cavity. Possibility of strong-coupling cavity-QED with
magnetic-dipole transitions opens up the possibility of extending previously
proposed quantum information processing protocols to spins in silicon or
graphene, without the need for single-electron confinement.Comment: 4 page
Ultra-long distance interaction between spin qubits
We describe a method for implementing deterministic quantum gates between two
spin qubits separated by centimeters. Qubits defined by the singlet and triplet
states of two exchange coupled quantum dots have recently been shown to possess
long coherence times. When the effective nuclear fields in the two asymmetric
quantum dots are different, total spin will no longer be a good quantum number
and there will be a large electric dipole coupling between the two qubit
states. We show that when such a double-quantum-dot qubit is embedded in a
superconducting microstrip cavity, the strong coupling regime of cavity quantum
electrodynamics lies within reach. Virtual photons in a common cavity mode
could mediate coherent interactions between two distant qubits embedded in the
same structure; the range of this two-qubit interaction is determined by the
wavelength of the microwave transition.Comment: 5 pages, 2 figures; final version v2 (minor changes
Squeezed thermal reservoirs as a resource for a nano-mechanical engine beyond the Carnot limit
The efficient conversion of thermal energy to mechanical work by a heat
engine is an ongoing technological challenge. Since the pioneering work of
Carnot, it is known that the efficiency of heat engines is bounded by a
fundamental upper limit, the Carnot limit. Theoretical studies suggest that
heat engines may be operated beyond the Carnot limit by exploiting stationary,
non-equilibrium reservoirs that are characterized by a temperature as well as
further parameters. In a proof-of-principle experiment, we demonstrate that the
efficiency of a nano-beam heat engine coupled to squeezed thermal noise is not
bounded by the standard Carnot limit. Remarkably, we also show that it is
possible to design a cyclic process that allows for extraction of mechanical
work from a single squeezed thermal reservoir. Our results demonstrate a
qualitatively new regime of non-equilibrium thermodynamics at small scales and
provide a new perspective on the design of efficient, highly miniaturized
engines.Comment: 5 pages, 3 figure
On the origin of strong photon antibunching in weakly nonlinear photonic molecules
In a recent work [T. C. H. Liew and V. Savona, Phys. Rev. Lett. {\bf104},
183601 (2010)] it was numerically shown that in a photonic 'molecule'
consisting of two coupled cavities, near-resonant coherent excitation could
give rise to strong photon antibunching with a surprisingly weak nonlinearity.
Here, we show that a subtle quantum interference effect is responsible for the
predicted efficient photon blockade effect. We analytically determine the
optimal on-site nonlinearity and frequency detuning between the pump field and
the cavity mode. We also highlight the limitations of the proposal and its
potential applications in demonstration of strongly correlated photonic systems
in arrays of weakly nonlinear cavities
Realization of an atomically thin mirror using monolayer MoSe2
Advent of new materials such as van der Waals heterostructures, propels new
research directions in condensed matter physics and enables development of
novel devices with unique functionalities. Here, we show experimentally that a
monolayer of MoSe2 embedded in a charge controlled heterostructure can be used
to realize an electrically tunable atomically-thin mirror, that effects 90%
extinction of an incident field that is resonant with its exciton transition.
The corresponding maximum reflection coefficient of 45% is only limited by the
ratio of the radiative decay rate to the linewidth of exciton transition and is
independent of incident light intensity up to 400 Watts/cm2. We demonstrate
that the reflectivity of the mirror can be drastically modified by applying a
gate voltage that modifies the monolayer charge density. Our findings could
find applications ranging from fast programmable spatial light modulators to
suspended ultra-light mirrors for optomechanical devices
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