499 research outputs found
Is there a no-go theorem for superradiant quantum phase transitions in cavity and circuit QED ?
In cavity quantum electrodynamics (QED), the interaction between an atomic
transition and the cavity field is measured by the vacuum Rabi frequency
. The analogous term "circuit QED" has been introduced for Josephson
junctions, because superconducting circuits behave as artificial atoms coupled
to the bosonic field of a resonator. In the regime with comparable
to the two-level transition frequency, "superradiant" quantum phase transitions
for the cavity vacuum have been predicted, e.g. within the Dicke model. Here,
we prove that if the time-independent light-matter Hamiltonian is considered, a
superradiant quantum critical point is forbidden for electric dipole atomic
transitions due to the oscillator strength sum rule. In circuit QED, the
capacitive coupling is analogous to the electric dipole one: yet, such no-go
property can be circumvented by Cooper pair boxes capacitively coupled to a
resonator, due to their peculiar Hilbert space topology and a violation of the
corresponding sum rule
Circuit Quantum Electrodynamics: Coherent Coupling of a Single Photon to a Cooper Pair Box
Under appropriate conditions, superconducting electronic circuits behave
quantum mechanically, with properties that can be designed and controlled at
will. We have realized an experiment in which a superconducting two-level
system, playing the role of an artificial atom, is strongly coupled to a single
photon stored in an on-chip cavity. We show that the atom-photon coupling in
this circuit can be made strong enough for coherent effects to dominate over
dissipation, even in a solid state environment. This new regime of matter light
interaction in a circuit can be exploited for quantum information processing
and quantum communication. It may also lead to new approaches for single photon
generation and detection.Comment: 8 pages, 4 figures, accepted for publication in Nature, embargo does
apply, version with high resolution figures available at:
http://www.eng.yale.edu/rslab/Andreas/content/science/PubsPapers.htm
Culture shapes how we look at faces
Background: Face processing, amongst many basic visual skills, is thought to be invariant across all humans. From as early as 1965, studies of eye movements have consistently revealed a systematic triangular sequence of fixations over the eyes and the mouth, suggesting that faces elicit a universal, biologically-determined information extraction pattern. Methodology/Principal Findings: Here we monitored the eye movements of Western Caucasian and East Asian observers while they learned, recognized, and categorized by race Western Caucasian and East Asian faces. Western Caucasian observers reproduced a scattered triangular pattern of fixations for faces of both races and across tasks. Contrary to intuition, East Asian observers focused more on the central region of the face. Conclusions/Significance: These results demonstrate that face processing can no longer be considered as arising from a universal series of perceptual events. The strategy employed to extract visual information from faces differs across cultures
Climbing the Jaynes-Cummings Ladder and Observing its Sqrt(n) Nonlinearity in a Cavity QED System
The already very active field of cavity quantum electrodynamics (QED),
traditionally studied in atomic systems, has recently gained additional
momentum by the advent of experiments with semiconducting and superconducting
systems. In these solid state implementations, novel quantum optics experiments
are enabled by the possibility to engineer many of the characteristic
parameters at will. In cavity QED, the observation of the vacuum Rabi mode
splitting is a hallmark experiment aimed at probing the nature of matter-light
interaction on the level of a single quantum. However, this effect can, at
least in principle, be explained classically as the normal mode splitting of
two coupled linear oscillators. It has been suggested that an observation of
the scaling of the resonant atom-photon coupling strength in the
Jaynes-Cummings energy ladder with the square root of photon number n is
sufficient to prove that the system is quantum mechanical in nature. Here we
report a direct spectroscopic observation of this characteristic quantum
nonlinearity. Measuring the photonic degree of freedom of the coupled system,
our measurements provide unambiguous, long sought for spectroscopic evidence
for the quantum nature of the resonant atom-field interaction in cavity QED. We
explore atom-photon superposition states involving up to two photons, using a
spectroscopic pump and probe technique. The experiments have been performed in
a circuit QED setup, in which ultra strong coupling is realized by the large
dipole coupling strength and the long coherence time of a superconducting qubit
embedded in a high quality on-chip microwave cavity.Comment: ArXiv version of manuscript published in Nature in July 2008, 5
pages, 5 figures, hi-res version at
http://www.finkjohannes.com/SqrtNArxivPreprint.pd
Coherent quantum state storage and transfer between two phase qubits via a resonant cavity
A network of quantum-mechanical systems showing long lived phase coherence of
its quantum states could be used for processing quantum information. As with
classical information processing, a quantum processor requires information bits
(qubits) that can be independently addressed and read out, long-term memory
elements to store arbitrary quantum states, and the ability to transfer quantum
information through a coherent communication bus accessible to a large number
of qubits. Superconducting qubits made with scalable microfabrication
techniques are a promising candidate for the realization of a large scale
quantum information processor. Although these systems have successfully passed
tests of coherent coupling for up to four qubits, communication of individual
quantum states between qubits via a quantum bus has not yet been demonstrated.
Here, we perform an experiment demonstrating the ability to coherently transfer
quantum states between two superconducting Josephson phase qubits through a
rudimentary quantum bus formed by a single, on chip, superconducting
transmission line resonant cavity of length 7 mm. After preparing an initial
quantum state with the first qubit, this quantum information is transferred and
stored as a nonclassical photon state of the resonant cavity, then retrieved at
a later time by the second qubit connected to the opposite end of the cavity.
Beyond simple communication, these results suggest that a high quality factor
superconducting cavity could also function as a long term memory element. The
basic architecture presented here is scalable, offering the possibility for the
coherent communication between a large number of superconducting qubits.Comment: 17 pages, 4 figures (to appear in Nature
Nonlinear response of the vacuum Rabi resonance
On the level of single atoms and photons, the coupling between atoms and the
electromagnetic field is typically very weak. By employing a cavity to confine
the field, the strength of this interaction can be increased many orders of
magnitude to a point where it dominates over any dissipative process. This
strong-coupling regime of cavity quantum electrodynamics has been reached for
real atoms in optical cavities, and for artificial atoms in circuit QED and
quantum-dot systems. A signature of strong coupling is the splitting of the
cavity transmission peak into a pair of resolvable peaks when a single resonant
atom is placed inside the cavity - an effect known as vacuum Rabi splitting.
The circuit QED architecture is ideally suited for going beyond this linear
response effect. Here, we show that increasing the drive power results in two
unique nonlinear features in the transmitted heterodyne signal: the
supersplitting of each vacuum Rabi peak into a doublet, and the appearance of
additional peaks with the characteristic sqrt(n) spacing of the Jaynes-Cummings
ladder. These constitute direct evidence for the coupling between the quantized
microwave field and the anharmonic spectrum of a superconducting qubit acting
as an artificial atom.Comment: 6 pages, 4 figures. Supplementary Material and Supplementary Movies
are available at http://www.eng.yale.edu/rslab/publications.htm
Resolving photon number states in a superconducting circuit
Electromagnetic signals are always composed of photons, though in the circuit
domain those signals are carried as voltages and currents on wires, and the
discreteness of the photon's energy is usually not evident. However, by
coupling a superconducting qubit to signals on a microwave transmission line,
it is possible to construct an integrated circuit where the presence or absence
of even a single photon can have a dramatic effect. This system is called
circuit quantum electrodynamics (QED) because it is the circuit equivalent of
the atom-photon interaction in cavity QED. Previously, circuit QED devices were
shown to reach the resonant strong coupling regime, where a single qubit can
absorb and re-emit a single photon many times. Here, we report a circuit QED
experiment which achieves the strong dispersive limit, a new regime of cavity
QED in which a single photon has a large effect on the qubit or atom without
ever being absorbed. The hallmark of this strong dispersive regime is that the
qubit transition can be resolved into a separate spectral line for each photon
number state of the microwave field. The strength of each line is a measure of
the probability to find the corresponding photon number in the cavity. This
effect has been used to distinguish between coherent and thermal fields and
could be used to create a photon statistics analyzer. Since no photons are
absorbed by this process, one should be able to generate non-classical states
of light by measurement and perform qubit-photon conditional logic, the basis
of a logic bus for a quantum computer.Comment: 6 pages, 4 figures, hi-res version at
http://www.eng.yale.edu/rslab/papers/numbersplitting_hires.pd
Single-shot qubit readout in circuit Quantum Electrodynamics
The future development of quantum information using superconducting circuits
requires Josephson qubits [1] with long coherence times combined to a
high-fidelity readout. Major progress in the control of coherence has recently
been achieved using circuit quantum electrodynamics (cQED) architectures [2,
3], where the qubit is embedded in a coplanar waveguide resonator (CPWR) which
both provides a well controlled electromagnetic environment and serves as qubit
readout. In particular a new qubit design, the transmon, yields reproducibly
long coherence times [4, 5]. However, a high-fidelity single-shot readout of
the transmon, highly desirable for running simple quantum algorithms or measur-
ing quantum correlations in multi-qubit experiments, is still lacking. In this
work, we demonstrate a new transmon circuit where the CPWR is turned into a
sample-and-hold detector, namely a Josephson Bifurcation Amplifer (JBA) [6, 7],
which allows both fast measurement and single-shot discrimination of the qubit
states. We report Rabi oscillations with a high visibility of 94% together with
dephasing and relaxation times longer than 0:5 \mu\s. By performing two
subsequent measurements, we also demonstrate that this new readout does not
induce extra qubit relaxation.Comment: 14 pages including 4 figures, preprint forma
Tripartite interactions between two phase qubits and a resonant cavity
The creation and manipulation of multipartite entangled states is important
for advancements in quantum computation and communication, and for testing our
fundamental understanding of quantum mechanics and precision measurements.
Multipartite entanglement has been achieved by use of various forms of quantum
bits (qubits), such as trapped ions, photons, and atoms passing through
microwave cavities. Quantum systems based on superconducting circuits have been
used to control pair-wise interactions of qubits, either directly, through a
quantum bus, or via controllable coupling. Here, we describe the first
demonstration of coherent interactions of three directly coupled
superconducting quantum systems, two phase qubits and a resonant cavity. We
introduce a simple Bloch-sphere-like representation to help one visualize the
unitary evolution of this tripartite system as it shares a single microwave
photon. With careful control and timing of the initial conditions, this leads
to a protocol for creating a rich variety of entangled states. Experimentally,
we provide evidence for the deterministic evolution from a simple product
state, through a tripartite W-state, into a bipartite Bell-state. These
experiments are another step towards deterministically generating multipartite
entanglement in superconducting systems with more than two qubits
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