1,631 research outputs found
Preparation of Subradiant States using Local Qubit Control in Circuit QED
Transitions between quantum states by photon absorption or emission are
intimately related to symmetries of the system which lead to selection rules
and the formation of dark states. In a circuit quantum electrodynamics setup,
in which two resonant superconducting qubits are coupled through an on-chip
cavity and driven via the common cavity field, one single-excitation state
remains dark. Here, we demonstrate that this dark state can be excited using
local phase control of individual qubit drives to change the symmetry of the
driving field. We observe that the dark state decay via spontaneous emission
into the cavity is suppressed, a characteristic signature of subradiance. This
local control technique could be used to prepare and study highly correlated
quantum states of cavity-coupled qubits.Comment: 5 pages, 4 figure
Electrolyte gate dependent high-frequency measurement of graphene field-effect transistor for sensing applications
We performed radiofrequency (RF) reflectometry measurements at 2.4 GHz on
electrolyte-gated graphene field-effect transistors (GFETs) utilizing a tunable
stub-matching circuit for impedance matching. We demonstrate that the gate
voltage dependent RF resistivity of graphene can be deduced even in the
presence of the electrolyte which is in direct contact with the graphene layer.
The RF resistivity is found to be consistent with its DC counterpart in the
full gate voltage range. Furthermore, in order to access the potential of
high-frequency sensing for applications, we demonstrate time-dependent gating
in solution with nanosecond time resolution.Comment: 14 pages, 4 figure
Protocols for optimal readout of qubits using a continuous quantum nondemolition measurement
We study how the spontaneous relaxation of a qubit affects a continuous
quantum non-demolition measurement of the initial state of the qubit. Given
some noisy measurement record , we seek an estimate of whether the qubit
was initially in the ground or excited state. We investigate four different
measurement protocols, three of which use a linear filter (with different
weighting factors) and a fourth which uses a full non-linear filter that gives
the theoretically optimal estimate of the initial state of the qubit. We find
that relaxation of the qubit at rate strongly influences the fidelity
of any measurement protocol. To avoid errors due to this decay, the measurement
must be completed in a time that decrease linearly with the desired fidelity
while maintaining an adequate signal to noise ratio. We find that for the
non-linear filter the predicted fidelity, as expected, is always better than
the linear filters and that the fidelity is a monotone increasing function of
the measurement time. For example, to achieve a fidelity of 90%, the box car
linear filter requires a signal to noise ratio of in a time
whereas the non-linear filter only requires a signal to noise ratio of .Comment: 12 pages, 6 figure
Studying Light-Harvesting Models with Superconducting Circuits
The process of photosynthesis, the main source of energy in the animate
world, converts sunlight into chemical energy. The surprisingly high efficiency
of this process is believed to be enabled by an intricate interplay between the
quantum nature of molecular structures in photosynthetic complexes and their
interaction with the environment. Investigating these effects in biological
samples is challenging due to their complex and disordered structure. Here we
experimentally demonstrate a new approach for studying photosynthetic models
based on superconducting quantum circuits. In particular, we demonstrate the
unprecedented versatility and control of our method in an engineered three-site
model of a pigment protein complex with realistic parameters scaled down in
energy by a factor of . With this system we show that the excitation
transport between quantum coherent sites disordered in energy can be enabled
through the interaction with environmental noise. We also show that the
efficiency of the process is maximized for structured noise resembling
intramolecular phononic environments found in photosynthetic complexes.Comment: 8+12 pages, 4+12 figure
Multi-photon transitions between energy levels in a current-biased Josephson tunnel junction
The escape of a small current-biased Josephson tunnel junction from the zero
voltage state in the presence of weak microwave radiation is investigated
experimentally at low temperatures. The measurements of the junction switching
current distribution indicate the macroscopic quantum tunneling of the phase
below a cross-over temperature of . At
temperatures below we observe both single-photon and
\emph{multi-photon} transitions between the junction energy levels by applying
microwave radiation in the frequency range between and to the junction. These observations reflect the anharmonicity of the
junction potential containing only a small number of levels.Comment: 4 pages, 5 figure
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