55 research outputs found
Manipulating the Quantum State of an Electrical Circuit
We have designed and operated a superconducting tunnel junction circuit that
behaves as a two-level atom: the ``quantronium''. An arbitrary evolution of its
quantum state can be programmed with a series of microwave pulses, and a
projective measurement of the state can be performed by a pulsed readout
sub-circuit. The measured quality factor of quantum coherence Qphi=25000 is
sufficiently high that a solid-state quantum processor based on this type of
circuit can be envisioned.Comment: 4 figures include
Coherent dynamics of a Josephson charge qubit
We have fabricated a Josephson charge qubit by capacitively coupling a
single-Cooper-pair box (SCB) to an electrometer based upon a single-electron
transistor configured for radio-frequency readout (RF-SET). Charge quantization
of 2e is observed and microwave spectroscopy is used to extract the Josephson
and charging energies of the box. We perform coherent manipulation of the SCB
by using very fast DC pulses and observe quantum oscillations in time of the
charge that persist to ~=10ns. The observed contrast of the oscillations is
high and agrees with that expected from the finite E_J/E_C ratio and finite
rise-time of the DC pulses. In addition, we are able to demonstrate nearly 100%
initial charge state polarization. We also present a method to determine the
relaxation time T_1 when it is shorter than the measurement time T_{meas}.Comment: accepted for publication in Phys. Rev.
Transferring the quantum state of electrons across a Fermi sea with Coulomb interaction
The Coulomb interaction generally limits the quantum propagation of
electrons. However, it can also provide a mechanism to transfer their quantum
state over larger distances. Here, we demonstrate such a form of teleportation,
across a metallic island within which the electrons are trapped much longer
than their quantum lifetime. This effect originates from the low temperature
freezing of the island's charge which, in the presence of a single
connected electronic channel, enforces a one-to-one correspondence between
incoming and outgoing electrons. Such high-fidelity quantum state imprinting is
established between well-separated injection and emission locations, through
two-path interferences in the integer quantum Hall regime. The added electron
quantum phase of can allow for strong and decoherence-free
entanglement of propagating electrons, and notably of flying qubits
Numerical analysis of the radio-frequency single-electron transistor operation
We have analyzed numerically the response and noise-limited charge
sensitivity of a radio-frequency single-electron transistor (RF-SET) in a
non-superconducting state using the orthodox theory. In particular, we have
studied the performance dependence on the quality factor Q of the tank circuit
for Q both below and above the value corresponding to the impedance matching
between the coaxial cable and SET.Comment: 14 page
Solving thermal issues in tensile-strained Ge microdisks
International audienceWe propose to use a Ge-dielectric-metal stacking to allow one to address both thermal management with the metal as an efficient heat sink and tensile strain engineering with the buried dielectric as a stressor layer. This scheme is particularly useful for the development of Ge-based optical sources. We demonstrate experimentally the relevance of this approach by comparing the optical response of tensile-strained Ge microdisks with an Al heat sink or an oxide pedestal. Photoluminescence indicates a much reduced temperature rise in the microdisk (16 K with Al pedestal against 200 K with SiO 2 pedestal under a 9 mW continuous wave optical pumping). An excellent agreement is found with finite element modeling of the temperature rise. This original stacking combining metal and dielectrics is promising for integrated photonics where thermal management is an issue
Quasiparticle Andreev scattering in the fractional quantum Hall regime
The scattering of exotic quasiparticles may follow different rules than
electrons. In the fractional quantum Hall regime, a quantum point contact (QPC)
provides a source of quasiparticles with field effect selectable charges and
statistics, which can be scattered on an 'analyzer' QPC to investigate these
rules. Remarkably, for incident quasiparticles dissimilar to those naturally
transmitted across the analyzer, electrical conduction conserves neither the
nature nor the number of the quasiparticles. In contrast with standard elastic
scattering, theory predicts the emergence of a mechanism akin to the Andreev
reflection at a normal-superconductor interface. Here, we observe the predicted
Andreev-like reflection of an quasiparticle into a hole
accompanied by the transmission of an quasielectron. Combining shot noise
and cross-correlation measurements, we independently determine the charge of
the different particles and ascertain the coincidence of quasielectron and
fractional hole. The present work advances our understanding on the
unconventional behavior of fractional quasiparticles, with implications toward
the generation of novel quasi-particles/holes and non-local entanglements
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