49 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
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
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
Observing the universal screening of a Kondo impurity
The Kondo effect, deriving from a local magnetic impurity mediating
electron-electron interactions, constitutes a flourishing basis for
understanding a large variety of intricate many-body problems. Its experimental
implementation in tunable circuits has made possible important advances through
well-controlled investigations. However, these have mostly concerned transport
properties, whereas thermodynamic observations - notably the fundamental
measurement of the spin of the Kondo impurity - remain elusive in test-bed
circuits. Here, with a novel combination of a "charge" Kondo circuit with a
charge sensor, we directly observe the state of the impurity and its
progressive screening. We establish the universal renormalization flow from a
single free spin to a screened singlet, the associated reduction in the
magnetization, and the relationship between scaling Kondo temperature and
microscopic parameters. In our device, a Kondo pseudospin is realized by two
degenerate charge states of a metallic island, which we measure with a
non-invasive, capacitively coupled charge sensor. Such pseudospin probe of an
engineered Kondo system opens the way to the thermodynamic investigation of
many exotic quantum states, including the clear observation of Majorana zero
modes through their fractional entropy