arXiv subjects

Christian Lupien

Publications and source records attributed to Christian Lupien.

28 records · Page 2Linked to original sources

arXiv2015

Discrete photon statistics from continuous microwave measurements

Photocount statistics are an important tool for the characterization of electromagnetic fields, especially for fields with an irrelevant phase. In the microwave domain, continuous rather than discrete measurements are the norm. Using a novel approach, we recover discrete photon statistics from the cumulants of a continuous distribution of field quadrature measurements. The use of cumulants allows the separation between the signal of interest and experimental noise. Using a parametric amplifier as the first stage of the amplification chain, we extract useful data from up to the sixth cumulant of the continuous distribution of a coherent field, hence recovering up to the third moment of the discrete statistics associated with a signal with much less than one average photon.

quant-ph↗

arXiv2015

Experimental Violation Of Bell-like Inequalities By Electronic Shot Noise

Quantum electronic noise, the random current fluctuations generated by electrons crossing a quantum conductor, has been thoroughly studied from the electron point of view. Recent experiments have shown that such noise may have striking properties in terms of the quantum electromagnetic field it generates. Both the existence of vacuum squeezing \cite{PAN_squeezing} and generation of correlated photon pairs \cite{C4quantique} have been observed. Here we demonstrate that this electromagnetic field also violates Bell-like inequalities, thereby proving its true quantum nature. This is achieved by the observation of two-mode quadrature squeezing of vacuum: the electromagnetic field quadratures at two different frequencies can be so correlated that their difference goes below vacuum fluctuations. Thus, electronic noise contains entangled photon pairs, a key ingredient for quantum computing and communication.

cond-mat.mes-hall↗

arXiv2014

Pauli-Heisenberg Blockade of Electron Quantum Transport

Conduction of electrons in matter is ultimately described by quantum mechanics. Yet at low frequency or long time scales, low temperature quantum transport is perfectly described by this very simple idea: electrons are emitted by the contacts into the sample which they may cross with a finite probability. Combined with Fermi statistics, this partition of the electron flow accounts for the full statistics of electron transport. When it comes to short time scales, a key question must be clarified: are there correlations between successive attempts of the electrons to cross the sample? While there are theoretical predictions and several experimental indications for the existence of such correlations, no direct experimental evidence has ever been provided. Here we show a direct experimental proof of how temperature and voltage bias control the electron flow: while temperature $T$ leads to a jitter which tends to decorrelate electron transport after a time $\hbar/k_BT$, the bias voltage $V$ induces strong correlations/anticorrelations which oscillate with a period $h/eV$. Our experiment reveals how time scales related to voltage and temperature operate on quantum transport in a coherent conductor. In complex quantum systems, the method we have developed might offer direct access to other relevant time scales related, for example, to internal dynamics, coupling to other degrees of freedom, or correlations between electrons.

cond-mat.mes-hall↗

arXiv2014

Emission of Microwave Photon Pairs by a Tunnel Junction

Generation and control of non-classical electromagnetic fields is of crucial importance for quantum information physics. While usual methods for the production of such fields rely on a non-linearity (of a crystal, a Josephson junction, etc.), a recent experiment performed on a normal conductor, a tunnel junction under microwave irradiation, has unveiled an alternative: the use of electron shot noise in a quantum conductor\cite{PAN_squeezing}. Here we show that such a device can emit \emph{pairs of microwave photons} of different frequencies with a rate as high as that of superconducting Josephson junctions\cite{Flurin}. This results in intensity fluctuations of the photon field at two different frequencies being correlated below the photon shot noise,i.e. two-mode amplitude squeezing. Our experiment constitutes a fundamental step towards the understanding of electronic noise in terms of quantum optics, and shows that even a normal conductor could be used as a resource for quantum information processing.

quant-ph↗

arXiv2013

Observation of Squeezing in the Electron Quantum Shot Noise of a Tunnel Junction

We report the measurement of the fluctuations of the two quadratures of the electromagnetic field generated by a quantum conductor, a dc- and ac-biased tunnel junction placed at very low temperature.We observe that the variance of the fluctuations on one quadrature can go below that of vacuum, i.e., that the radiated field is squeezed. This demonstrates the quantum nature of the radiated electromagnetic field.

quant-ph↗

arXiv2013

Observation of Quantum Oscillations in the Photo-assisted Shot Noise of a Tunnel Junction

We report measurements of the low frequency current fluctuations of a tunnel junction placed at very low temperature biased by a time-dependent voltage $V(t)=V(1+\cos 2πνt)$. We observe that the excess noise generated by the ac excitation exhibits quantum oscillations as a function of the dc bias, with a period given by $hν/e$ with $e$ the charge of a single electron. This is a direct consequence of the quantum nature of electricity in a normal conductor.

cond-mat.mes-hall↗

arXiv2013

Noise Intensity-Intensity Correlations and the Fourth Cumulant of Current Fluctuations

We report measurements of the correlation between intensities of noise at different frequencies on a tunnel junction under ac excitation, which corresponds to two-mode intensity correlations in optics. We observe positive correlation, \emph{i.e.} photon bunching, which exist only for certain relations between the excitation frequency and the two detection frequencies, depending on the dc bias of the sample. We demonstrate that these correlations are proportional to the fourth cumulant of current fluctuations.

cond-mat.mes-hall↗

arXiv2003

Transport in Ultraclean YBa$_2$Cu$_3$O$_7$: neither Unitary nor Born Impurity Scattering

The thermal conductivity of ultraclean YBa$_2$Cu$_3$O$_7$ was measured at very low temperature in magnetic fields up to 13 T. The temperature and field dependence of the electronic heat conductivity show that two widespread assumptions of transport theory applied to unconventional superconductors fail for clean cuprates: impurity scattering cannot be treated in the usual unitary limit (nor indeed in the Born limit), and scattering of quasiparticles off vortices cannot be neglected. Our study also sheds light on the long-standing puzzle of a sudden onset of a "plateau" in the thermal conductivity of Bi-2212 versus field.

cond-mat.supr-con↗

arXiv2000

Low-Energy Quasiparticles in Cuprate Superconductors: A Quantitative Analysis

A residual linear term is observed in the thermal conductivity of optimally-doped Bi-2212 at very low temperatures whose magnitude is in excellent agreement with the value expected from Fermi-liquid theory and the d-wave energy spectrum measured by photoemission spectroscopy, with no adjustable parameters. This solid basis allows us to make a quantitative analysis of thermodynamic properties at low temperature and establish that thermally-excited quasiparticles are a significant, perhaps even the dominant mechanism in suppressing the superfluid density in cuprate superconductors Bi-2212 and YBCO.

cond-mat.supr-con↗

arXiv1999

Quasiparticle transport in the vortex state of YBa_2Cu_3O_6.9

The effect of vortices on quasiparticle transport in cuprate superconductors was investigated by measuring the low temperature thermal conductivity of YBa_2Cu_3O_6.9 in magnetic fields up to 8 T. The residual linear term (as T \to 0) is found to increase with field, directly reflecting the occupation of extended quasiparticle states. A study for different Zn impurity concentrations reveals a good agreement with recent calculations for a d-wave superconductor, thereby shedding light on the nature of scattering by both impurities and vortices. It also provides a quantitative measure of the gap near the nodes.

cond-mat.supr-con↗