Memristors are resistive elements retaining information of their past dynamics. They have garnered substantial interest due to their potential for representing a paradigm change in electronics, ...information processing and unconventional computing. Given the advent of quantum technologies, a design for a quantum memristor with superconducting circuits may be envisaged. Along these lines, we introduce such a quantum device whose memristive behavior arises from quasiparticle-induced tunneling when supercurrents are cancelled. For realistic parameters, we find that the relevant hysteretic behavior may be observed using current state-of-the-art measurements of the phase-driven tunneling current. Finally, we develop suitable methods to quantify memory retention in the system.
Shot noise in ballistic graphene Danneau, R; Wu, F; Craciun, M F ...
Physical review letters,
05/2008, Letnik:
100, Številka:
19
Journal Article
Recenzirano
Odprti dostop
We have investigated shot noise in graphene field effect devices in the temperature range of 4.2-30 K at low frequency (f=600-850 MHz). We find that for our graphene samples with a large width over ...length ratio W/L, the Fano factor F reaches a maximum F ~ 1/3 at the Dirac point and that it decreases strongly with increasing charge density. For smaller W/L, the Fano factor at Dirac point is significantly lower. Our results are in good agreement with the theory describing that transport at the Dirac point in clean graphene arises from evanescent electronic states.
We introduce a setup which realises a tunable engineered environment for experiments in circuit quantum electrodynamics. We illustrate this concept with the specific example of a quantum bit, qubit, ...in a high-quality-factor cavity which is capacitively coupled to another cavity including a resistor. The temperature of the resistor, which acts as the dissipative environment, can be controlled in a well defined manner in order to provide a hot or cold environment for the qubit, as desired. Furthermore, introducing superconducting quantum interference devices (SQUIDs) into the cavity containing the resistor, provides control of the coupling strength between this artificial environment and the qubit. We demonstrate that our scheme allows us to couple strongly to the environment enabling rapid initialization of the system, and by subsequent tuning of the magnetic flux of the SQUIDs we may greatly reduce the resistor-qubit coupling, allowing the qubit to evolve unhindered.
We have investigated electrical transport and shot noise in graphene field effect devices. In large width over length ratio
W
/
L
graphene strips, we have measured shot noise at low frequency (
f
...=600–850 MHz) in the temperature range of 4.2–30 K. We observe a minimum conductivity of
and a finite and gate dependent Fano factor reaching the universal value of
at the Dirac point, i.e. where the density of states vanishes. These findings are in good agreement with the theory describing that transport at the Dirac point should occur via evanescent waves in perfect graphene samples with large
W
/
L
. Moreover, we show and discuss how disorder and non-parallel leads affect both conductivity and shot noise.
Quantum driving and work Salmilehto, J; Solinas, P; Möttönen, M
Physical review. E, Statistical, nonlinear and soft matter physics
89, Številka:
5
Journal Article
Odprti dostop
As quantum systems become more experimentally accessible, we are forced to reconsider the notions of control and work to fully account for quantum effects. To this end, we identify the work injected ...into a quantum system during a general quantum-mechanical driving protocol and quantify the relevant heat flows. The known results that are applicable in the limit of a classical drive are shown to emerge from our equations as a special case. Using the established framework, we show that the Bochkov-Kuzovlev identity for the exclusive work distribution is modified in a nontrivial way by the accumulation of system-drive correlations resulting from quantum back action. Our results accentuate the conceptual and discernible differences between a fully quantum-mechanical and classical driving protocols of quantum systems.
Shot noise measurements in graphene Danneau, R.; Wu, F.; Craciun, M.F. ...
Solid state communications,
07/2009, Letnik:
149, Številka:
27
Journal Article, Conference Proceeding
Recenzirano
Odprti dostop
We have investigated shot noise at microwave frequencies in wide-aspect-ratio graphene sheets in the temperature range of 4.2–30 K. We find that for our short
(
L
<
300
nm
)
graphene samples with ...width over length ratio
W
/
L
>
3
, the Fano factor
F
reaches a maximum
F
∼
1
/
3
at the Dirac point and that it decreases substantially with increasing charge density. Our results agree with the theoretical prediction that electrical transport at the Dirac point is governed by evanescent electronic states.
We extend the quantum jump method to nearly adiabatically driven open quantum systems in a way that allows for an accurate account of the external driving in the system-environment interaction. Using ...this framework, we construct the corresponding trajectory-dependent work performed on the system and derive the integral fluctuation theorem and the Jarzynski equality for nearly adiabatic driving. We show that such identities hold as long as the stochastic dynamics and work variable are consistently defined. We numerically study the emerging work statistics for a two-level quantum system and find that the conventional diabatic approximation is unable to capture some prominent features arising from driving, such as the continuity of the probability density of work. Our results reveal the necessity of using accurate expressions for the drive-dressed heat exchange in future experiments probing jump time distributions.
We derive a fundamental conservation law of operator current for master equations describing reduced quantum systems. If this law is broken, the temporal integral of the current operator of an ...arbitrary system observable does not yield in general the change of that observable in the evolution. We study Lindblad-type master equations as examples and prove that the application of the secular approximation during their derivation results in a violation of the conservation law. We show that generally any violation of the law leads to artificial corrections to the complete quantum dynamics, thus questioning the accuracy of the particular master equation.
Cooper-pair current in the presence of flux noise Solinas, P.; Möttönen, M.; Salmilehto, J. ...
Physical review. B, Condensed matter and materials physics,
01/2012, Letnik:
85, Številka:
2
Journal Article
Odprti dostop
We study the effect of flux noise on the Cooper pair current of a superconducting charge pump. We generalize the definition of the current in order to take into account the contribution induced by ...the environment. It turns out that this dissipative current vanishes for charge noise but it is finite in general for noise operators that do not commute with the charge operator. We discuss in a generic framework the effect of flux noise and present a way to engineer it by coupling the system to an additional external circuit. We calculate numerically the pumped charge through the device by solving the master equation for the reduced density matrix of the system and show how it depends on the coupling to the artificial environment.
PDF
