9 research outputs found
Phonon Decoherence of a Double Quantum Dot Charge Qubit
We study decoherence of a quantum dot charge qubit due to coupling to
piezoelectric acoustic phonons in the Born-Markov approximation. After
including appropriate form factors, we find that phonon decoherence rates are
one to two orders of magnitude weaker than was previously predicted. We
calculate the dependence of the Q-factor on lattice temperature, quantum dot
size, and interdot coupling. Our results suggest that mechanisms other than
phonon decoherence play a more significant role in current experimental setups.Comment: RevTex, 7 pages, 5 figures. v2: appendix added, more details
provided. Accepted for publication in PR
Coulomb Blockade Oscillations of Conductance at Finite Energy Level Spacing in a Quantum Dot
We find an analytical expression for the conductance of a single electron
transistor in the regime when temperature, level spacing, and charging energy
of a grain are all of the same order. We consider the model of equidistant
energy levels in a grain in the sequential tunneling approximation. In the case
of spinless electrons our theory describes transport through a dot in the
quantum Hall regime. In the case of spin-1/2 electrons we analyze the line
shape of a peak, shift in the position of the peak's maximum as a function of
temperature, and the values of the conductance in the odd and even valleys.Comment: RevTex, 13 pages, 13 figure
Spin Qubits in Multi-Electron Quantum Dots
We study the effect of mesoscopic fluctuations on the magnitude of errors
that can occur in exchange operations on quantum dot spin-qubits. Mid-size
double quantum dots, with an odd number of electrons in the range of a few tens
in each dot, are investigated through the constant interaction model using
realistic parameters. It is found that the constraint of having short pulses
and small errors implies keeping accurate control, at the few percent level, of
several electrode voltages. In practice, the number of independent parameters
per dot that one should tune depends on the configuration and ranges from one
to four.Comment: RevTex, 6 pages, 5 figures. v3: two figures added, more details
provided. Accepted for publication in PR
Coulomb Blockade Peak Spacings: Interplay of Spin and Dot-Lead Coupling
For Coulomb blockade peaks in the linear conductance of a quantum dot, we
study the correction to the spacing between the peaks due to dot-lead coupling.
This coupling can affect measurements in which Coulomb blockade phenomena are
used as a tool to probe the energy level structure of quantum dots. The
electron-electron interactions in the quantum dot are described by the constant
exchange and interaction (CEI) model while the single-particle properties are
described by random matrix theory. We find analytic expressions for both the
average and rms mesoscopic fluctuation of the correction. For a realistic value
of the exchange interaction constant J_s, the ensemble average correction to
the peak spacing is two to three times smaller than that at J_s = 0. As a
function of J_s, the average correction to the peak spacing for an even valley
decreases monotonically, nonetheless staying positive. The rms fluctuation is
of the same order as the average and weakly depends on J_s. For a small
fraction of quantum dots in the ensemble, therefore, the correction to the peak
spacing for the even valley is negative. The correction to the spacing in the
odd valleys is opposite in sign to that in the even valleys and equal in
magnitude. These results are robust with respect to the choice of the random
matrix ensemble or change in parameters such as charging energy, mean level
spacing, or temperature.Comment: RevTex, 11 pages, 9 figures. v2: Conclusions section expanded.
Accepted for publication in PR
<title>Phonon decoherence in quantum dot qubits</title>
The spin of an electron confined into a lateral semiconductor quantum dot has been proposed as a possible physical realization of a qubit. While the spin has the advantage of large decoherence times, operations with more than one qubit will necessarily involve orbital degrees of freedom, namely, charge, which is much more prone to decoherence. There are also alternative quantum dot qubit proposals that are entirely based on charge. We have used a realistic model to quantify the limitations imposed by acoustic phonons on the operation of quantum dot-based qubits. Our treatment includes essential aspects of the setup geometry, wave function profile and materials characteristics. The time dependence of the qubit density matrix is the presence of a phonon bath solved within the Born-Markov approximation. We find that the inclusion of geometric form factors makes the phonon-induced decoherence rates in double dot charge qubits nearly one order of magnitude lower than estimates previously in the literature. Moreover, our theoretical prediction for the quality factor of coherent charge oscillations based on phonon decoherence are higher than the values recently observed experimentally. This allows us to conclude that phonons are not the primary source of decoherence in double quantum dot qubits
Phonon Decoherence In Quantum Dot Qubits
The spin of an electron confined into a lateral semiconductor quantum dot has been proposed as a possible physical realization of a qubit. While the spin has the advantage of large decoherence times, operations with more than one qubit will necessarily involve orbital degrees of freedom, namely, charge, which is much more prone to decoherence. There are also alternative quantum dot qubit proposals that are entirely based on charge. We have used a realistic model to quantify the limitations imposed by acoustic phonons on the operation of quantum dot-based qubits. Our treatment includes essential aspects of the setup geometry, wave function profile and materials characteristics. The time dependence of the qubit density matrix is the presence of a phonon bath solved within the Born-Markov approximation. We find that the inclusion of geometric form factors makes the phonon-induced decoherence rates in double dot charge qubits nearly one order of magnitude lower than estimates previously in the literature. Moreover, our theoretical prediction for the quality factor of coherent charge oscillations based on phonon decoherence are higher than the values recently observed experimentally. This allows us to conclude that phonons are not the primary source of decoherence in double quantum dot qubits
A Review of the Magnetic Forces in the CMS Magnet Yoke
A summary of the magnetic forces, acting on various ferromagnetic parts of the flux - return yoke of the CMS magnetic system is presented. The latest information about the parameters of the system has been taken into account: coil revision 5/98, yoke revision 6/98, hadronic forward calorimeter revision 6/98. The Vector Fields TOSCA code and the modified CERN POISCR code were used for two - and three - dimensional finite - element calculations of the magnetic field. The forces on ferromagnetic elements were computed using the Maxwell surface integral method. The obtained results are compared with the estimates of the magnetic forces presented in the Magnet Technical Design Report