Institut für Theoretische Festkörperphysik
Stephan André, Jared Cole, Pei-Qing Jin, Michael Marthaler, Alessandro Romito, Gerd Schön
Ongoing collaboration with members of TKM: Alexander Shnirman, Clemens Müller
We also work closely with the experimental groups of A. Ustinov (Karlsruhe), J. Wrachtrup (Stuttgart)
Quantum computing, as an area of study, has grown enormously in the last 15 years,
linking quantum physics with computer science, information theory and
computer engineering. As the principles and models used to describe quantum information
processing are found in many physical systems, the study of these processes tells us
much about the fundamental rules and limitations of quantum theory.
In the quantum computing subgroup of the Institut für Theoretische Festkörperphysik,
we tackle many problems, including; the control and description of quantum coherent solid-state devices, the effects of decoherence, measurement theory in solid-state, and the application of these ideas beyond that of quantum information processing.
Quantum computing using solid-state devices
Quantum state engineering, i.e., active control over the coherent
dynamics of suitable quantum mechanical systems, opens fascinating
perspectives including the ideas of quantum computation. For this
purpose a number of individual two-state quantum systems (qubits)
should be manipulated in a controlled way. Nano-electronic devices
appear particularly promising because they can be embedded in
electronic circuits and scaled up to large numbers of qubits.
Ultrasmall quantum dot systems with charge or spin degrees of
freedom have been suggested. Low-capacitance Josephson junction
devices with logical states differing by one Cooper-pair charge
(Josephson charge qubit) or one flux quantum (flux qubit) futher
exploit the phase coherence of the superconducting state to achieve
long phase coherence times. We proposed a design, with controlled
which is close to ideal. Single- and two-bit operations can
be performed by applying a sequence of gate voltages and currents.
Design of a register of qubits
controlled by gate voltages and fluxes,
coupled by the oscillations in the LC oscillator
Quantum measurement theory and its application to quantum devices
The standard picture of a 'projective' measurement is only valid in some situations.
Many modern experiments in quantum physics investigate a range of regimes and therefore
rely on more sophisticated models of the measurement process. In such situations, concepts
such as imperfect collapse, weak continuous measurement and weak values come into play.
We consider these issues within a range of physical systems, including solid-state devices.
While understanding the measurement process is central to the operation of a quantum computer,
such considerations have increased our understanding of the interplay of measurement in quantum physics in general.
A practical example is superconducting devices, where, in
addition to the controlled manipulation of the quantum
state of the system the resulting state
must be read out. This can be accomplished by coupling a
single-electron transistor capacitively to the (quantum dot or
Josephson) charge qubit or a SQUID to the flux qubit. We analyze
the process by evaluating the time-evolution of the density matrix
of the coupled system. The quantum measurement process is
characterized by three time scales:
a fast dephasing time, a longer time needed to read out the signal,
and a third, even longer time scale when the measurement induced
transitions destroy the information about the inital quantum state.
The theory of decoherence and the cross-over between the quantum and classical worlds
When considering the boundary between the quantum and classical descriptions of the world around us,
decoherence plays a fundamental role. How a quantum system looses phase coherence, or how this
process can be prevented, is the key to realising interesting quantum mechanical effects in practice.
Motivated by recent experiments with Josephson-junction circuits we
reconsidered decoherence effects in quantum two-level systems (TLS).
On one hand, the experiments demonstrate the importance of 1/f noise,
on the other hand, by operating at symmetry points one can suppress noise
effects in linear order. We, therefore, analyzed noise sources with
a variety of power spectra, with linear or quadratic coupling, which are
longitudinal or transverse relative to the eigenbasis of the
Manipulations of the quantum state of the
TLS define characteristic time scales. We discussed the consequences
for relaxation and dephasing processes.
The application of quantum computing ideas in controllable quantum devices
Recently, an additional focus has been the application of some of the above ideas from quantum computing
to the design of quantum devices. In this case, the power of
superposition or entanglement is used in the operation of novel solid-state devices.
Recent examples include, single-qubit lasing, Sisyphus heating and cooling,
parameteric amplification, photon number squeezing and decoherence microscopy.
For an introduction see
Qubits (fast) zum Anfassen
G. Schön und A. Shnirman, Physik Journal 11/2005, p. 51-56
Scanning Quantum Decoherence Microscopy
J.H. Cole and L.C.L. Hollenberg
Nanotechnology 20, 495401 (2009)
Phase diffusion and locking in single qubit lasers
S. André, V. Brosco, A. Shnirman, and G. Schön
Phys. Rev. A 79, 053848 (2009)
Modeling two-spin dynamics in a noisy environment
M.J. Testolin, J.H. Cole, and L.C.L. Hollenberg
Phys. Rev. A 80, 042326 (2009)
Sensing of Fluctuating Nanoscale Magnetic Fields Using Nitrogen-Vacancy Centers in Diamond
L.T. Hall, J. H. Cole, C.D. Hill, and L.C.L. Hollenberg
Phys. Rev. Lett. 103, 220802 (2009)
damping and amplification by a superconducting qubit
M. Grajcar, S.H.W. van der Ploeg, A. Izmalkov, E. Il'ichev, H.-G.
Meyer, A. Fedorov, A. Shnirman, and G. Schön
Physics 4, 612-616 (2008)
Single-qubit lasing and cooling at the Rabi frequency
J. Hauss, A. Fedorov, C. Hutter, A. Shnirman, and G. Schön
Phys. Rev. Lett. 100, 037003 (2008)
squeezing in circuit quantum electrodynamics
M. Marthaler, G. Schön, and A. Shnirman
Phys. Rev. Lett. 101, 147001 (2008)
Decoherence from ensembles of two-level fluctuators
J. Schriefl, Yu. Makhlin, A. Shnirman, and G. Schön
Journal of Physics 8, 1 (2006).
Decoherence in a superconducting quantum bit circuit
G. Ithier, E. Collin, P. Joyez, P.J. Meeson, D. Vion, D. Esteve,
F. Chiarello, A. Shnirman, Y. Makhlin, J. Schriefl, and G. Schön
Phys. Rev. B 72, 134519 (2005)
Low- and high-frequency noise from coherent two-level systems
A. Shnirman, G. Schön, I. Martin, and Y. Makhlin
Phys. Rev. Lett. 94 , 127002 (2005)
Dephasing of solid-state qubits at optimal points
Yu. Makhlin and A. Shnirman
Phys. Rev. Lett. 92, 178301 (2004)
Quantum State Engineering with Josephson-Junction Devices
Yu. Makhlin, G. Schön, and A. Shnirman
Rev. Mod. Phys. 73, 357-400 (2001)
Josephson-Junction Qubits with Controlled Couplings
Y. Makhlin, G. Schön, and A. Shnirman
Nature 398, 305-307 (1999).
Quantum Measurements Performed with a Single-Electron Transistor
A. Shnirman and G. Schön
Phys. Rev. B 57, 15400 (1998).
Quantum Manipulations of Small Josephson Junctions
A. Shnirman, G. Schön, and Z. Hermon
Phys. Rev. Lett. 79, 2371 (1997).
Relevant grants and some links
top of page
- EU Information Societies Technologies (IST) Programme
(Project Nr: IST FP6-015708 EuroSQIP)
Integrated Project for developing a ``European Superconducting
Quantum Information Processor''
Partners: Chalmers Univ., CEA-Saclay, TU Delft, PTB Braunschweig, CNRS
Pisa, Univ. Karlsruhe, IPHT Jena, MATIS Catania, Univ. Bari,
Univ. Innsbruck, LMU München, Landau Institute
1.11.2005 - 30.4.2010
- EU Information Communication Technologies (ICT) Programme
(Project FP7-ICT-FET - 248629 Solid)
Solid State Systems for Quantum Information Processing
Partners: Chalmers Univ., CEA-Saclay, TU Delft, ETH Zürich, KIT, CNRS
Grenoble, Univ. Basel, TU München, Univ. Stuttgart, SNS
Pisa, UPV-EHU Bilbao
expected starting date 1. Jan. 2010
Center for Functional Nanostructures (CFN)
Teilprojekt B3.3: Modelling of Quantum Information Devices
3. Förderungszeitraum: 01.07.2009 - 30.06.2013
- Landesstiftung Baden-Württemberg:
Kompetenznetz Funktionelle Nanostrukturen
Teilprojekt B6 (J. Weis, G. Schön, A. Shnirman)
QED auf dem Chip: Wechselwirkung von Quantenpunkten mit einem
3. Förderzeitraum: 01.01.2009 - 31.12.2011
- IARPA-Project: Reducing Decoherence in the Flux Qubit
A. Shnirman, G. Schön
Partners: John Clarke and Irfan Siddiqi, UC Berkeley, Dale van Harlingen and Jim Eckstein, Univ. of Illinois at Urbana Champaign
1.7.2009 - 30.6.2012