



GROUP MEMBERS
PRESENT GROUP MEMBERS (2012)
Fumika Suzuki 
(MSc student) 
Tim Cox 
(PhD student) 
Ryan MacKenzie 
(PhD student) 
Zhen Zhu 
(PhD student) 


Maritza Hernandez 
(Postdoc) 
JeanSebastien Bernier 
(Postdoc) 


Igor Tupitsyn 
(Research Associate) 
PAST GROUP MEMBERS (20022011)
Matthew Hasselfield 
MSc, 200406 
Now PhD, Astronomy 
Lara Thompson 
MSc, 200305 


PhD, 200510 
Now Postdoc, MIT 
Dominic Marchand 
MSc, 200406 


PhD, 200611 
Now Postdoc, UBC 
Zhen Zhu 
MSc, 200709 
See above 



Moshe Schechter 
Postdoc, 200409 
Now Prof, BenGurion Univ (Beersheva) 
Inanc Adagideli 
Postdoc, 200507 
Now Prof, Univ of Istanbul 
Andrea Morello 
Postdoc, 200507 
Now Prof, Univ of New South Wales 
Andrew Hines 
Postdoc, 200608 
Now in industry, Australia 
Alejandro GaitaArino 
Postdoc, 200710 
Now Prof, Univ of Valencia 
RESEARCH AREAS of CURRENT INTEREST
The following is intended to give a guide to what is going on
in the group at right now, and some of the new directions we are
looking at. However, prospective students and postdocs with
interests and ideas in other directions are always welcome!
(A) STRONGLYCORRELATED CONDENSED MATTER SYSTEMS
These are systems in which interactions (eg., between
electrons) play a key role, and can in fact change the physics in
fundamental ways.
A1. SUPERFLUIDITY and SUPERCONDUCTIVITY: Two key problems in this
field concern the nature of the 'normal' state underlying
superconductivity in highTc superconductors (is it a Fermi Liquid,
or some more peculiar state?), and the physics of the quantum
vortices that exist in the superfluid state.

(i) Our most important recent work in this area has been to give
the solution to a 60year old fundamental problem, to find the
correct equation of motion for a quantum vortex [1]. The results
show that the standard HallVinenIordanski (HVI) equations are
valid in a 'classical regime' of low frequency and high temperature,
provided inertial and noise fluctuation terms are added. However
at low T and/or higher frequency of vortex motion, one enters a
'quantum regime' where the physics is very different. These results
open up very new physics for superfluids and superconductors, which
we have begun to explore [2,3]. This will involve looking
at this new physics in cold BoseEinstein condensed gases, in
superfluid He4 and He3, and in a variety of superconductors 
notably in highTc superconductors [4].

(ii) Our recent work on the normal state has focussed on the
theory of quantum oscillations in 2d and quasi2d systems: the key
problem has been to understand the role of interactions on these
oscillations [5]. Such theory is essential to the interpretation of
dHvA and SdH experiments, which provide the best way of currently
probing the normal state of, eg., highTc superconductors.
Other related work has been on roomtemperature BoseEinstein
condensation of magnons in magnetic insulators (see below).
Future Work: This will undoubtedly focus on vortices (see
above).
[1] L. Thompson, P.C.E. Stamp, "Equation of Motion of a Superfluid Vortex",
Phys Rev Lett 108, 184501 (2012)
[2] L. Thompson, P.C.E. Stamp, "Vortex Dynamics: Quantum vs Classical
Regimes", (J Low Temp. Phys., submitted June 2012); and 3 other papers
in preparation
[3] T Cox, P.C.E. Stamp, "Inertial and Fluctuational effects on the motion
of a superfluid Bose Vortex" (J Low Temp. Phys, submitted June 2012);
and one other paper in preparation.
[4] JC Seamus Davis, PCE Stamp, "Quantum Vortices", Physics in Canada 67,
no. 2, 126135 (2011)
[5] L Thompson, P.C.E. Stamp, "dHvA oscillations in highTc compounds",
Phys. Rev. B81, 100514 (2010)
A2. QUANTUM MAGNETISM: The main question of current interest in this
field concerns the new kinds of order that can exist in magnetic
systems where interactions can frustrate classical ordering; and the
quantum dynamics of these systems. This field strongly overlaps with
'quantum nanomagnetism', the field concerned with spin qubits,
tunneling spins, etc. (see A3. below).

(i) Our main recent work in this area has looked at Quantum Spin
Glasses. The archetypal quantum spin glass system is the famous
'Quantum Ising' magnetic insulator LiHo_xY_{1x}F_4, wherein spin8
Ho ions interact via strong magnetic dipolar interactions, which
try to frustrate conventional magnetic ordering when the spins are
disordered. Our key advance here [13] was to show how in this system
(and many like it), the ordering and dynamics are controlled to a
great extent by the hyperfine coupling of the electronic spins to
the nuclear 'spin bath'; this completely changes the phase diagram.
The story here remains to be finished, as does the link to the
physics of quantum dielctric glasses [4]. There is an even more
important link to quantum information and the dynamics of spin qubits
(see A3. below).

(ii) Roomtemperature BoseEinstein condensation of magnons was
recently discovered by experimentalists in magnetic insulators, and
we immediately showed [5] that '4magnon' interactions between magnons
played a crucial role in such systems, and that they could be
tuned by external fields. We await experimental confirmation of the
initial discovery  if supefluiduty can be demonstrated, this
promises to be exciting.
[1] M Schechter P.C.E. Stamp, "Significance of the hyperfine interactions
in the phase diagram of LiHoxY1xF4", Phys. Rev Lett 95 , 267208
(2005)
[2] M Schechter, P.C.E. Stamp, "Quantum Spin Glass in Anisotropic Dipolar
Systems", J Phys CM 19, 145218 (2007)
[3] M Schechter, P.C.E. Stamp, "The lowT phase diagram of LiHoxY1xF4 ",
Phys Rev B78, 054438 (2008)
[4] M Schechter, P.C.E. Stamp, "Correlated Random fields in dielectric and
spin glasses", Europhys. Lett. 88, 66002 (2009)
[5] I.S. Tupitsyn, P.C.E. Stamp, A.L. Burin, "Stability of BoseEinstein
condensates of hot magnons in YIG"; Phys Rev. Lett. 100, 257202
(2008)
A3. QUANTUM SPIN NETS and SPIN QUBITS: One of the most exciting
challenges in physics is to devise networks of 'qubits' (ie.,
quantum 2level systems) which can behave as a quantum information
processing system. In our view the most promising candidates for
the qubits are electronic and/or nuclear spins, provided the
fundamental problem of decoherence (see also XXXX below) can be
brought under control. This problem has various aspects, as follows:

(i) The dynamics of such quantum spin nets is the central problem.
The problem of this dynamics in the 'incoherent tunneling' regime
was already solved by us in the period 19932004; there is a
fascinating interplay between the electronnuclear hyperfine
coupling, and the longrange dipolar interaction between electronic
spins [1]. But the key has always been to understand how decoherence
works in the 'quantum coherent regime', and we have recently succeeded
[2] in demonstrating (in a collaboration with an experimental group)
that we finally have a proper predictive theory of the decoherence
mechanisms (this work verified theoretical predictions given [3] in 2006).
A key role was played, in all this work, by the 'spin bath' theory [1]
of dissipation and decoherence from localized modes in the environment
(see XXXX below). It will be interesting to see what other 'spin net'
systems can be explored  spin chains are one possibility [4].

(ii) The physics of the spin qubits themselves is also important 
here we have concentrated on 'chemistrybased' molecular magnets (ie.,
the 'bottomup' approach to nanofabrication of spin qubits). The
microscopic physics and dynamics of these molecules is of key interest 
and here again work with experimenters is important [5,6]. We expect
that such chemistrybased approaches may be quite competitive in the
search for solidstate quantum computation schemes [7,8].
Future Directions: We expect to be looking at decoherence
and spin dynamics for spins in semiconducting systems, and also
in hybrid quantum optical/solidstate spin systems.
[1] N.V. Prokof'ev, P.C.E. Stamp, "Theory of the Spin Bath" Rep. Prog. Phys.
63, 669726 (2000)
[2] S Takahashi, I.S. Tupitsyn, C.C. Beedle, D Hendrickson, P.C.E. Stamp,
"Decoherence in Crystals of Quantum Molecular Magnets", Nature 476, 76
(2011); and 2 papers in preparation.
[3] A Morello, P.C.E. Stamp, I.S. Tupitsyn, "Pairwise decoherence in coupled
spin qubit networks", Phys. Rev. Lett 97, 207206 (2006).
[4] E Mills, P.C.E. Stamp, to be published.
[5] J.J. Henderson, C Koo, P.L. Feng, E del Barco, S Hill, I.S. Tupitsyn,
P.C.E. Stamp, D Hendrickson, "Manifestation of Spin selection rules on
the quantum tunneling of magnetization in single molecule magnets" Phys
Rev Lett 103, 017202 (2009)
[6] E. del Barco, S. Hill, CC Beedle, D.N. Hendrickson, I.S. Tupitsyn,
P.C.E.
Stamp, "Tunneling and Inversion symmetry in single molecule magnets:
the
case of the Mn12 wheel molecule", Phys. Rev. B82, 104426 (2010)
[7] P.C.E. Stamp, "Quantum Information: Stopping the Rot", Nature 453, 167
(2008)
[8] P.C.E. Stamp, A. GaitaArino, "Spinbased Quantum Computers made by
Chemistry: Hows and Whys"; J Mat. Chem. 19, 17181730 (2009)
A4. QUANTUM COHERENCE PHENOMENA IN BIOLOGY: In the last few years
experiments have shown that some key biological processes rely on
roomtemperature quantum coherence. Key examples are photosynthesis,
and the use of 'magnetoreception molecules' for navigation by birds
and some mammals. Our work is focussing on 2 aspects of this:

(i) The underlying microscopic description of the interactions
causing decoherence in electron transport in lightharvesting
photosynthesis molecules is not clear. We think that the
'nondiagonal' coupling to both bulk and quasilocalized phonons
is important, and we recently clarified this, by solving the key
problem of the dynamics of polarons coupled nondiagonally to
phonons [1]. We can also map the problem a polaron, or exciton,
coupled to localized phonons to one of a hopping particle coupled
to a 'spin bath' environment [2]. This model can also be used
to describe entangled free radicals, coupled to nuclear spins,
hopping around a molecule (the key problem in bird navigation).

(ii) The biological function of these preocesses depends on the
competition between coherent electron spin dynamics and
decoherence from phonons and nuclear spins. This dynamics is
a complex manybody problem, wich we can solve by mapping to a
spin bath model [3,4]. We hope to be able to to test this
kind of theory very soon on some real biological molecules,
working in collaboation with experimenters.
Future: There is a possibility that we are on the verge of
a revolutionary new development in biology, whereby largescale
quantum coherence is found to be crucial to many biological
processes. If so, then this is likely to be a key focus of
future work in our group.
[1] D. Marchand, G. de Fillipis, V. Cautadella, M. Berciu, N. Nagaosa,
N.V. Prokof'ev, A.S. Mischenko, P.C.E. Stamp, "Sharp transition for
single polarons in the 1dimensional SuSchriefferHeeger model", Phys Rev
Lett. 105, 266605 (2010)
[2] Z. Zhu, P.C.E. Stamp, to be published
[3] Z. Zhu, A. Aharony, O. EntinWohlman, P.C.E. Stamp, "Pure Phase
decoherence
in a Ring Geometry", Phys Rev A81, 062127 (2010); Z Zhu, P.C.E.
Stamp, to be published.
[4] M Hernadez, P.C.E. Stamp, to be published.
A5. QUANTUM GLASSES: The main problem here (described by both
PW Anderson and AJ Leggett as a really central problem in
physics) is to understand the dynamics of a set of localised
modes (defects) in a disordered solid, coupled by strain fields,
electric dipole interactions, phonons, etc. A key mystery
is the existence of lowT 'universality' in this dynamics.
In our work so far on this problem we have tried to quantify
the key interactions in these systems [1], and their role in
both controlling the effecive random field Hamiltonian and
the distribution of these fields [2]. We have more recently
given a new theory of the universality properties [3], which
hypothesizes two kinds of defect, differing in their inversion
symmetry and in their coupling to phonons  universality arises
because level repulsion forces the collective levels of the
inversion symmetric levels to very low energies.
Future Work: A key problem is to give a scaling theory
of the crossover to the lowT regime. We are developing
a field theory for this purpose. All new ideas are welcome.
[1] M Schechter, P.C.E. Stamp, " What are the interactions in a Quantum
Glass?",
J Phys CM 20, 244136 (2008)
[2] M Schechter, P.C.E. Stamp, "Correlated Random fields in dielectric
and spin
glasses", Europhys. Lett. 88, 66002 (2009)
[3] M Schechter, P.C.E. Stamp, "Low Temperature Universality in
disordered solids",
submitted to Phys Rev Lett (original version: /condmat: 0910.1283)
A6. GRAPHENE: The discovery of graphene in 2004, the demonstration
that its 'Dirac electron' properties were those predicted by
Semenoff in 1984 using topological field theory, and the promise of
a new generation of graphenebased electronic devices, has led to an
explosion of activity.
We have just begun a collaboration with GW Semenoff, to try and
solve a key problem, viz., the role of electronelectron interactions
in this system. We will be using a combination of condensed matter
manybody theory and string theory methods to do this.
B. DECOHERENCE, QUANTUM INFORMATION, and GRAVITY
Some of the condensed matter questions described above lead to much
more general problems in physics. A key question is to understand
decoherence in Nature, and also in quantum information processing
We are dealing here not just with conventional environmental
decoherence, caused by coupling to spin and oscillator bath modes,
but also other possible 'intrinsic decoherence' mechanisms. One such
mechanism is suggested by the clash between Quantum Mechanics and
General Relativity, the 2 foundational pillars of 20th century physics;
this mechanism is sometimes called 'gravitational decoherence'. We are
thus led to the most important unsolved problem in physics, viz., how
to go beyond these 2 central theories, to find a new theory embracing
both.
B1. MECHANISMS of ENVIRONMENTAL DECOHERENCE: There are two
standard models which describe quantum environments: the
'oscillator bath' model, for extended environmental modes
(phonons, photons, spin waves, electronhole pairs, etc), and
the 'spin bath' model [1], developed in the 1990's by Prokof'ev
and Stamp, which describes localized modes (defects, nuclear
and paramegnetic spins, local phonons, etc.). Our more recent
work has focussed on the application of variants of these
models to different physical systems (see A3,A4 above, and
B2 below). However, there are still general questions remaining:
notably, what other environmental decoherence mechanisms exist
in Nature. Two of these we have focussed on [2] are
'3rd party decoherence' (where systemenvironment
interactions are mediated by a 3rd system) and intrinsic
decoherence, a process which, if it exists, would amount
to a breakdown of quantum mechanics. The specific example of
graviational decoherence is discussed in B3 below.
[1] N.V. Prokof'ev, P.C.E. Stamp, "Theory of the Spin Bath" Rep. Prog. Phys.
63, 669726 (2000)
[2] P.C.E. Stamp, "The decoherence puzzle", Studies Hist. Phil. Mod. Phys.
37, 467497 (2006)
B2. DECOHERENCE and QUANTUM COMPUTING: Decoherence is the key
obstacle facing efforts to make a quantum computer. Our main
work has focussed on decoherence caused by spin bath systems,
with the quantum computer represented by a quantum walk
Hamiltonian [1]. Some remarkable results are found, including
the simultaneous existence of ballistic and anomalous diffusion
in the quantum diffusion [2]. A much more through investigation
(which requires numerical work in the intermediate regime [3])
shows just how different the resuts are from oscillator bath
decoherence.
Many questions about the dynamics of decoherence remain to be
answered; some of the mathemtical problems are rather severe,
and we have found that methods imported from string theory
can be very useful [4].
[1] N.V. Prokof'ev, P.C.E. Stamp, "Decoherence and Quantum Walks: Anomalous
Diffusion and Ballistic Tails", Phys Rev A74, 020102(R) (2006)
[2] A. Hines, P.C.E. Stamp, "Quantum Walks, Quantum Gates, and Quantum
Computers", Phys Rev A75, 0623231 (2007); see also A Hines, P.C.E.
Stamp,
"Decoherence in Quantum Walks and Quantum Computers", Can J Phys 84,
541548 (2008)
[3] M Hasselfield, T Lee, G.W. Semenoff, P.C.E. Stamp, "Critical boundary
SineGordon revisited", Annals of Physics (NY), 321, 28492875 (2006)
[4] M Hernandez, P.C.E. Stamp, Z. Zhu, to be published.
B3. GRAVITATIONAL DECOHERENCE and QUANTUM GRAVITY: The basic idea
here, which has been discussed by Diosi, Penrose, and others, is
to modify quantum mechanics so as to accommodate superpositions of
different spacetimes, in a way which doesn't do too much to damage
to General Relativity. This idea, of intrinsic decoherence [1],
has yet to find a precise theoretical formulation. Our idea [2],
still in its early stages, is to introduce correlations between
paths in a path integral formulation of quantum mechanics coming
from gravitational interaction between these paths.
Future work: The further development of ideas about
gravitational decoherence will be a major focus of future work.
Some of this will be done in a joint project with W,G Unruh.
[1] P.C.E. Stamp, "The decoherence puzzle", Studies Hist. Phil. Mod. Phys.
37, 467497 (2006)
[2] P.C.E. Stamp, "Intrinsic Decoherence vs. Environmental Decoherence"
(Phil. Trans. Roy. Soc., in press); /arXiV 1205.5307

