140 research outputs found
New class of quantum error-correcting codes for a bosonic mode
We construct a new class of quantum error-correcting codes for a bosonic mode
which are advantageous for applications in quantum memories, communication, and
scalable computation. These 'binomial quantum codes' are formed from a finite
superposition of Fock states weighted with binomial coefficients. The binomial
codes can exactly correct errors that are polynomial up to a specific degree in
bosonic creation and annihilation operators, including amplitude damping and
displacement noise as well as boson addition and dephasing errors. For
realistic continuous-time dissipative evolution, the codes can perform
approximate quantum error correction to any given order in the timestep between
error detection measurements. We present an explicit approximate quantum error
recovery operation based on projective measurements and unitary operations. The
binomial codes are tailored for detecting boson loss and gain errors by means
of measurements of the generalized number parity. We discuss optimization of
the binomial codes and demonstrate that by relaxing the parity structure, codes
with even lower unrecoverable error rates can be achieved. The binomial codes
are related to existing two-mode bosonic codes but offer the advantage of
requiring only a single bosonic mode to correct amplitude damping as well as
the ability to correct other errors. Our codes are similar in spirit to 'cat
codes' based on superpositions of the coherent states, but offer several
advantages such as smaller mean number, exact rather than approximate
orthonormality of the code words, and an explicit unitary operation for
repumping energy into the bosonic mode. The binomial quantum codes are
realizable with current superconducting circuit technology and they should
prove useful in other quantum technologies, including bosonic quantum memories,
photonic quantum communication, and optical-to-microwave up- and
down-conversion.Comment: Published versio
Dissipationless counterflow currents above T_c in bilayer superconductors
We report the existence of dissipationless currents in bilayer
superconductors above the critical temperature , assuming that the
superconducting phase transition is dominated by phase fluctuations. Using a
semiclassical lattice gauge theory, we show that thermal fluctuations
cause a transition from the superconducting state at low temperature to a
resistive state above , accompanied by the proliferation of unbound
vortices. Remarkably, while the proliferation of vortex excitations causes
dissipation of homogeneous in-plane currents, we find that counterflow
currents, flowing in opposite direction within a bilayer, remain
dissipationless. The presence of a dissipationless current channel above
is attributed to the inhibition of vortex motion by local superconducting
coherence within a single bilayer, in the presence of counterflow currents. Our
theory presents a possible scenario for the pseudogap phase in bilayer
cuprates.Comment: Main text : 4 pages, 4 figures. Supplement: 8 pages, 9 figure
Self-similar dynamics of order parameter fluctuations in pump-probe experiments
Upon excitation by a laser pulse, broken-symmetry phases of a wide variety of
solids demonstrate similar order parameter dynamics characterized by a dramatic
slowing down of relaxation for stronger pump fluences. Motivated by this
recurrent phenomenology, we develop a simple non-perturbative effective model
of dynamics of collective bosonic excitations in pump-probe experiments. We
find that as the system recovers after photoexcitation, it shows universal
prethermalized dynamics manifesting a power-law, as opposed to exponential,
relaxation, explaining the slowing down of the recovery process. For strong
quenches, long-wavelength over-populated transverse modes dominate the
long-time dynamics; their distribution function exhibits universal scaling in
time and space, whose universal exponents can be computed analytically. Our
model offers a unifying description of order parameter fluctuations in a regime
far from equilibrium, and our predictions can be tested with available
time-resolved techniques
Principles of 2D terahertz spectroscopy of collective excitations: the case of Josephson plasmons in layered superconductors
Two-dimensional terahertz spectroscopy (2DTS), a terahertz analogue of
nuclear magnetic resonance, is a new technique poised to address many open
questions in complex condensed matter systems. The conventional theoretical
framework used ubiquitously for interpreting multidimensional spectra of
discrete quantum level systems is, however, insufficient for the continua of
collective excitations in strongly correlated materials. Here, we develop a
theory for 2DTS of a model collective excitation, the Josephson plasma
resonance in layered superconductors. Starting from a mean-field approach at
temperatures well below the superconducting phase transition, we obtain
expressions for the multidimensional nonlinear responses that are amenable to
intuition derived from the conventional single-mode scenario. We then consider
temperatures near the superconducting critical temperature , where
dynamics beyond mean-field become important and conventional intuition fails.
As fluctuations proliferate near , the dominant contribution to nonlinear
response comes from an optical parametric drive of counter-propagating
Josephson plasmons, which gives rise to 2D spectra that are qualitatively
different from the mean-field predictions. As such, and in contrast to
one-dimensional spectroscopy techniques, such as third harmonic generation,
2DTS can be used to directly probe thermally excited finite-momentum plasmons
and their interactions. Our theory provides a clear interpretation of recent
2DTS measurements on cuprates, and we discuss implications beyond the present
context of Josephson plasmons
Periodic dynamics in superconductors induced by an impulsive optical quench
A number of experiments have evidenced signatures of enhanced superconducting correlations after photoexcitation. Initially, these experiments were interpreted as resulting from quasi-static changes in the Hamiltonian parameters, for example, due to lattice deformations or melting of competing phases. Yet, several recent observations indicate that these conjectures are either incorrect or do not capture all the observed phenomena, which include reflectivity exceeding unity, large shifts of Josephson plasmon edges, and appearance of new peaks in terahertz reflectivity. These observations can be explained from the perspective of a Floquet theory involving a periodic drive of system parameters, but the origin of the underlying oscillations remains unclear. In this paper, we demonstrate that following incoherent photoexcitation, long-lived oscillations are generally expected in superconductors with low-energy Josephson plasmons, such as in cuprates or fullerene superconductor K3C60. These oscillations arise from the parametric generation of plasmon pairs due to pump-induced perturbation of the superconducting order parameter. We show that this bi-plasmon response can persist even above the transition temperature as long as strong superconducting fluctuations are present. Our analysis offers a robust framework to understand light-induced superconducting behavior, and the predicted bi-plasmon oscillations can be directly detected using available experimental techniques
Mechanisms for Long-Lived, Photo-Induced Superconductivity
Advances in the control of intense infrared light have led to the striking
discovery of metastable superconductivity in at
100K, lasting more than 10 nanoseconds. Inspired by these experiments, we
discuss possible mechanisms for long-lived, photo-induced superconductivity
above . We analyze a minimal model of optically-driven Raman phonons
coupled to inter-band electronic transitions. Using this model, we develop a
possible microscopic mechanism for photo-controlling the pairing interaction by
displacively shifting the Raman mode. Leveraging this mechanism, we explore two
pictures of long-lived, light-induced superconductivity far above . We
first investigate long-lived, photo-induced superconductivity arising from the
metastable trapping of a displaced phonon coordinate. We then propose an
alternate route to long-lived superconductivity. Within this paradigm, the slow
equilibration of quasi-particles enables a long-lived, non-thermal
superconducting gap. We conclude by discussing implications of both scenarios
to experiments that can be used to discriminate between them. Our work provides
falsifiable, mechanistic explanations for the nanosecond scale photo-induced
superconductivity found in , while also offering a
theoretical basis for exploring long-lived, non-equilibrium superconductivity
in other quantum materials.Comment: 7 pages Main Text, 9 pages Supplementary Material, 4 figure
Probing Inhomogeneous Cuprate Superconductivity by Terahertz Josephson Echo Spectroscopy
Inhomogeneities play a crucial role in determining the properties of quantum
materials. Yet methods that can measure these inhomogeneities are few, and
apply to only a fraction of the relevant microscopic phenomena. For example,
the electronic properties of cuprate materials are known to be inhomogeneous
over nanometer length scales, although questions remain about how such disorder
influences supercurrents and their dynamics. Here, two-dimensional terahertz
spectroscopy is used to study interlayer superconducting tunneling in
near-optimally-doped La1.83Sr0.17CuO4. We isolate a 2 THz Josephson echo signal
with which we disentangle intrinsic lifetime broadening from extrinsic
inhomogeneous broadening. We find that the Josephson plasmons are only weakly
inhomogeneously broadened, with an inhomogeneous linewidth that is three times
smaller than their intrinsic lifetime broadening. This extrinsic broadening
remains constant up to 0.7Tc, above which it is overcome by the
thermally-increased lifetime broadening. Crucially, the effects of disorder on
the Josephson plasma resonance are nearly two orders of magnitude smaller than
the in-plane variations in the superconducting gap in this compound, which have
been previously documented using Scanning Tunnelling Microscopy (STM)
measurements. Hence, even in the presence of significant disorder in the
superfluid density, the finite frequency interlayer charge fluctuations exhibit
dramatically reduced inhomogeneous broadening. We present a model that relates
disorder in the superfluid density to the observed lifetimes
Terahertz parametric amplification as a reporter of exciton condensate dynamics
Condensates are a hallmark of emergence in quantum materials with
superconductors and charge density wave as prominent examples. An excitonic
insulator (EI) is an intriguing addition to this library, exhibiting
spontaneous condensation of electron-hole pairs. However, condensate
observables can be obscured through parasitic coupling to the lattice.
Time-resolved terahertz (THz) spectroscopy can disentangle such obscurants
through measurement of the quantum dynamics. We target , a
putative room-temperature EI where electron-lattice coupling dominates the
structural transition (=326 K), hindering identification of excitonic
correlations. A pronounced increase in the THz reflectivity manifests following
photoexcitation and exhibits a BEC-like temperature dependence. This occurs
well below the , suggesting a novel approach to monitor exciton
condensate dynamics. Nonetheless, dynamic condensate-phonon coupling remains as
evidenced by peaks in the enhanced reflectivity spectrum at select
infrared-active phonon frequencies. This indicates that parametric reflectivity
enhancement arises from phonon squeezing, validated using Fresnel-Floquet
theory and density functional calculations. Our results highlight that coherent
dynamics can drive parametric stimulated emission with concomitant
possibilities, including entangled THz photon generation.Comment: 51 pages, 14 figures, 1 tabl
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