Theory of Cold Atoms @ Seoul National University

# Home

## About Our Group

Quantum many-body physics investigates the complex and highly entangled behavior of interacting (elementary or composite) particles at ultralow temperatures. Our group explores the intricate and unexpected manifestations of the resulting emergent phenomena when many quantum degrees of freedom cooperate, which go fundamentally beyond those expected from the simple single-particle picture familiar from textbook scenarios.

## Selected Publications

Probing the Scale Invariance of the Inflationary Power Spectrum in Expanding Quasi-Two-Dimensional Dipolar Condensates

Phys. Rev. Lett. 118, 130404 (2017).
"Photonic" Cat States from Strongly Interacting Matter Waves

Phys. Rev. Lett. 115, 260404 (2015).
Consistent perturbative treatment of the subohmic spin-boson model yielding arbitrarily small $T_2/T_1$ decoherence time ratios

Phys. Rev. B 90, 220302 (Rapid Communication) (2014).
Revealing Single-Trap Condensate Fragmentation by Measuring Density-Density Correlations after Time of Flight

Phys. Rev. Lett. 113, 140404 (2014).
Ultrafast quantum random access memory utilizing single Rydberg atoms in a Bose-Einstein condensate

Phys. Rev. Lett. 111, 240504 (2013).
Fragmented many-body ground states for scalar bosons in a single trap

Phys. Rev. Lett. 103, 060402 (2009).
Vortex quantum creation and winding number scaling in a quenched spinor Bose gas

Phys. Rev. Lett. 99, 120407 (2007).
Sweeping from the superfluid to the Mott phase in the Bose-Hubbard model

Phys. Rev. Lett. 97, 200601 (2006).
Stability of quasi-two-dimensional Bose-Einstein condensates with dominant dipole-dipole interactions

Phys. Rev. A 73, 031602 (Rapid Communication) (2006).
Quantum simulation of cosmic inflation in two-component Bose-Einstein condensates

Phys. Rev. A 70, 063615 (2004).
"Cosmological" quasiparticle production in harmonically trapped superfluid gases

Phys. Rev. A 69, 033602 (2004).
Gibbons-Hawking Effect in the Sonic de Sitter Space-Time of an Expanding Bose-Einstein-Condensed Gas

Phys. Rev. Lett. 91, 240407 (2003).
Vortex states of rapidly rotating dilute Bose-Einstein condensates

Phys. Rev. Lett. 90, 140402 (2003).
Existence of Long-Range Order for Trapped Interacting Bosons

Phys. Rev. Lett. 89, 280402 (2002).

# Members

## Postdocs

Relativistic Quantum Information
56-519

최석영 (Seok-yeong Chä)
Analogue Quantum Cosmology
56-520
윤태웅 (Tae-Woong Youn)
Hybrid Quantum Gates
56-517
신성호 (Seongho Shin)
Topological Defects in Ultracold Quantum Gases
56-517
NRF Global PhD Fellowship Holder
박상신 (Sangshin Park)
Analogue Gravity
56-520
정환철 (Hwan Chul Jung)
Quantum Memories
56-517
배승리 (Seunglee Bae)
Many-Body Effects in Quantum Information Processing
56-517
박재균 (Jae-gyun Park)
Quantum Metrology
56-520
곽영진 (YeongJin Gwak)
Self-Consistent Many-Body Physics
56-515

## Postdocs

강명균 (Myung-Kyun Kang)
Measurement Theory of Quantum Many-Body States
BK Thesis Excellence Award 2016

노경주 (Kyung-Joo Noh)
Theory of Open Quantum Systems, Quantum Decoherence, and Quantum Thermalization
Now at Yale University

# Research Interests

## Hybrid Quantum Systems

This is the study of a long-lived and rapidly accessible quantum memory unit, for which the operational Hilbert space is spanned by states involving the two macroscopically occupied hyperfine levels of a miscible binary atomic Bose-Einstein condensate and the Rydberg state of a single atom. Our group have shown that an arbitrary qubit state, initially prepared using a flux qubit, can be rapidly transferred to and from the trapped atomic ensemble in approximately 10 ns and with a large fidelity of 97%, via an effective two-photon process using an external laser for the transition to the Rydberg level. The achievable ultrafast transfer of quantum information enables a large number of storage and retrieval cycles from the highly controllable quantum optics setup of a dilute ultracold gas, even within the typically very short flux qubit lifetimes of the order of microseconds.
[Related Publications]
, Ultrafast quantum random access memory utilizing single Rydberg atoms in a Bose-Einstein condensate, Phys. Rev. Lett. 111, 024504 (2013).
, Transfer and storage of qubits in the presence of decoherence, EPL 102, 20001 (2013).
, Hybrid of superconducting quantum interference device and atomic Bose-Einstein condensate: An architecture for quantum information processing, Phys. Rev. A 87, 052303 (2013).

## Fragmented Condensates of Interacting Bosons as a Many-body Phenomenon

We investigate whether the many-body ground states of bosons in a generalized two-mode model with localized inhomogeneous single-particle orbitals and anisotropic long-range interactions (e.g., dipole-dipole interactions) are coherent or fragmented. It is demonstrated that fragmentation can take place in a single trap for positive values of the interaction couplings, implying that the system is potentially stable. Furthermore, the degree of fragmentation is shown to be insensitive to small perturbations on the single-particle level.
[Related Publications]
, Fragmented many-body ground states for scalar bosons in a single trap, Phys. Rev. Lett. 103, 060402 (2009).
, Robustness of fragmented condensate many-body states for continuous distribution amplitudes in Fock space, Phys. Rev. A 88, 053602 (2013).
, Revealing Single-Trap Condensate Fragmentation by Measuring Density-Density Correlations after Time of Flight, Phys. Rev. Lett. 113, 140404 (2014).

## Atoms in Optical Lattices as Quantum Simulators for New Phases of Matter

We consider the rapid quench of a one-dimensional strongly correlated supersolid to a localized density wave (checkerboard) phase, and calculate the first-order coherence signal following the quench. It is shown that unique coherence oscillations between the even and odd sublattice sites of the checkerboard are created by the quench, which are absent when the initial state is described by a Gutzwiller product state. This is a striking manifestation of the versatility of the far-from-equilbrium and nonperturbative collapse and revival phenomenon as a microscope for quantum correlations in complex many-body states. For the present example, this opens up the possibility to discriminate experimentally between mean-field and many-body origins of supersolidity.
[Related Publications]
, Identifying strongly correlated supersolid states on the optical lattice by quench-induced $\pi$-states, EPL 99, 66003 (2012).
, Interaction-induced coherence among polar bosons stored in triple-well potentials, Phys. Rev. A 88, 063608 (2013).
, Many-site coherence revivals in the extended Bose-Hubbard model and the Gutzwiller approximation, Phys. Rev. A 84, 063635 (2011).

## Analogue Gravity and Quantum Field Theory of Bogoliubov Excitations

A variety of cosmologically motivated effective quasiparticle space-times can be produced in harmonically trapped superfluid Bose and Fermi gases. We have studied the analog of cosmological particle production in these effective space-times, induced by trapping potentials and coupling constants possessing an arbitrary time dependence. The WKB probabilities for phonon creation from the superfluid vacuum have been calculated, and an experimental procedure to detect quasiparticle production by measuring density-density correlation functions was proposed.
[Related Publications]
, "Cosmological" quasiparticle production in harmonically trapped superfluid gases, Phys. Rev. A 69, 033602 (2004).
, Quantum simulation of cosmic inflation in two-component Bose-Einstein condensates, Phys. Rev. A 70, 063615 (2004).
, Gibbons-Hawking Effect in the Sonic de Sitter Space-Time of an Expanding Bose-Einstein-Condensed Gas, Phys. Rev. Lett. 91, 240407 (2003).

# Publications

## 2017

Phase-fluctuating condensates are fragmented: An experimental benchmark for self-consistent quantum many-body calculations

arXiv:1701.06821v1 [cond-mat.quant-gas] (2017).
Probing the Scale Invariance of the Inflationary Power Spectrum in Expanding Quasi-Two-Dimensional Dipolar Condensates

Phys. Rev. Lett. 118, 130404 (2017).

## 2016

How precisely can the speed of light and the metric of space-time be determined in principle?

arXiv:1502.04979v2 [quant-ph] (2016).
Amplification of the quantum superposition macroscopicity of a flux qubit by a magnetized Bose gas

Phys. Rev. A 94, 042320 (2016).
Quantum sine-Gordon dynamics on analogue curved spacetime in a weakly imperfect scalar Bose gas

Phys. Rev. D 94, 024051 (2016).

## 2015

"Photonic" Cat States from Strongly Interacting Matter Waves

Phys. Rev. Lett. 115, 260404 (2015).
Condensate fragmentation as a sensitive measure of the quantum many-body behavior of bosons with long-range interactions

Phys. Rev. A 91, 063621 (2015).

## 2014

Consistent perturbative treatment of the subohmic spin-boson model yielding arbitrarily small $T_2/T_1$ decoherence time ratios

Phys. Rev. B 90, 220302 (Rapid Communication) (2014).
Revealing Single-Trap Condensate Fragmentation by Measuring Density-Density Correlations after Time of Flight

Phys. Rev. Lett. 113, 140404 (2014).
Universality of the Berezinskii–Kosterlitz–Thouless type of phase transition in the dipolar XY-model

New J. Phys. 16, 053011 (2014).
Truncated many-body dynamics of interacting bosons: A variational principle with error monitoring

Int. J. Mod. Phys. B 28, 1550021 (2014).

## 2013

Ultrafast quantum random access memory utilizing single Rydberg atoms in a Bose-Einstein condensate

Phys. Rev. Lett. 111, 024504 (2013).
Robustness of fragmented condensate many-body states for continuous distribution amplitudes in Fock space

Phys. Rev. A 88, 053602 (2013).
Transfer and storage of qubits in the presence of decoherence

EPL 102, 20001 (2013).
Interaction-induced coherence among polar bosons stored in triple-well potentials

Phys. Rev. A 88, 063608 (2013).
Stability of spherically trapped three-dimensional Bose-Einstein condensates against macroscopic fragmentation

Phys. Rev. A 87, 023632 (2013).
Hybrid of superconducting quantum interference device and atomic Bose-Einstein condensate: An architecture for quantum information processing

Phys. Rev. A 87, 052303 (2013).

## 2012

Verifying the observer dependence of quasiparticle counts in the analogue gravity of dilute ultracold quantum gases

arXiv:1205.5548 [cond-mat.quant-gas] (2012).
Identifying strongly correlated supersolid states on the optical lattice by quench-induced $\pi$-states

EPL 99, 66003 (2012).

## 2011

Many-site coherence revivals in the extended Bose-Hubbard model and the Gutzwiller approximation

Phys. Rev. A 84, 063635 (2011).
Emergence of a new pair-coherent phase in many-body quenches of repulsive bosons

Phys. Rev. A 84, 011604 (Rapid Communication) (2011).

## 2010

System size scaling of topological defect creation in a second-order dynamical quantum phase transition

New J. Phys. 12, 095020 (2010).
Interacting trapped bosons yield fragmented condensate states in low dimensions

Phys. Rev. A 82, 013607 (2010).
Increased Cation Conductance in Human Erythrocytes Artificially Aged by Glycation

J. Membrane Biol. 235, 177-189 (2010).
$O(N)$ symmetry-breaking quantum quench: Topological defects versus quasiparticles

Phys. Rev. D 81, 025017 (2010).

## 2009

Fragmented many-body ground states for scalar bosons in a single trap

Phys. Rev. Lett. 103, 060402 (2009).

## 2008

Tunneling-induced damping of phase coherence revivals in deep optical lattices

Phys. Rev. A 78, 061603 (Rapid Communication) (2008).
Effect of fluctuations on the superfluid-supersolid phase transition on the lattice

Phys. Rev. A 78, 033604 (2008).
Bogoliubov theory of quantum correlations in the time-dependent Bose-Hubbard model

Phys. Rev. A 77, 043615 (2008).
Coherent single atom shuttle between two Bose-Einstein condensates

Phys. Rev. A 77, 031602 (Rapid Communication) (2008).

## 2007

Vortex quantum creation and winding number scaling in a quenched spinor Bose gas

Phys. Rev. Lett. 99, 120407 (2007).
Quantum top inside a Bose-Einstein-condensate Josephson junction

Phys. Rev. A 75, 053605 (2007).
Dynamical Aspects of Analogue Gravity: The Backreaction of Quantum Fluctuations in Dilute Bose-Einstein Condensates

Lect. Notes Phys. 718, 93-113 (2007).

## 2006

Global singularity-free solution of the Iordanskiĭ force problem

Phys. Rev. B 74, 224503 (2006).
Sweeping from the superfluid to the Mott phase in the Bose-Hubbard model

Phys. Rev. Lett. 97, 200601 (2006).
Mean-field expansion in Bose-Einstein condensates with finite-range interactions

Int. J. Mod. Phys. B 20, 3555-3565 (2006).
Stability of quasi-two-dimensional Bose-Einstein condensates with dominant dipole-dipole interactions

Phys. Rev. A 73, 031602 (Rapid Communication) (2006).

## 2005

Quantum backreaction in dilute Bose-Einstein condensates

Phys. Rev. D 72, 105005 (2005).
Reply to Comment on "Dynamical role of anyonic excitation statistics in rapidly rotating Bose gases"

Phys. Rev. Lett. 94, 208904 (2005).
Maximal length of trappped one-dimensional Bose-Einstein condensates

J. Low Temp. Phys. 138, 723-728 (2005).

## 2004

Quantum simulation of cosmic inflation in two-component Bose-Einstein condensates

Phys. Rev. A 70, 063615 (2004).
Dynamical role of anyonic excitation statistics in rapidly rotating Bose gases

Phys. Rev. Lett. 93, 160403 (2004).
Quasiparticle universes in Bose-Einstein condensates

Mod. Phys. Lett. A 19, 1789-1812 (2004).
"Cosmological" quasiparticle production in harmonically trapped superfluid gases

Phys. Rev. A 69, 033602 (2004).
Observer dependence for the phonon content of the sound field living on the effective curved space-time background of a Bose-Einstein condensate

Phys. Rev. D 69, 064021 (2004).
Vortex liquids and vortex quantum Hall states in trapped rotating Bose gases

J. Phys. B: At. Mol. Opt. Phys. 37, S301-S310 (2004).

## 2003

Zero-temperature damping of Bose-Einstein condensate oscillations by vortex-antivortex pair creation

Phys. Rev. A 68, 011602 (Rapid Communication) (2003).
Gibbons-Hawking Effect in the Sonic de Sitter Space-Time of an Expanding Bose-Einstein-Condensed Gas

Phys. Rev. Lett. 91, 240407 (2003).
Warped space-time for phonons moving in a perfect nonrelativistic fluid

Europhys. Lett. 62, 1-7 (2003).
On the space-time curvature experienced by quasiparticle excitations in the Painlevé-Gullstrand effective geometry

Ann. Phys. (N.Y.) 304, 22-39 (2003).
Vortex states of rapidly rotating dilute Bose-Einstein condensates

Phys. Rev. Lett. 90, 140402 (2003).

## Before 2003

Existence of Long-Range Order for Trapped Interacting Bosons

Phys. Rev. Lett. 89, 280402 (2002).
Riemannian geometry of irrotational vortex acoustics

Phys. Rev. Lett. 88, 110201 (2002).
Electromagnetomotive force fields in noninertial reference frames and accelerated superconducting quantum interferometers

Phys. Rev. B 64, 214509 (2001).
Short wavelength spectrum and Hamiltonian stability of vortex rings

Phys. Rev. E 64, 016306 (2001).
Specific heat of the Kelvin modes in low temperature superfluid turbulence

Phys. Rev. B 63, 212504 (2001).
Hall state quantization in a rotating frame

Europhys. Lett. 54, 502-507 (2001).
Thermal quasi-equilibrium states across Landau horizons in the effective gravity of superfluids

Int. J. Mod. Phys. D 10, 57-88 (2001).
Tunnelling of topological line defects in strongly coupled superfluids

Ann. Phys. (Leipzig) 9, 523-570 (2000).
Motion of Quantized Vortices as Elementary Objects

Ann. Phys. (N.Y.) 278, 62-85 (1999).
On the theory of vortex quantum tunnelling in the dense Bose superfluid helium II

Physica B 255, 41-54 (1998).
Geometric Laws of Vortex Quantum Tunneling

Phys. Rev. B 58, 105-108 (1998).
Massive Charged Strings in the Description of Vortex Ring Quantum Nucleation

J. Low Temp. Phys. 110, 39-44 (1998).
Transition probabilities for a Rydberg atom in the field of a gravitational wave

Class. Quantum Grav. 11, 463-474 (1994).