Measuring statistics-induced entanglement entropy with a Hong-Ou-Mandel interferometer
Gu Zhang,1, 2, ∗Changki Hong,3, ∗Tomer Alkalay,3, ∗Vladimir
Umansky,3Moty Heiblum,3, †Igor Gornyi,2, ‡and Yuval Gefen4, §
1Beijing Academy of Quantum Information Sciences, Beijing 100193, China
2Institute for Quantum Materials and Technologies, Karlsruhe Institute of Technology, 76021 Karlsruhe, Germany
3Braun Center for Submicron Research, Department of Condensed Matter Physics,
Weizmann Institute of Science, Rehovot 761001, Israel
4Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot 761001, Israel
(Dated: December 27, 2023)
Despite its ubiquity in quantum computation and quantum information, a universally applicable definition
of quantum entanglement remains elusive. The challenge is further accentuated when entanglement is associ-
ated with other key themes, e.g., quantum interference and quantum statistics. Here, we introduce two novel
motifs that characterize the interplay of entanglement and quantum statistics: an ‘entanglement pointer’ and a
‘statistics-induced entanglement entropy’. The two provide a quantitative description of the statistics-induced
entanglement: (i) they are finite only in the presence of quantum entanglement underlined by quantum statis-
tics; (ii) their explicit form depends on the quantum statistics of the particles (e.g., fermions, bosons, anyons).
We have experimentally implemented these ideas by employing an electronic Hong-Ou-Mandel interferome-
ter fed by two highly diluted electron beams in an integer quantum Hall platform. Performing measurements
of auto-correlation and cross-correlation of current fluctuations of the scattered beams (following ‘collisions’),
we quantify the statistics-induced entanglement by experimentally accessing the entanglement pointer and the
statistics-induced entanglement entropy. Our theoretical and experimental approaches pave the way to study
entanglement in various correlated platforms, e.g., those involving anyonic Abelian and non-Abelian states.
Introduction. A pillar of quantum mechanics – quantum
entanglement – prevents us from obtaining a full independent
knowledge of subsystem Aentangled with another subsystem
B. Indeed, the state of subsystem Amay be influenced or
even determined following a measurement of B, even when
both are distant apart. This feature, known as the non-locality
of quantum entanglement, is at the heart of the fast-developing
field of quantum information processing (see, e.g., Refs. [1–
5]). An apt example is a system comprising two particles
with opposite internal magnetic moments (“spin up” and “spin
down”). Imagine we put one particle on Earth (subsystem A)
and the other on Mars (subsystem B). If measurement on A
reveals the particle is in the “up” state, this instantaneously
dictates that the Bparticle is “down”. Following Bell [6]
and CHSH [7] inequalities, measurements of the respective
spins in different directions may unambiguously demonstrate
the quantum nature of the entanglement of Aand B.
An essential way in which quantum entanglement reveals
itself is the entangled subsystem’s entropy. The entanglement
entropy (EE) of subsystem Acan be found when the complete
information of Bis discarded. This amounts to summing over
all possible states of B. Formally, the von Neumann EE is
defined as Sent =−Tr(ρAln ρA), where ρA=TrB(ρAB)is
the reduced density matrix of Aafter tracing over the states of
B, where ρAB is the density matrix in the combined Hilbert
space A⊕B. When the subsystems share common entangled
pairs of particles, such pairs are effectively counted by the EE.
Another pillar of quantum mechanics is the quantum statis-
tics of indistinguishable particles, whose wavefunction might
acquire a non-trivial phase upon exchanging the particles’ po-
sitions (braiding). This phase is pertinent in classifying quasi-
particles as fermions, bosons, and, most interestingly, anyons.
Being instrumental in realizing platforms for quantum infor-
mation processing (see, e.g., Ref. [8]), it motivated several
insightful experiments [9–15] that intended to detect any-
onic statistics [16–23]. Among such experimental setups, the
Hong-Ou-Mandel (HOM) interferometer [24] was employed
as one of the simplest platforms to manifest bosonic [25],
fermionic (see, e.g., Refs. [26,27]), and anyonic (Laughlin
quasiparticles) [14,21,28] statistics. Despite their impor-
tance, the interplay of entanglement with quantum statistics
has hardly been studied, either theoretically or experimentally
(see, however, Refs. [27,29]).
In an attempt to address EE in the context of quantum trans-
port, it has been theoretically proposed [30] to focus on a sin-
gle quantum point contact (QPC) geometry (with partitioning
Tof the incident beam), which allows partial separation of
two subsystems (“arms”), Aand B. Following partial trac-
ing over states in one subsystem, the EE can, in principle, be
obtained indirectly via a weighted summation over even cu-
mulants of particle numbers extracted from the current-noise
measurements (see the discussion of noise cumulants in, e.g.,
Ref. [31]). However, even measurement of the fourth cumu-
lant is not straightforward [32] in mesoscopic conductors [33].
To our knowledge, no study of EE through measurements of
quantum transport has been reported. We note that the EE had
been measured in localized atomic systems (see Ref. [34] for
a review). In addition, the “impurity entropy” (not an EE), in-
duced by frustration at quantum criticality was most recently
reported in Refs. [35,36].
Perspective of our analysis. In the present study, we
fuse two foundational quantum-mechanical notions: quan-
tum statistics and entanglement, and propose the concept of
statistics-induced entanglement. We introduce two functions
quantifying entanglement arising from the quantum statistics
of indistinguishable particles: (i) the “entanglement pointer”
arXiv:2210.15520v3 [cond-mat.mes-hall] 25 Dec 2023