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Acknowledgments: We thank R. Ionicioiu, S. Pironio,
T. Rudolph, N. Sangouard, and D. R. Terno for useful
discussions, and acknowledge financial support from the UK
Engineering and Physical Sciences Research Council (EPSRC),
European Research Council (ERC), the Quantum Integrated
Photonics (QUANTIP) project, A Toolbox for Photon Orbital
Angular Momentum Technology (PHORBITECH) project, the
Quantum InterfacES, SENsors, the Communication based on
Entanglement (Q-ESSENCE) integrating project, Nokia, the
Centre for Nanoscience and Quantum Information (NSQI), the
Templeton Foundation, and the European Union Union
Device-Independent Quantum Information Processing (DIQIP)
project. J.L.O. and S.P. acknowledge a Royal Society Wolfson
Merit Award. A.P. holds a Royal Academy of Engineering
Materials and Methods
28 June 2012; accepted 18 September 2012
Florian Kaiser,1 Thomas Coudreau,2 Pérola Milman,2,3 Daniel B. Ostrowsky,1 Sébastien Tanzilli1*
Wave-particle complementarity is one of the most intriguing features of quantum physics. To
emphasize this measurement apparatus–dependent nature, experiments have been performed
in which the output beam splitter of a Mach-Zehnder interferometer is inserted or removed after
a photon has already entered the device. A recent extension suggested using a quantum beam
splitter at the interferometer’s output; we achieve this using pairs of polarization-entangled
photons. One photon is tested in the interferometer and is detected, whereas the other allows
us to determine whether wave, particle, or intermediate behaviors have been observed. Furthermore,
this experiment allows us to continuously morph the tested photon’s behavior from wavelike to
particle-like, which illustrates the inadequacy of a naive wave or particle description of light.
Although the predictions of quantum me- chanics have been verified with marked precision, subtle questions arise when
attempting to describe quantum phenomena in
classical terms (1, 2). For example, a single quantum object can behave as a wave or as a particle.
This concept is illustrated by Bohr’s complementarity principle (3) which states that, depending
on the measurement apparatus, either wave or
particle behavior is observed (4, 5). This is demonstrated by sending single photons into a Mach-Zehnder interferometer (MZI) followed by two
detectors (Fig. 1A) (6). If the MZI is closed [that
is, if the paths of the interferometer are recombined at the output beam splitter (BS2)], the probabilities for a photon to exit at detectors Da and
Db depend on the phase difference q between
the two arms. The which-path information remains
unknown, and wavelike intensity interference patterns are observed (Fig. 1B). On the other hand,
if the MZI is open (i.e., if BS2 is removed), each
1Laboratoire de Physique de la Matière Condensée, CNRS UMR
7336, Université de Nice–Sophia Antipolis, Parc Valrose, 06108
Nice Cedex 2, France. 2Laboratoire Matériaux et Phénomènes
Quantiques, Université Paris Diderot, Sorbonne Paris Cité, CNRS,
UMR 7162, 75013 Paris, France. 3Institut de Sciences Moléc-ulaires d’Orsay (CNRS) Bâtiment 210, Université Paris Sud 11,
Campus d’Orsay, 91405, Orsay Cedex, France.
*To whom correspondence should be addressed. E-mail:
photon’s path can be known, and consequently, no
interference occurs. Particle behavior is said to be
observed, and the detection probabilities at Da
and Db are equal to ½, independent of the value
of q (Fig. 1C). In other words, these two different
configurations—BS2 present or absent—give different experimental results. Recently, Jacques et al.
have shown that, even when performing Wheeler’s
original gedanken experiment (7) in which the
configuration for BS2 is chosen only after the
photon has passed the entrance beam splitter BS1,
Bohr’s complementarity principle is still obeyed
(8). Intermediate cases, in which BS2 is only partially present, have been considered in theory and
led to a more general description of Bohr’s complementarity principle expressed by an inequality
limiting the simultaneously available amount of
interference (signature of wavelike behavior) and
which-path information (particle-like behavior)
(9, 10). This inequality has also been confirmed
experimentally in delayed-choice configurations
We take Wheeler’s experiment one step fur-
ther by replacing the output beam splitter by a
quantum beam splitter (QBS), as theoretically pro-
posed of late (13, 14). In our experiment (Fig. 2),
we exploit polarization entanglement as a re-
source for two reasons. First, doing so permits
implementing the QBS. Second, it allows us
to use one of the entangled photons as a test
photon sent to the interferometer and the other
one as a corroborative photon. Here, as opposed
to previous experiments (8, 11), the state of the
interferometer remains unknown, as does the
wave or particle behavior of the test photon, until
we detect the corroborative photon. By continuous-
ly modifying the type of measurement performed
on the corroborative photon, we can morph the
test photon from wave to particle behavior, even
after the test photon was detected. To exclude
interpretations based on either mixed states, as-
sociated with preexisting state information (15),
or potential communication between the two pho-
tons, the presence of entanglement is verified via
the violation of the Bell inequalities with a space-
like separation (16–18).