Since the dawn of the quantum era, physicists have armed themselves with brilliantly conceived 'gedanken' (thought) experiments for their intellectual duels. A counterintuitive facet of the quantum laws is tested, or disputed, by considering in one's mind a minimal real-world set-up in which simple devices and operations yield the ultimate meaning of the first principles. The actual implementation of such intellectual instances is particularly exciting, as on the one hand it provides tangible evidence of the overwhelming explanatory power of quantum mechanics, and on the other it exposes the limits of the everyday terminology built on our classical intuition.
Reporting in Nature Photonics, Jian-Shun Tang and co-workers now present the experimental realization1 of the refined version of one of these fascinating mental challenges — namely Wheeler's delayed-choice experiment2, following the prescriptions of a recent theoretical proposal3. In particular, Tang et al. have observed a photon being in a quantum superposition of both a particle and a wave1.
For three centuries, the concepts of particle and wave have been dichotomous. Famous is the dispute about the nature of light between Newton, who believed light had a particle nature, and Hooke and Huygens, who championed the wave hypothesis. When quantum theory was introduced, it became clear that light can behave sometimes as a particle, and other times as a wave, depending on the test performed. This wave–particle duality was somewhat solved by Bohr through his celebrated complementarity principle: mutually exclusive conditions prevent complementary features from being revealed in a single experimental set-up4. This statement was fertile ground for supporters of hidden-variable theories, who predicated that an additional unknown quantity should carry information that discriminates the photon between either a particle or a wave.
Wheeler ruled out such hypotheses by conceiving the following thought experiment. Consider a Mach–Zehnder interferometer with beamsplitters BS1 and BS2, as depicted in Fig. 1a. A single photon enters BS1, with the two arms of the interferometer representing the two possible paths through which the photon may travel. BS2 is either present or absent, and the photon arrives at the signal detectors Da and Db. If the interferometer is complete (that is, BS2 is present), interference between the two possible paths is observed, revealing the wave nature of the initial photon. On the other hand, without BS2, the detectors reveal the path taken by the photon, and a deterministic pattern typical of a classical particle would be seen. So far, this is analogous to Young's famous double-slit experiment. Wheeler extended this by postulating that postponing the insertion of BS2 until after the photon has entered the interferometer will not change the result. When BS2 is put in place, photons will behave as waves; otherwise they will behave as particles. The photon cannot have a predetermined nature and its character is decided upon observation — therefore no hidden variable is required. This traditional version of the experiment has been implemented in conditions of space-like event separation between the entrance of the photon in the circuit and the choice of measurement5, thereby confirming the predictions of quantum theory.
a, A Mach–Zehnder interferometer. A photon, split at a beamsplitter BS1, is eventually detected as having travelled through either both arms (displaying wave-like behaviour) or just one arm (displaying particle-like behaviour), depending respectively on whether or not the second beamsplitter BS2 is in place. If BS2 is engineered to be in a quantum superposition of being present and absent, then this experimental set-up reveals the photon to be in a quantum superposition of being both a wave and a particle2, 6. This has now been demonstrated by Tang et al.1, who exploited an ancillary photon to control the action of the Hadamard gate used to realize BS2. b, The implemented circuit schematic of Tang et al.
The experiment performed by Tang et al.1, which is based on the proposal by Ionicioiu and Terno2, 6, sheds further light on the fundamental nature of photons. The rationale behind the experiment is that complementarity is not due to set-in-stone physical principles, but rather to the limitations of our classical experience. Specifically, the researchers reveal both the particle and wave nature of single photons in a single set-up by introducing a quantum device that regulates the presence of BS2. This means there is no longer a classical operator making the delayed choice about inserting or removing the second beamsplitter, but rather an ancillary photon is employed to realize a superposition of BS2 being both present and absent. A description in terms of quantum circuits is useful for clarifying this point. In such language, a Mach–Zehnder interferometer is written as a network containing Hadamard gates (H) and a phase shifter. The Hadamard gate performs a type of Fourier transform, and acting on a single photon will transform it into a superposition of the photon being in both horizontal and vertical polarization states.
The experiment conducted by Tang et al.1 can be viewed as a network comprising a Hadamard gate H1 (corresponding to BS1), a phase shifter placed in one of the arms, and a second Hadamard gate H2 (corresponding to BS2), acting in sequence on the initial state of the entering photon. Control over the presence of H2 can be regulated in two ways. The first — the classical technique — associates the presence of H2 with the result of measuring the polarization of an ancillary photon; the ancillary photon is considered to be a statistical mixture of both horizontal and vertical polarizations. The second way is quantum in the sense that H2 is implemented as a control-Hadamard gate, with the ancillary photon being the control qubit (Fig. 1b). Here, the superposition of horizontal and vertical polarization states of the ancillary photon determines a superposition of BS2 being both present and absent in the circuit.
In the practical implementation of this technique, beam displacers, which are used in place of beamsplitters, operate on single photons produced by InGa/GaAs quantum dots. Tang et al. compared both scenarios: the ancillary photon in the quantum superposition and in the classical mixture of two orthogonal polarizations. They then measured the probabilities associated with detecting single photons in either one of the paths and compared the results. Striking differences in the experimental data between both scenarios are in agreement with theoretical expectations. These differences are exemplified by the observation of interference between the particle and wave states, which appears only in the case of a quantum controller.
Researchers have reported an analogous effect in implementations based on nuclear magnetic resonance7,8. The demonstrated morphing between particle and wave characteristics of photons within a single experimental setting1 somehow shatters the acclaimed (although poorly understood) duality between them, and calls for a re-visitation of Bohr's classic complementarity principle9. The debate about the nature of light cannot be settled because it is simply ill-defined: the description of a photon cannot be reduced to just 'particle' or 'wave'6.
These feats reveal that, given the striking success of quantum mechanics, attributes such as 'particle' and 'wave' cannot sustain any higher epistemic meaning. Preparing a photon in a coherent superposition of particle and wave must be accepted as no more special than preparing it in a superposition of horizontal and vertical polarizations. Quantum technology may well come to take proper advantage of the wave-versus-particle character of the photon (or of an atom or molecule) as an additional degree of freedom to encode information, joining the likes of polarization, spin, momentum and so on.
In this respect, Tang et al.1 have demonstrated the first ever 'character qubit', or whatever you may wish to call it. More recently, in an independent realization of a quantum version of Wheeler's delayed-choice experiment based on re-configurable integrated optics, researchers also verified the creation of entanglement in the 'character' degrees of freedom between the ancillary photon and the photon entering the circuit10. Another independent implementation of a similar experiment that has appeared even more recently involves preparing the photon to be tested and the ancillary photon in a polarization-entangled state, thereby providing an alternative demonstration of the morphing between particle and wave states11.
Ultimately we may need to abandon our best hopes to reconcile everyday language with the laws of science. Forget the terms 'particle' or 'wave' — let there be only 'quantum light'. Stay tuned for the next gentle blow to common sense.
Gerardo Adesso & Davide Girolami
Nature Photonics 6, 579–580 (2012) | doi:10.1038/nphoton.2012.214
Published online 03 September 2012
- References
- Author Information
- Tang, J.-S. et al. Nature Photon. 6, 600–604 (2012).
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- Ionicioiu, R. & Terno, D. R. Phys. Rev. Lett. 107, 230406 (2011).
- Bohr, N. Nature 121, 580–590 (1928).
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- Roy, S. S., Shukla, A. & Mahesh, T. S. Phys. Rev. A 85, 022109 (2012).
- Auccaise, R. et al. Phys. Rev. A 85, 032121 (2012).
- Tang, J.-S., Li, Y.-L., Li, C.-F. & Guo, G.-C. Preprint at http://arxiv.org/abs/1204.5304 (2012).
- Peruzzo, A., Shadbolt, P., Brunner, N., Popescu, S. & O'Brien, J. L. Preprint athttp://arxiv.org/abs/1205.4926 (2012).
- Kaiser, F., Coudreau, T., Milman, P., Ostrowsky, D. B. & Tanzilli, S. Preprint athttp://arxiv.org/abs/1206.4348 (2012).