Over the last decade significant resources have been dedicated to building integrate linear optics photonic platforms that demonstrate various quantum information processing and sensing schemes. Generally, the details of a particular platform is determined by the application, but there are certain universal requirements for any such scheme: (a) deterministic generation of single-photons, more generally entangled photons, (b) efficient nano-photonic elements for routing and manipulating photons and (c) on-chip single photon detectors. Among these, the technologies for routing and detecting single photons, especially in visible and NIR wavelengths, have attained a high level of maturity. In contrast integrated platforms for deterministic generation of single-photons are still in its infancy and in this context the capabilities enabled by DNA origami, and DNA origami placement (DOP), can be invaluable.
An ideal Single Photon Source (SPS) emits exactly one photon at a time, on-demand, into a unique optical mode such that all photons are indistinguishable. Currently, the most promising integrated, deterministic, SPSes are based on individual light emitters like quantum dots, color centers, molecules and ions since they are inherently incapable of multi-photon emission. Although each emitter class has its own unique advantages and disadvantages, the absence of a scalable method for deterministic integration is a challenge common to all.
Figure (Origami SPS) depicts one possible scheme, using DNA origami, for creating an on-chip single photon source. First DNA origami is localized to a particular location on a substrate using DOP. Second, emitters are bound to the origami and finally nanophotonic elements (e.g. an optical cavity) is built around the immobilized emitter. While this description is quiet straightforward, in reality there are several details that would need to be optimized and prime among them is the choice of emitter. Chelated metal ions (Lanthanides or other transition metals) or molecules like Terrylene or Pyrelene would be ideal since they have, in one form or the other, been used for quantum optics experiments and are also compatible with DNA labeling. However, other emitters like colloidal quantum dots, color centers and dye molecules might also be worth being explored. Eventually the choice of emitter depends will depend on a lot of factors like;
(a) Ease and yield of organization on immobilized DNA origami.
(b) Detailed photophysics of the emitters, like quantum yield, emission wavelength, de-coherence, radiative lifetime etc.
(c) Detailed excitation modality, i.e. can the emitter be excited electrically or optically and details of the same.
(d) Available methods for tuning the emission.
(e) Temperature dependence of emission profile and the photophysics.
Further, the details of the nano-photonic elements will also need to be optimized. Although, this is not of principle importance since there is a large body of existing literature that has painstakingly investigated role of various nanophotonic elements within linear optics quantum information processing models.
In the long run, one of the main aim would be the demonstration of an entirely on-chip Hong-Ou-Mandel (HOM) indistinguishability experiment; the basic target device will have independent emitters on two arms of the interferometer that are independently triggered (and tuned) to emit indistinguishable single-photons that are quantified by correlating photons that are interacting at the interferometer, using integrated single photon detector. While the successful demonstration HOM experiment, in and of itself, isn't necessarily a landmark achievement. However, doing it using two independent on-chip SPSes, is a crucial first step towards creating integrated linear optics quantum optical systems. Of particular importance is the well accepted fact that a DNA origami based framework is scalable and modular which means that creating large networks of of SPSes would be straight forward. While I am tempted to claim that this would immediately usher in the age of quantum superiority, the reality is that it would merely add more parameters to the list of optimizations that need to be performed.
The DNA origami based approach described could also open up other methods engineering on-chip quantum light sources. For instance, we can now position emitters at spacing as small as ∼5nm to many microns (using DOP) within arbitrary nanophotonic environment which can directly be used to explore Dicke’s super-radiance in all it’s intricacies.
Dicke’s model of superradiance (SR), introduced in 1954, describes a collective behavior of emitters, similar to lasing, well before the first lasers were invented. In his seminal paper Dicke described how optical dipoles within a closely packed cluster can interact with each other through their common electromagnetic field and radiate coherently. In the ideal case, this phenomenon will cause a cluster of N dipoles, excited coherently, to emit a pulse whose intensity and decay rate has been enhanced by a factor of N^2 and N respectively when compared to an isolated dipole (fig (Origami SR)). Since the original description by dicke, the field of superradiance has flourished with an abundance of theoretical and experimental results relevant for engineering light absorption, developing new classes of ultrastable light sources, quantum metrology as well as quantum computing. While the reality of SR has clearly been established, many facets of the phenomena have not been experimentally demonstrated. This is because detailed exploration of dicke's SR requires deterministic positioning (and orientation control) of light emitting dipoles at deep-subwavelength regime which is currently unavailable using conventional nanofabrication approach. However, DNA origami and DOP presents a tremendous degree of freedom toexplore various facets of Dicke's superradiance. Specifically,
a) Digitally increasing the number of light emitters in tight clusters (with or without dipole interaction).
b) Preserving the net emitter but changing the spatial density to understand temporal interaction but also phase interaction
c) Embedding the well-defined emitter clusters within nanophotonic resonators.
d) Exploring SR interaction between emitters with similar emission properties but dramatically different lifetime
These represents but a small subset of the potentially giant set of experiments and eventually functional devices enabled by Engineering Dicke's superradiance using DNA origami.
One of the possible arguments against this entire line of research is the perceived inferior photophysics of dye molecules, colloidal quantum dots or chelated ions when compared to solid state quantum dots, color centers or neutral atoms/ions in high vacuum atom traps. While this line of reasoning is partially valid, I believe the fundamentally superior organization capabilities and modularity enabled by DNA origami makes this approach wothwhile and interesting. Especially, since the structures and architectures that are proposed are almost impossible to be realized in optical (or NIR) regime using any solid state emitters or atom traps. A couple of other factor that is often overlooked about DNA origami compatible emitters are, (a) The photo-physics of these emitters have not been studied precisely in cryogenic temperature, (b) These emitter (not colloidal quantum dots) are are atomically precise with respect to each other.