Proof-of-concept experiments for quantum physics in space

Famous experiments in quantum physics demonstrate that the way a scientist sets up his experimental apparatus will determine the result that he then measures. And Heisenberg’s Uncertainty Principle tells us that the questions we ask create the answers that we get. Thus quantum physics tells us we live in a “Participatory Universe” in which we human beings, through the thoughts that we think, and the actions that we take, cocreate reality. We make the world that we live in, and thus we are responsible for the world we have made. And, we have the capacity to make a different or better world simply by changing our thinking and thus the actions that follow from our thoughts. As Harry Truman once said, “There ain’t nobody here but us chickens.” We will be the authors of the post-Covid world.

Entangled photon sources play a pivotal role in most of the optical systems that utilize quantum effects. Those include quantum key distribution, quantum computing, metrology, sensing and imaging. Photon pair generation using spontaneous parametric down conversion (SPDC) is one of the most popular routes for producing entangled photons. While SPDC has become more popular, the creation of entangled photon pairs in a degree of freedom, such as polarization, is more challenging. The difficulty is related in no small part to the birefringence of the structure where the generation takes place. Birefringence results in distinguishing information that hinders the production of entanglement in the platforms where SPDC has been most popular including ferroelectric crystals.Despite this limitation, entangled sources have been realized in bulk crystals via well designed interference techniques, which often include additional compensating optics. As the boundaries of optical quantum information science continue to be extended, multi-photon entanglement is now required. Solutions based on SPDC have been attempted, and while some outstanding results have been achieved, the interferometers and compensation procedures become increasingly complex as the size of the entangled state grows. Such scalability and stability issues are directly addressed by integrated entangled photon sources, and while still in their infancy, they promise to help a great deal towards the practical utilization and the production of entanglement on a larger scale.Semiconductors are an ideal platform to fabricate such integrated entangled photon sources. Recently, it was demonstrated that the gallium-arsenide (GaAs) based Bragg refection waveguide (BRW), could efficiently produce photon pairs via SPDC. This platfrom was shown to have a distinct advantage over other semiconductor sources, due largely to its monolithic architecture and its layered epitaxy, which underpins many photonic devices. In this work we demonstrate yet another advantage: the intrinsic capability of the BRW to directly produce polarization entangled photon pairs, without any additional interferometry, spectral filtering, compensation or post-selection. Not only do we show that the BRW can produce entangled photons, but we wish to emphasize that the traditional compensation and interferometric methods used to create entanglement, which presumably would have occurred on chip, may no longer be necessary. Our experimental results confirm that the BRW has the potential to be one of the very first self-contained integrated room temperature resources of entanglement.In this talk various approaches for utilizing SPDC processes in various integration platforms will be discussed. The root cause of entanglement generation is the lack of birefringence in these material systems, which otherwise makes exact phase matching impossible in bulk crystals. Because of this, cross-polarized photons propagate at almost identical group velocities, making off-chip path compensation unnecessary.

Space offers a very unique environment for quantum physics experiments at regimes of distance and velocity not possible on ground. In the recent years there have been a range of theoretical and experimental studies towards the feasibility of performing quantum physics and quantum information science experiments in space. Such a system is possible with today's technologies and would allow to demonstrate the application of quantum secured transmission, and perform quantum optics science tests. In the near term the most promising approach is using a low-earth-orbiting (LEO) satellite either as an independent nano-sat, a piggyback payload on a larger mission, or even on the International Space Station (ISS). I will outline the envisioned quantum experiments and the requirements for the technology.

Entangled photon pairs are a crucial resource in quantum information processing. However, common sources of entangled photon pairs, such as generation by spontaneous parametric down-conversion, impose a Poisson distribution on the pairs, limiting their applicability to quantum information processing. Therefore, generating entangled photon pairs exhibiting a sub-Poisson distribution (i.e., entangled photon bundle) would be crucial/an important advancement in the current research context. Recently, it has been proposed that antibunched photon pairs can be generated using pulsed excitation of quantum dots based on photon blockade. In this work we propose a scheme to generate entangled photon pairs with strong antibunching properties via continuous driving. Under two-photon resonant continuous pumping, a quantum dot is first excited to a biexciton state and then emits a pair of entangled photons into a two-mode cavity. Owing to the strong coupling between the quantum dot and optical modes of the cavity, at most one pair of left-/right-polarized photons exists at a time in the cavity. Specifically, the antibunching effect, average photon number, and entanglement of the left- or right-polarized photons can be resonantly enhanced when the coupling strengths between the exciton (biexciton) transition and cavity modes are equal. Moreover, our results show that the binding energy of the biexciton substantially affects the antibunching effect, average photon number, and entanglement of the left-/right-polarized photons. With suitable parameters, we can realize photon pairs with strong antibunching, a large average photon number, and deep entanglement. Hence, the studied quantum dot-cavity system is a promising source of antibunched entangled photon pairs.

Production of entangled photon pairs is important in secure communication systems, quantum computing, and fundamental physics experiments. Achieving efficient generation of such photon pairs with low-loss parametric oscillators is a key objective in advancing integrated quantum technologies. However, spatially separating the generated photons while preserving their entanglement represents a significant technical challenge. In this work, we demonstrate nonlinear generation of correlated optical harmonics based on non-degenerate four-wave mixing with an optimally pumped optical microcavity with Kerr nonlinearity. The phase matching of the process is achieved with self-injection locked lasers producing parametric oscillation while locked to two different modes of the microresonator. This condition is reminiscent of slow-light technique developed for coherent atomic systems. The experimental design, utilizing counterpropagating light from two self-injection locked lasers, also effectively addresses the challenge of spatial separation of the generated harmonics. Additionally, we demonstrate correlation mediated by self injection locking and Kerr nonlinearity between the two lasers. We validate the theoretical predictions using two self-injection locked semiconductor lasers integrated with a crystalline whispering gallery mode resonator with optimized spectral structure. ©2025 All Rights Reserved

Low-energy neutrons have been a useful probe in fundamental physics studies for more than 70 years. With advances in accelerator technology, many new sources are spallation based. These new, high-flux facilities are becoming the sites for many next-generation fundamental neutron physics experiments. In this review, we present an overview of the sources and the current and upcoming fundamental neutron physics programs.

This article deals with the Copenhagen interpretation of quantum mechanics and its epistemological discussion by Ernst Cassirer and Richard Hönigswald. The starting point is Cassirer's treatise Determinism and Indeterminism in Modern Physics, published in Stockholm. Both philosophers and several physicists were involved in the subsequent exchange of letters. From a physical point of view, the Copenhagen interpretation was particularly criticised by v. Laue and Einstein. Both demanded a revision of the foundations of quantum mechanics or a critical examination of essential physical concepts. The epistemological implications of the Copenhagen interpretation were rejected, in particular that the interactions between observation, the observed and the observer in quantum physics experiments should lead to a renunciation of the concept of causality or an objectively unambiguous description of natural processes. Cassirer argues here that quantum mechanics fulfils the epistemological requirement of transforming ontological “concepts of things” into epistemological concepts of relations and is therefore compatible with the neo-critical approach he advocates. At the same time, Hönigswald was working on two major treatises on the structure of physics and the concept of causality. Here he undertakes an epistemological foundation of physics as a whole. A transcendental foundation of the concept of experience and its specifications in principles such as contemplation, observation or experiment appears to be called for. Hönigswald therefore essentially rejects the Copenhagen interpretation, as it draws epistemological consequences by physical means and thus inadmissibly reverses the transcendentally necessary relations of justification. These arguments are examined and could be used for a current discussion on the epistemological foundations of quantum mechanics and physics as a whole. A post-Neo-Kantian, transcendental philosophical foundation of scientific knowledge, especially of modern physics, seems possible and necessary.

Recent experiments in neutron interferometry and optics at NIST

Space offers unique experimental conditions and a wide range of opportunities to explore the foundations of modern with an accuracy far beyond that of ground-based experiments. Space-based experiments today can uniquely address important questions related to the fundamental laws of Nature. In particular, high-accuracy experiments in space can test relativistic gravity and probe the beyond the Standard Model; they can perform direct detection of gravitational waves and are naturally suited for investigations in precision cosmology and astroparticle physics. In addition, atomic has recently shown substantial progress in the development of optical clocks and atom interferometers. If placed in space, these instruments could turn into powerful high-resolution quantum sensors greatly benefiting fundamental physics. We discuss the current status of space-based research in fundamental physics, its discovery potential, and its importance for modern science. We offer a set of recommendations to be considered by the upcoming National Academy of Sciences' Decadal Survey in Astronomy and Astrophysics. In our opinion, the Decadal Survey should include space-based research in fundamental as one of its focus areas. We recommend establishing an Astronomy and Astrophysics Advisory Committee's interagency Fundamental Physics Task Force to assess the status of both ground- and space-based efforts in the field, to identify the most important objectives, and to suggest the best ways to organize the work of several federal agencies involved. We also recommend establishing a new NASA-led interagency program in fundamental that will consolidate new technologies, prepare key instruments for future space missions, and build a strong scientific and engineering community. Our goal is to expand NASA's science objectives in space by including laboratory research in fundamental physics as an element in the agency's ongoing space research efforts.

Promising experiments from the Moon using particle detectors are discussed, noting the advantage of the large flux collecting power Pc∼AΩt(m2‐sr‐years) offered by the remote, stable environment of a lunar base. An observatory class of particle experiments is presented, based upon proposals at NASA’s recent Stanford workshop. They vary from neutrino astronomy, particle astrophysics, and cosmic ray experiments to space physics and fundamental physics experiments such as proton decay and ‘‘table‐top’’ arrays. This research is background‐limited on Earth, and it is awkward and unrealistic in Earth orbit. It is particularly suited for the Moon where Pc can be quite large and the instrumentation is not subject to atmospheric erosion as it is (for large t) in low Earth orbit.

The invention of the 3He/4He dilution refrigerator opened a new chapter in experimental ultra-low temperature physics. Dilution refrigerators became essential for providing ultra-low temperature environments for nuclear demagnetisation experiments, superconducting-qubit quantum processors and highly sensitive bolometers used in fundamental physics experiments. Development of dilution refrigeration technology requires thorough understanding of the quantum mechanical processes that take place in liquid helium at ultra-low temperatures. For decades the quantum fluids research community provided valuable information to engineers and designers involved in the development of advanced dilution refrigerators. However, the lack of methods that allow the measurement of physical parameters of liquid helium during the operation of a dilution refrigerator was hindering development of the technology. Here we show direct imaging of an operational dilution refrigerator using neutron radiography. This allows direct observation of the dilution process in 3He/4He mixtures and opens an opportunity for direct measurement of the 3He concentration. We observe the refrigerator behaviour in different regimes, such as continuous circulation and single shot, and show that our method allows investigation of various failure modes. Our results demonstrate that neutron imaging applied to the study of dilution refrigeration processes can provide essential information for developers of ultra-low temperature systems. We expect that neutron imaging will become instrumental in the research and development of advanced dilution refrigerators.

The utility of quantum light as a spectroscopic tool is demonstrated for frequency-dispersed pump-probe, integrated pump-probe, and two-photon fluorescence signals which show Ramsey fringes. Simulations of the frequency-dispersed transmission of a broadband pulse of entangled photons interacting with a three-level model of matter reveal how the non-classical time-bandwidth properties of entangled photons can be used to disentangle congested spectra, and reveal otherwise unresolved features. Quantum light effects are most pronounced at weak intensities when entangled photon pairs are well separated, and are gradually diminished at higher intensities when different photon pairs overlap.

Quantum computing is an extremely promising research combining theoretical and experimental quantum physics, mathematics, quantum information theory and computer science. Classical simulation of quantum computations will cover part of the gap between the theoretical mathematical formulation of quantum mechanics and the realization of quantum computers. One of the most important problems in “quantum computer science” is the development of new symbolic languages for quantum computing and the adaptation of existing symbolic languages for classical computing to quantum algorithms. The present paper is devoted to the adaptation of the Mathematica symbolic language to known quantum algorithms and corresponding simulation on the classical computer. Concretely we shall represent in the Mathematica symbolic language Simon’s algorithm, the Deutsch-Josza algorithm, Grover’s algorithm, Shor’s algorithm and quantum error-correcting codes. We shall see that the same framework can be used for all these algorithms. This framework will contain the characteristic property of the symbolic language representation of quantum computing and it will be a straightforward matter to include this framework in future algorithms.KeywordsDeutsch-Josza algorithmGrover’s algorithmQuantum computingQuantum error-correctingShor’s algorithmSimon’s algorithmSimulation of quantum algorithms

Quantum networks are composed of quantum nodes which coherently interact by way of quantum channels. They offer powerful capabilities for quantum computation, communication, and metrology. A generic requirement for these realizations is the capability to generate and store quantum states among multiple quantum nodes, and to disseminate these resources throughout the network via the quantum channels. In this thesis, I describe a series of experiments whereby single excitations in atomic ensembles are strongly coupled to optical modes and provide efficient means for the coherent control of entangled states between matter and light. By following the seminal proposal by Duan et al., we have generated measurement-induced entanglement of an excitation between two cold atomic ensembles. Using this system, we investigated the relationship for the global bipartite entanglement and local correlations in its subsystems. In addition, we achieved functional quantum nodes for entanglement distribution. Two pairs of remote ensembles at two quantum nodes were prepared into entangled states in a heralded and asynchronous fashion by the conditional controls of the entanglement. The quantum states of the ensembles were then distributed into polarization entangled states of photons. We also prepared an analogous quantum state and transferred the nonlocal coherence between two pairs of heralded entangled atomic ensembles, providing a step towards entanglement connection. Beyond such probabilistic approaches, we demonstrated an experiment where entanglement between two quantum memories is created by the reversible and deterministic mapping of an entangled state of light via dynamic electromagnetically induced transparency. This experiment opens novel prospects of integrating hybrid quantum systems by way of reversible quantum interfaces between light and matter. Then, we extended our work to multipartite quantum systems. We theoretically investigated the characterization of multipartite mode-entangled states by way of quantum uncertainty relations, and introduced theoretical tools to verify the entanglement orders in multipartite systems. In particular, we achieved entanglement for one delocalized photon among multiple optical modes (N > 2). Finally, we have achieved measurement-induced entanglement of spin waves among four quantum memories. The individual atomic components for the entangled W state of the four ensembles were then coherently converted into four propagating entangled beams of light via superradiant emissions. We observed the statistical and dynamic transitions for the multipartite entangled spin waves. Experiments described in this thesis thereby represent significant advances of experimental and theoretical capabilities to generate, store, transfer, and characterize entanglement of matter and light over quantum networks.

Investigations into the foundations of quantum mechanics and their link to gravity and general relativity require sensitive quantum experiments. To provide ultimate insight, the realization of such experiments in space will sooner or later become a necessity. With their advanced state and backed by decades of development, quantum technologies and among them quantum memories are providing novel approaches to reach conclusive experimental results. We therefore want to highlight the use case of quantum memories for investigations of fundamental physics in space, and discuss both concrete experiments as well as platforms and performance of candidate quantum memories