Welcome to Hong Y. Ling's  Website

I was drawn to physics, much like many of my students, by a curious disposition not unlike that of an ancient shepherd who wondered what were the “twinkle, twinkle, little stars” high above the night sky.  It is this primal instinct of curiosity that drives me to extend, with delight, my research interest beyond my comfort zone, from (quantum) light & matter, the area of my graduate training, to (cold) atomic and molecular physics, and condensed matter physics.  Presently, I focus my research on collective phenomena, such as phases and phase transitions, in interacting many-body quantum systems at low temperatures. 

For example,  there are particles, called bosons (fermions), which, even in the absence of any attractive (repulsive) forces, have the propensity to aggregate (disperse), much like their human counterparts, 'extroverts' ('introverts'), who enjoy parties (value independence).  How do particles gain personal-like qualities?  Believe it or not, theoretical physicists trace it to the phase of a wave function - a function of complex number describing a quantum system - acquired by exchanging two identical particles.  Particles are classified as bosons (fermions) if the phase is 0 (π), (and can even take the unique identity of anyons if the phase lies between 0 and π).   

As if this were not strange enough, there exists the mind-boggling phenomenon of quantum entanglement, through which particles communicate in ways that may even invite jealousy from the gods dwelling in Mount Olympus.   It is said that for a pair of entangled particles,  an action on one particle may be felt by another particle instantaneously, even when the two particles are far apart from each other.  Phrasing it differently, knowing the state of the particle at hand, a fortune teller will be able to predict the 'fortune' of the distant particle with a perfect accuracy, i.t., devoid of any uncertainty. 

An analogy may further help to magnify the peculiarity of this concept.  Imagine that misfortune befalls a far-away, normally tranquil and peaceful, kingdom shortly after the kingdom, plebeians and nobility alike, celebrated, with a fairytale-like fanfare, the arrival of adorable identical twins to the royal family - one twin is taken to a distant planet by invading aliens.   But, thanks to the twins being born entangled, the twin, who may be light-years away, feels the gentle caress of their mother whenever she, pining and lamenting for her lost child, impresses a heart-felt and melancholy kiss on the cheek of the twin on her arms.  Does this sound natural or supernatural, and science or alchemy?  Sleep tight and hope that this does not give you any nightmares.

This monstrous, shocking, and mysterious idea, much like a priceless treasure that lay hidden in the most secrete chamber within the innermost bosom of nature, was 'excavated' by three amigos led by Einstein in 1935 in his attempt to regain his footing in his skirmishes in continental Europe with the camp led by Bohr regarding the foundation of quantum mechanics.  It came like a thunderbolt, giving a great jolt to Bohr who was still basking, half dreaming and half smiling (with no lack of  tints of self-satisfied smirk), in the afterglow of his recent triumph (in 1927) when he brushed aside waves of onslaughts by Einstein, armed with his most lethal weapon - ingeniously crafted Gedanken (thought) experiments, with such nonchalance and effortlessness as if such attacks were merely child's play.  

As an epilogue, Bohr appeared to never recover his early coolness, unable to come up with a satisfactory rebuttal before he departed from this world.  Meanwhile, scientific community moves on with its own rhythm of life beyond the Einstein-Bohr debate.  Especially, in recent decades, great attentions have been paid to whether quantum paradoxes can help to further transcend humanity's asperation for technological superiority, leading to breakthroughs that have hitherto eluded even mankind's imaginary world.  While the unearth of Rosetta Stone unlocked the secrets to the forgotten hieroglyphs in the distant past, the discovery of the seemingly physics-defying phenomenon of quantum entanglement has opened a fresh new vista of possibilities in the future, including quantum computers which promise to solve problems that leave even the most powerful supercomputers available in the state of deep despair. 

My apologies for being overly garrulous and sidetracked.  Let's return to the comparisons related to murmuration. In the many-body quantum world, capacities akin to “coordination” and “synchronization” in bird's murmuration are realized by striking a delicate balance between hopping and push & pull among particles, aided by a host of oddities, such as wave-particle duality, tunneling, quantum statistics (whether particles are bosons, fermions, or anyons), and entanglement, that do not have their classical analogs and are thus far more subtle and difficult to fathom.  In consequence, a many-body quantum system has the inherent capacity to create collective quantum phases that feast eyes and defy imaginations in manners no less spectacular than murmuration.

An example that immediately comes to mind is the celebrated superconductor where current can flow without resistance bellow a critical temperature. This implies that with a superconducting laptop, one is spared the trouble of looking for electric outlets wherever one goes and the unwholesome routine of having to charge the laptop from time to time, irrespective of how many hours one spends playing video games, watching movies, etc..   How does nature manage to pull through such a stunt?  It may come as a shock that the answer lies actually in an unlikely domain, symmetry, which serves more as an esthetic means to value and appreciate beauty, than as an investigative tool to unravel the mystery in physics.   Nature can sometimes behave as if it were a mischievous and deceitful child.  Contrary to pedestrian impression, symmetry is an indispensable and unifying concept across, virtually, all areas of scientific inquiry.   

Consider, for instance, water when it changes from a liquid fluid, which quenches thirsty, to a solid ice, which ameliorates gut-wrenching toothache.  Picture that you could hold, in the palms of your hands, the liquid water, just as you do with a piece of ice; it would look the same no matter at what angles you rotate the liquid.  On the other hand, an astute observer may be aware that only at specific angles (not at an arbitrary direction) does a solid ice, a crystal structure, return to its original appearance.  It is said that in a liquid-solid phase transition, continuous rotational symmetry is spontaneously broken, giving way to a lower symmetry - discrete rotational symmetry. 

An analogous, albert somewhat more abstruse, symmetry breaking occurs in the superconductor phase transition.  It traces its root to a mathematical object mentioned previously: the phase inherited from a complex wave function, which is simply the polar angle in a complex plane.  A normal conductor, which remains invariant under arbitrary phase rotations - adding to or subtracting from the phase angle by any amount, no matter how minute - above the critical temperature, settles down to a state with a particular phase below the critical temperature when it enters a superconducting state. In doing so, it singles out a specific phase angle from all possible phases, spontaneously breaking the phase rotation symmetry. In a nutshell, spontaneous symmetry breaking, transitioning a state from unbroken symmetry to broken symmetry, lies at the heart of the magic phenomenon of superconductivity.

It would have caught Plato completely off guard had Plato witnessed, in the material world, a demonstration of a superconductor, which, in all respects, is no less than an ideal conductor with the perfect “conduct-ness”, and therefore should, according to his theory of “forms”,  float around an empyrean realm of "forms" - an immaterial world inaccessible to the direct human perception.  I cannot help but concoct the following comical scene to have a good laugh about it.  

Imagine Plato in a tunic and sandals, with overgrown beard and shaggy hair, looking disheveled and exhausted after a day of toil, full of Socratic style of debates, especially with his precocious and tenacious pupil, Aristotle, in the Academy.  In this weary state, he was making his way back to his lodge at night when he suddenly spotted a faint glow suspended in the air, dotted with golden sparkles.  Being both dazzled and intrigued by it, he inched, surreptitiously, closer towards this mysterious glow.  No sooner did he recognize that it was a superconductor than he ran away from it as fast as his legs could carry him, as if he had just undergone a near-death otherworldly experience, all the while, crying frantically, with not so much trepidation as a profound sense of injustice, that the world must have gone mad, his theory was impregnable, etc..