Quantum Darwinism: Survival of the fittest

 “If you are not completely confused by quantum mechanics, you do not understand it.” – John Wheeler

Quantum mechanics describes the laws and behaviour of subatomic particles^a. What we perceive at the macroscopic level is pretty different from the behaviour of the individual microscopic particles. It is inconceivable that the pizza you are eating might have a non-zero probability of being elsewhere and not on your plate! But such seemingly bizarre possibilities are commonplace when we look at the atomic scale.

The large and the very small have always been at two extremes and a different set of laws are used to explain their conflicting behaviours. Throughout decades, scientists have tried to bridge the conceptual gap that exists while transitioning from the small to the large and vice-versa. Quantum Darwinism, proposed by W. H. Zurek in 2003^[1], is one such attempt to explain how the macroscopic classical world emerges out of the quantum.  

How different are the quantum and classical worlds?

The two main features of quantum theory that are missing from the classical world are superposition and the measurement problem. Superposition refers to property whereby a microscopic entity can exist in a general state that can be written as a probabilistic sum of all possible states. In a classical world, it can be in only one localized state. Before measurement, the entity behaves as a wave with the constituent states (in the superposition) interfering with each other. The superposition state of a quantum particle collapses to the eigenstate^b of the measured observable. This makes it fragile and susceptible to even minute environmental noise, giving rise to the difficulty of measuring the state – the measurement problem.

For several centuries, using separate laws for quantum and classical and quantizing certain quantities (after initially starting with classical assumptions) as and when required proved to be an easy respite from these two disturbing difficulties. In such quantization techniques, a classical field is converted into an operator that acts on the quantum states of the field. Several such examples can be found in quantum optics, condensed matter physics, and nuclear and particle physics. This fared well so far. Yet, there lies the possibility that the laws of the two worlds may not be entirely different.

Decoherence – making a quantum state fragile

When a microscopic particle^a interacts with its environment, it can entangle with it. This destroys the initial superposition state that it was in. Not surprisingly, its properties can also get entangled with any measuring device. This results in decoherence or the vanishing of quantum properties (quantumness). Decoherence is a very fast process which also makes the detection of quantumness very hard. Thus, classically observable results lack some features of what is predicted by the rigorous mathematical calculations.

W. H. Zurek, along with his collaborators, postulated that the states that are most stable and quite resistant against decoherence are the ones that survive at the macroscopic level^[1]. Such states are called pointer states – the name comes from the “pointer” of a measuring instrument. Decoherence can either keep the pointer states unchanged or change it to a form that is similar to the original one. Thus, many superposition states get destroyed and only the pointer states survive when the quantum system interacts with the environment. Zurek called this environmentally-induced superselection or einselection.

However, the question that still persists is how can several observers agree on the same classical outcome if the state can collapse to any of the possible pointer states. This is where Darwin’s theory of natural selection comes to play!

Quantum Darwinism

Charles Darwin proposed the theory of evolution by natural selection in 1859^[2]. A species evolves over time, developing traits that make it more adaptable to the environment. Future generations inherit these traits. During evolution, organisms with traits that are more favorable for existence will leave more offspring, thereby ensuring survival.

Quantum Darwinism (QD) is similar in a sense that the observables of the pointer states that make the maximum number of copies^c of itself are the ones that can be detected classically. Furthermore, the dynamics that cause decoherence is also capable of creating multiple copies of the information about the system^[3][4].

Experimental evidence: a step towards the success of a theory

Since its inception, the theory of Quantum Darwinism has been very well developed by several notable physicists. However, the correctness of a theory can only be verified experimentally. Famous physicist Asher Peres once stated, “Quantum phenomena do not occur in a Hilbert space. They occur in a laboratory.” Fortunately, QD has passed some tests of verifiability, albeit the initial ones.

For experimentally testing QD, scientists create a quantum system with an artificial environment consisting of only a few qubits. This is because studying a large number of particles that belong to the original surrounding poses several complications. The environment interacts with the system and information about the latter is extracted by investigating the former^[5]. An environment consisting of even one entity that strongly interacts with the quantum system can provide enough information about the system.

Working with quantum dots, photons, and nitrogen-vacancy centers

In 2010, researchers from Arizona State University and the Naval Research Laboratory in Washington, D.C. studied the images of “scars” in a quantum dot^[6]. Quantum scars are traces of classical behaviour that are left behind by a chaotic system when scaled down to the quantum level. Scars occur on the wave function of the quantum system, which causes the probability densities to be concentrated along classical orbits. Even though scars are highly unstable, when they evolve according to QD, they form multiple mother-daughter states that ultimately stabilize into pointer states^[7].

Three other experiments were carried out independently by researchers from different parts of the world^[8][9][10] in 2018, one of which included W.H. Zurek^[10]. In two of these experiments^[8][9], the quantum system and the environment consisted of a single photon and a few photons, respectively. Information about the pointer state of the quantum system – in this case, the polarization of the photon – was extracted from the environment. In the third experiment^[10], nitrogen-vacancy defects in diamond were used to create the quantum system and its environment. The interaction of the spins of the nitrogen-vacancy centers with the environment was studied.

All these experiments agree well with the predictions of Quantum Darwinism, even though in a fairly constrained environment. Far superior and sophisticated equipment might be needed to map the environment in its entirety. We can hope that with the efforts of the researchers and the technological advancements, this will be achieved someday. And we will be a step closer to a better understanding of the universe we live in.

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