The Dawn of Quantum Information

Henry and Eve have finally tested their quantum computer (QC) with resounding success! It may enable much faster and better modelling of complex pharmaceutical designs, long-term weather forecasts or brain process simulations than classical computers. A 1,000-qubit QC can process in a single step 21000 possible superposition states: its speedup is exponential in the number of qubits. Yet this wondrous promise requires overcoming the enormous hurdle of decoherence, which is why progress towards a large-scale QC has been painstakingly slow. To their dismay, their QC is “expropriated for the quantum revolution” in order to share quantum information among all mankind and thus impose a collective entangled state of mind. They set out to foil this totalitarian plan and restore individuality by decohering the quantum information channel. The appendix to this chapter provide a flavor of QC capabilities through a quantum algorithm that can solve problems exponentially faster than classical computers.

Can Dephasing be Controlled?

Henry is in peril after bailing out from a burning airplane, because his quantum suit has uncontrollably split him into four quantum versions, only one of which can get hold of the parachute. In order to survive, all his versions must recombine via interference exactly where the parachute is. To this end, the phases of all his versions must be fully in control. Alas, Henry’s ejection has scrambled (randomized) his phases by decoherence (dephasing), which is common in quantum systems. A tip from Eve in mid-air concerning phase reversal proves to be a life saver! This decoherence control, which is indispensable in MRI, is termed sin echo, as the dynamics after the phase reversal echoes the dynamics before this operation. The much further-reaching potential goal of decoherence control may be to influence metabolism and even the process of dying. The appendix to this chapter explains dephasing and its control by the spin-echo method.

Can Quantum Measurements Prevent Change?

In a strange dream, Henry is coherently transported towards his bride down the aisle. But just as a small portion of him arrives next to her, that portion disappears in a flash of light caused by a snapshot! Henry keeps trying to be united with his bride, but repeated snapshots cause Henry’s collapse to being far away from her. This dream illustrates the quantum Zeno effect (QZE): if a measurement collapses the quantum state with high probability to the initial state, then frequent repeated measurements can almost stop the change of the quantum state. Yet less frequent measurements cause the opposite, anti-Zeno effect (AZE), whereby change or decay increases. Thus, decay is controllable. These effects confirm Zeno’s argument that change is an illusion, as it is up to the observer to prevent or induce it by appropriate observation. The appendix to this chapter explains the QZE for coherent and decay processes.

What is Time–Energy Uncertainty?

Henry scores a surprise win over Eve thanks to his quantum rocket that is powered by a quantum-chargeable battery. This gadget is subject to the time–energy uncertainty relation that may result in the battery having more energy than expected. This occurs if an energy measurement within a short time “collapses” the battery randomly to the highest energy state. Intriguingly, time is not a quantum observable. This raises the question that was hotly debated by Bohr and Einstein: how can time be uncertain and affect the energy uncertainty? The more general question is: what is the meaning of time, energy and their uncertainty in physics and in human experience? Attempts to define time have been the subject of philosophical controversy throughout millennia. The appendix to this chapter introduces the Schrödinger equation that governs the dynamics of quantum systems and their time–energy uncertainty.

What is Quantumness?

Henry Bar is about to become the first quantum superhero, having discovered the incredible yet true principle that all things, large and small, are subject to the laws of quantum physics. He finds out that it may be possible, albeit extremely challenging, even for us humans to manifest “quantumness”. This principle underlies Henry’s implementation of his quantum suit that allows him to act as a quantum object. In order to understand this principle, the historical route from early atomism to the emergence of quantum mechanics (QM) as a revolutionary theory of radiation and matter is presented. The inception of QM was a landmark in the age-old quest for answers to the question: is reality, in its complexity, reducible to simple constituents? Alternative questions include: How far up the complexity ladder can QM be pushed as a framework for explaining reality? The appendix to this chapter introduces mathematical notations for QM phenomena.

What is Quantum Teleportation?

Eve (E) entangles herself to Schred (S) before taking off to a conference. There she barely escapes an assault, and calls Henry. Is he too far away to help? Henry (H) resorts to quantum teleportation: a split-second later, H and E, aided by S, have traded places despite being thousands of miles apart! Schematically, the unknown state of H is teleported to E upon measuring the entangled state of E and S and communicating the result to E, who then trades states with H. Since the measured result is communicated at the speed of light, there is no faster-than-light signaling involved. In the distant future, space travel may be partly replaced by large-scale teleportation. Teleportation has intriguing philosophical implications, as it separates the object essence, its quantum information, from its non-essential material substance. The appendix to this chapter presents a quantum teleportation protocol that involves qubits.

What is Quantum Tunneling?

Henry and Eve have been locked behind bars by their captors. Eve recalls that Henry accidentally stepped into the focal area of multiple laser beams and found himself in her office, having gone through the wall! This effect is called “quantum tunneling”. Eve’s reminiscence makes Henry realize that enhanced tunneling through the jail bars is achievable by a periodic force at the tunneling resonance frequency. Tunneling underlies diverse processes: nuclear radioactive decay, transistor action, superconducting junction operation and ultracold gas dynamics. It may be explained as predominantly destructive interference between quantum wavepacket portions inside the barrier, and can be drastically enhanced or suppressed via interplay between environment and control effects. Tunneling exemplifies the shattering of spacetime concepts that may profoundly affect human existence. The appendix to this chapter discusses Schrödinger’s wavefunctions in tunneling.

What are Quantum Measurements?

Chapter 4 introduces a great QM mystery: the notion of quantum measurements. Henry is in a superposition of versions localized in several places, but when Eve measures Henry’s position she (as a classical observer) either sees Henry or she does not. Physical reality is made of such measurements. Eve’s measurement projects or collapses Henry’s superposition state to a single location. The meaning of quantum-state or wavefunction “collapse” and the role of the observer have been at the heart of the historical debate concerning the interpretation of QM. Whereas Von Neumann and Wigner stressed the inseparability of the observed (measured) world from the human mind, alternative “observer-free” views were suggested, such as Everett’s many-world interpretation or Zurek’s quantum Darwinism that replaces the observer by the environment. In the appendix to this chapter the notion of probability amplitudes is elucidated, new notations for operators are introduced and projection operators are presented.

Can Quantum Measurements Control Temperature?

Henry is trapped in a burning building because of Eve’s mischief. Henry’s entanglement with the hot environment is ominous: his states receive excessive energy from the environment, threatening his physiology. Can quantum effects rescue Henry from the fire? Unexpectedly, Eve comes to his rescue. She frequently measures his energy in a “quantum non-demolition” (QND) fashion which is expected to keep his state intact. Surprisingly, Henry grows hotter or colder depending on Eve’s measurement rate! These quantum effects seem to violate the laws of thermodynamics. Engraved in stone as these laws and the ensuing time directionality (time arrow) may be, they fail if the system is examined too frequently, whence time arrow loses its meaning. One may speculate over the role of such anomalies in the Big Bang. These anomalies support Parmenides’ view that time flow is determined by the observer’s choices. The appendix to this chapter elaborates on the dynamics induced by system–environment interaction changes.

What is Quantum Entanglement?

In this adventure, two quantum characters interact, because Henry has constructed a quantum suit for his cat, Schred. Henry and Schred end up in a quantum-entangled state. Remarkably, by measuring one of two entangled systems, the state of the other system is immediately collapsed, even if they are far apart. This bizarre feature of entanglement implies non-locality—synchronization or “collusion” between quantum objects, regardless of their distance. Cosmology provides an explanation: the universe emerged from a unified state describable as a quantum-entangled “hologram”. This notion resonates with the ancient Hindu view that the common essence of all things, the Brahman, can be revealed at every level of the natural hierarchy. The appendix to this chapter discusses operators that create entanglement.