Quantum leap

Print edition : November 16, 2012

KIMBERLY WHITE/AFP

The Physics Nobel has been awarded for experiments that have tried to resolve the paradoxes of quantum theory and help unravel the fuzzy boundary between the quantum and classical worlds.

EVENTS in the submicroscopic world of atoms, electrons, photons and other subatomic particles, which are described by quantum mechanics, are counter-intuitive and often contrary to our experiences with the physical phenomena in the macroscopic, classical world of planets, chairs, balls, people, cats, and so on. Physics in the quantum world has some inherent uncertainty or randomness to it. The theory only assigns a probability that a quantum system can exist in a particular state. This uncertainty essentially arises from the dual nature of quantum objects: particles behave like waves, oscillating and interfering, and conversely, light waves bounce about like particles called photons.

At any given instant, the state of a quantum object is described by a wave function which incorporates the entire range of all its possible energies, positions, momenta and its internal states with a well-defined probability for every combination of these attributes consistent with the laws of physics. It is in a superposition of several states simultaneously. Quantum particles with a well-defined superposition of distinctly different states are said to be in a coherent state. That is, a coherent quantum state exists in all those possibilities at once. For example, an atom may simultaneously be in two locations or in two states with different energies. This may sound weird but that is how the quantum world is. However, an experimental observation with a macroscopic measuring device to determine its state invariably gives only one result, and quantam theory precisely states the probability of getting that result.

So why do these strange facets of the quantum world not manifest themselves in everyday life, say a ball being at two places at the same time? When does the quantum description end and the classical description begin? Erwin Schrdinger, the famous Austrian physicist and one of the pioneers of quantum theory who devised the idea of a wave function in 1926, spent much of his later life trying to understand and interpret the implications of the strange quantum behaviour. As late as 1952 he wrote: We never experiment with just one electron or atom or (small) molecule. In thought experiments, we sometimes assume that we do; this invariably entails ridiculous consequences.

It was to illustrate such absurd consequences of moving between the quantum world and the macro world that Schrdinger devised in 1935 the thought experiment with a cat, now famously known as Schrdingers cat. The cat is inside a box and is completely isolated from the outside world. The box also contains a bottle of deadly poison, which is released when a radioactive source, also inside the box, decays. Radioactive decay is governed by laws of quantum mechanics, according to which the unobserved radioactive substance is in a superposition of decayed state and not-yet-decayed state. Therefore, the cat must also be in a superposed state of being both alive and dead. Now, if you open the box to see if the cat is alive or dead, quantum theory says that you are making a measurement and the quantum cat state will collapse to one of the two possibilities.

Then which is the exact act of measurement? Opening the box? Or when light reaches the eye and is processed by the brain? The current theoretical understanding of the collapse of a quantum system is that any interaction with the environment destroys the coherence of the quantum state. As any attempt to measure the quantum state constitutes an interaction with the environment, the perturbation removes the isolation in which the coherent quantum state existed.

In recent times, there has been a spate of Schrdingers cat experiments that try to clarify some of these fundamental questions about quantum theory. These attempts are an indication that the transition from the quantum to classical domain (described as the collapse of the wave function into one state) is no longer confined to thought experiments a la Schrdinger. They have moved into the realm of real laboratory studies. The underlying principle in all these experiments, called quantum non-demolition (QND) experiments, has been to preserve the coherence by minimising the external perturbation and studying the quantum state.

Scientists who carried out some of the path-breaking experiments that enable direct observation of individual quantum particles without destroying them are this years Physics Nobel laureates. Serge Haroche of the College de France and Ecole Normale Superieur in Paris and David J. Wineland of the National Institute of Standards and Technology, Maryland, United States, both 68 years old, have been awarded the Nobel Prize for, as the citation says, ground-breaking experimental methods that enable measuring and manipulation of individual quantum systems. The two have independently invented and developed methods that preserve the quantum mechanical nature of the quantum particles observed in ways that were previously thought unattainable. Their respective research groups have managed to devise clever experiments that allow them to measure and control very fragile quantum states. Their methods have enabled them to examine and count the particles, and they have been able to show in great detail how the act of measuring actually causes the quantum state to collapse and lose its coherence.

Haroche and Wineland trapped quantum particles and put them in cat-like superposition states. Their methods are in a sense complementary but have a lot of features in common. Wineland trapped ions or charged atoms (atoms stripped off an electron from the outermost orbit) and controlled and measured them with particles of light (photons). Haroche did the opposite. He used trapped photons and controlled and measured them by sending atoms through a trap. The quantum systems they work with are not large macroscopic objects but are still fairly large by quantum standards.

In Winelands approach, ions are kept inside a trap created by surrounding them by an appropriate configuration of electric fields. The experiment is performed in a vacuum at extremely low temperatures so that the quantum system is isolated from the heat and radiation in the environment. Ingenious use of laser and laser pulses to control the quantum system is behind the success of Winelands experiment. A laser is used to suppress the ions thermal motion in the trap so that it settles down in its lowest energy state (Figure 1). A carefully tuned laser pulse is then used to put the ion in a superposition state. For example, the ion can be prepared to occupy different energy states simultaneously. How is that done? Quantum systems have discrete energy states. The laser pulse is tuned in such a way that it only gives the ion a gentle kick from its lowest energy state so that it moves only midway from the higher energy level and is left in between the two levels, in a superposition of two energy states, with an equal probability of ending up in either of them.

HarocheS Method

Haroche and his associates use a different method, which they proposed way back in 1990 to measure the number of photons in a cavity. Measuring the number of photons in a coherent state is not easy because Heisenbergs Uncertainty Principle tells us that coherence would be lost if one wished to count the exact number of photons. After nearly two decades of painstaking efforts, it was only in 2007 that they could realise the same.

As Haroche has explained in an article titled Life and Death of a Photon, the experiment crucially required two conditions to be met, which were not fulfilled when they made the proposal. One, they had to trap light in a box for a tenth of a second on an average in order to be able to perform repeated observations. They also had to develop a new kind of atomic detector to record the imprint of a single photon without absorbing its energy. We have been able to observe hundreds of times the same photon trapped in a box. After a perceptible delay, which can reach half a second, the light particle finally disappears, in a sudden process occurring at a random time. We have thus witnessed in this way the story of single photons and recorded the times of their birth and death, Haroche wrote. The photon box that they used is made of two metallic mirrors facing each other at a distance of 2.7 centimetres (Figure 2). The photons (which are microwave photons with a wavelength of about 6 millimetres) bounce more than a billion times between the mirrors before escaping through scattering because of the mirrors imperfections or absorption by the metal. Such a high reflectivity, thousands of times higher than the best optical mirrors, which enables a single photon to bounce back and forth inside the box for nearly a tenth of a second before it is lost or absorbed, is obtained by coating the mirrors with a layer of superconducting metal and cooling them down to 0.8 Kelvin. The mirror substrate is polished with a roughness of a few nanometres only. The appearance of a photon in the box is due to the radiation of its walls, whose atoms have, even at these very low temperatures, residual thermal excitation which causes emission of a photon every now and then in accordance with Plancks radiation law. This record means that the photon would have travelled 40,000 km, equivalent to about one trip around the earth. The long lifetime of photons per se is not surprising because we receive light from the far reaches of the universe travelling through empty space for billions of years. Storing a trapped photon for a long time in a cavity is extremely difficult because it will interact with the walls of the box and will be absorbed in no time. What prevented this is the ingenious design of the experiment, with superconducting mirrors as cavity walls.

During the trapped photons long lifetime, Haroche and his colleagues performed many quantum manipulations. Haroche used specially prepared atoms, called Rydberg atoms, of rubidium to both control and measure the microwave photon in the cavity. A Rydberg atom has a radius of about 125 nanometres, which is roughly 1,000 times larger than the radius of a typical atom. This is achieved by exciting one of the atomic electrons with a laser to move it into an orbit of large size. These gigantic doughnut-shaped atoms are sent into the cavity one by one at a carefully chosen speed so that the interaction with the photon occurs in a controlled manner. If one allowed the Rydberg atoms to absorb the photon and undergo a resonant transition from an initial state to a higher excited state, the final state energy would tell one that a photon was detected, but the resonant absorption would also destroy the photon.

For it to be a QND measurement, the trapped photon must have a more subtle but measurable effect on the atom. By adjusting the mirrors separation, the photon frequency is slightly detuned away from the atoms resonant frequency. There is no absorption then, and the Rydberg atom leaves the microwave photon intact. But the interaction between the photon and the atom causes a change in the phase of the atoms quantum state. Phase refers to the position (measured in degrees) of a given point on an oscillating wave relative to a reference point. If we think of an atom, a quantum system, as a wave, phase shift means the shifting of positions of its peaks and troughs. This phase shift can be measured when the atom exits the cavity, thereby revealing the presence or absence of a photon in the box. When there is no photon there is no phase shift. So the experiment measures a single photon without destroying it.

The same year, taking this method forward, Haroches group counted the number of photons inside the cavity, the experiment that was originally proposed nearly two decades ago. In this, photons of a coherent microwave field are trapped in the superconducting mirror cavity. This means that, before the measurement, the system is in a superposition of several states corresponding to different photon numbers. As in the earlier experiment, Rydberg atoms sent into the cavity interact with the photons without destroying them. Electrons in these atoms oscillate between two excited states, with the rate of oscillation depending on the photon number.

Each measurement gives a different oscillation rate and hence a different photon number as it should be because the photons are still in a superposition state. But after many measurements, the distribution of measurements settles at a particular number, indicating that from a superposition state the photon system has collapsed into a well-defined state with a definite number of photons. This experiment thus traced step-by-step the evolution of an individual quantum state, in real time. If this experiment were to be repeated, the measured number of photons would be different, as the probabilistic nature of quantum theory predicts.

Fascinating perspectives

This new QND way of observing a quantum system opens up fascinating perspectives. For example, information carried by a photon can be shared by a large number of atoms interacting one at a time with the photon field. It is also possible, as Haroche has pointed out, to realise the cavity in a superposition of two states, one in which it is empty and the other in which it contains a photon. This strange quantum superposition at the macroscopic level would be analogous to the ambiguous state of Schrdingers cat. It is not that these experiments have fully resolved the paradoxes of quantum theory but they have helped us understand better the nature of the fuzzy boundary between the quantum and classical worlds.

Will such understanding of the quantum behaviour lead to any applications? I dont know, remarked Haroche in his post-Prize announcement interview. What will be the use to detect single photons without destroying them, I dont know. I hope there will be some applications. However, Winelands manipulation of trapped ions is already finding application in extremely precise clocks and time standards. Another possible application that is envisaged is in the quantum computer.

Quantum computers could exploit the unusual behaviour of quantum systems and would be able to perform highly intractable computational tasks much faster than conventional computers. The best example of such a task is the factoring of a large number into its two prime factors. This is an extraordinarily difficult task for conventional computational methods and it forms the basis for all forms of data encryption in use today. The basis for the potential power of a quantum computer is the principle of superposition in a quantum system. In classical computers, the smallest unit of information, or a bit, can take on the value of either 0 or 1. In a quantum computer, however, the basic unit of information is a quantum bit or a qubit, which can simultaneously assume values of 0 and 1. Similarly, two qubits can simultaneously assume for values 00, 01, 10, 11. Thus, for N qubits, there would be 2N possible states, and a quantum computer of only 300 qubits can assume 2,300 values simultaneously, which is greater than the number of particles in the universe. But, as Wineland said, A useful quantum computer is a long way [away][but] will eventually happen.

His group was the first to demonstrate in 1995 a quantum operation with two qubits, which were two trapped ions. Since then, many proof-of-principle experiments in quantum computing have been performed. Quantum systems with eight qubits have been produced, and scientists have shown that these rudimentary computers can run simple algorithms. Wineland and others believe that this should be scaleable to much larger number of qubits with trapped ions though it would be technically very challenging.

The first challenge in making a universal quantum computer that performs all possible computations is to create reliable memory. If a qubit is put in a superposition of 0 and 1, with the ions magnetic orientation correspondingly pointing up and down at the same time, it must remain in that state until the data are processed. Ions held in electromagnetic traps can act as good qubit memory registers, with coherence times exceeding 10 minutes. It is the extremely weak interaction between the ion trap and its environment that results in such long superposition lifetimes. The second essential requirement for quantum computing is the manipulation of a single qubit. For qubits based on magnetic orientations of trapped ions, oscillating magnetic fields could be thought of to flip a qubit from 01 to 1 and vice versa or to put it in a superposition state. But this is a difficult task because, given the small distances between trapped ions, typically a few micrometres, it would be difficult to localise the magnetic field on an individual ion. This problem can be solved by using laser beams that are focussed on a particular qubit, says Wineland. Scientists have devised appropriate logic gates, analogous to the AND/OR gates in conventional processors, between the two qubits necessary to build a quantum computer. It is quite likely that an operational quantum computer will be built this century. It will change our lives much more radically than the classical computer did in the last century.

Wineland and his team have also used ion traps to develop a clock that is 100 times more precise than the caesium-based atomic clocks that are the current standards for time measurement. Caesium clocks function in the microwave range, whereas ion-trap-based clocks use visible light and are, therefore, called optical clocks. An optical clock will consist of only one or two ions in a trap. With two ions, one is used as a lock and the other to read the clock without destroying its state or causing it to skip a tick. The precision of an optical clock is better than one in 1,017, which means that it would be off only by five seconds during the lifetime of the universe of 14 billion years. Using precise time measurements, some subtle effects involving time have been measured, such as changes in the flow of time with motion and gravity as required by Einsteins Theory of Relativity. For example, when we use the global positioning system such corrections are routinely made because gravity is somewhat weaker several hundred kilometres up in the sky. An optical clock can measure such differences in the passage of time even when the speed change is less than 10 m/s or with gravity change over a height difference of only 30 m.

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