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Electronics is the field of electron movement in the circuits governed by the use of wires, silicon and electricity. All modern electronic devices contain transistors as the fundamental building blocks. Until recently, electronics had been based on a single property of electrons—their charge. But now physicists have begun to exploit another property—electron spin.

So-called spintronics promises to revolutionise electronics because it allows information to be encoded in an entirely new way. Though electronics is bestowed with a large number of inherited advantages, but in the era of quantum electronics it is facing new challenges. Because electrons lose any possible initial quantum state as they bounce around through the energy-dissipating semiconductor or metallic systems, they are ill-equipped for quantum computing.

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Atomtronics
We all have heard of electronics, mechatronics and maybe even spintronics, but the latest buzzword doing the rounds of physics labs is atomtronics—the science of creating circuits, devices and materials using ultra-cold atoms instead of electrons. What if atoms could be used to perform the functions that are currently the province of electronic devices? The goal of atomtronics is to do just that by creating analogues to the common items found in electronic and spintronic devices.

Physicists Satyendra Nath Bose and Albert Einstein proposed in 1924 that large numbers of atoms could be chilled to the point that they joined together in a single quantum state, bringing subatomic effects to a scale accessible by laboratory experiments. But it wasn’t until 1995 that scientists made a Bose-Einstein condensate (ultra-cold gas) using lasers to carefully cool rubidium-87 atoms down to temperatures less than a millionth of a degree above absolute zero. The 2001 Nobel Prize in Physics celebrated this accomplishment, which was also achieved using sodium atoms.

Atomtronics is a young and mostly theoretical field based on the idea that atoms in unusual quantum states of matter may provide an alternative to the tried-and-true electron for making useful devices. The field’sproponents have drawn up blueprints for atomic versions of many traditional electronic components—from wires and batteries to transistors and diodes. The idea is to manipulate neutral atoms using lasers in a way that mimics the behaviour of electrons in wires, transistors and logic gates.

In atomtronics, the current carriers in electronics (electrons) are replaced with neutral, ultra-cold atoms; the semiconductor material that the electrons traverse is replaced with an optical lattice; and the electric potential difference, which induces the flow of electrons around the circuit, is replaced by a chemical potential difference.

Over the last decade or two, physicists have become masters at creating optical lattices in which atoms can be pushed, pulled and prodded at will. This optical property of atoms has not attracted much attention of workers but now people have begun a program to put tame atoms to work. The problem is that atoms don’t behave like electrons. So, building the atomtronic equivalent of something even as straightforward as a simple circuit consisting of a battery and resistor in series requires some thinking out of the box.

The dynamics of atoms in optical lattices, which are basically crystals of light, has been studied theoretically and experimentally for many years now. It is just a further addition to this field by theoretically demonstrating that the electronic properties of the diode and transistor can be observed in specifically tailored optical lattices.

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Researchers believe that it is possible to emulate the behaviour of a semiconductor diode in these atomic systems. For example, simulations show that this augmented optical lattice will allow atoms to flow acros it from left to right, but forbids the atoms to traverse the lattice going the other way.

Ultra-cold atoms have interesting properties that conventional materials lack—superfuidity, superconductivity and coherence, to name just three. Being cold, they are also well-behaved enough to be manipulated by lasers. When several are held in a line or an array, these can link in a way that is governed by the laws of quantum mechanics and then the fun really starts.

These can be used to measure time on unimaginably short time-scales, can carry out simple calculations and may even form the basis of future quantum computers. Almost all of the atomtronics pioneers hope that for certain applications atoms will prove to be more interesting than electrons.

Fig. 1: Atoms spun by laser beams
Fig. 1: Atoms spun by laser beams

The motivation to construct and study atomtronic analogues of electronic systems comes from several directions:
1. The experimental atomtronic realisations promise to be extremely clean. Imperfections such as lattice defects or phonons can be completely eliminated. This allows one to study an idealised system from which all inessential complications have been stripped.

Consequently, one may obtain an improved understanding of the essential requirements that make certain electronic devices work. It is possible that a deeper understanding may provide feedback to the design of conventional electronic systems leading to future improvements. This lies parallel to the recent interest in single-electron transistors in mesoscopic systems and molecules, where many themes common with atomtronics emerge.

A consequence of the near-ideal experimental conditions for optical lattice systems is that theoretical descriptions for atomtronic systems can be developed from firstprinciples. This allows theorists to develop detailed models that can reliably predict the properties of devices.

2. Atomtronic systems are richer than their electronic counterparts because atoms possess more internal degrees of freedom than electrons. Atoms can be either bosons or fermions, and the interactions between these can be widely varied from short to long range and from strong to weak. This can lead to behavior that is qualitatively different to that of electronics.

Consequently, one can study repulsive, attractive or even non-interacting atoms in the same experimental setup. Additionally, current experimental techniques allow the detection of atoms with fast, state-resolved and near-unit quantum efficiency. Thus it is possible in principle, to follow the dynamics of an atomtronic system in real time.

3. Neutral atoms in optical lattices can be well isolated from the environment, reducing de-coherence. These combine a powerful means of state readout and preparation, with methods for entangling atoms. Such systems have all the necessary ingredients to be the building blocks of quantum signal processors. The close analogies with electronic devices can serve as a guide in the search for new quantum information architectures, including novel types of quantum logic gates that are closely tied with the conventional architecture in electronic computers.

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4. Recent experiments studying transport properties of ultra-cold atoms in optical lattices can be discussed in the context of the atomtronics framework. In particular, one can model the short-time transport properties of an optical lattice with the open quantum system formalism discussed here.

Latest developments
The atoms placed in an optical lattice, when super-cooled to form Bose-Einstein condensates, may form states analogous to electrons in solid-state crystalline media such as semiconductors. Impurity doping allows the creation of n- and p-type semiconductor analogue states, and an atomtronic battery can be created by maintaining two contacts at different chemical potentials. Analogues to diodes and transistors have also been theoretically demonstrated.

Although atomtronic devices have yet to be realised experimentally, the properties of condensed atoms offer a wide range of possible applications. The use of ultra-cold atoms allows for circuit elements, which further allow for the coherent flow of information and may be useful in connecting classical electronic devices and quantum computers.

The use of atomtronics may allow for quantum computers that work on macroscopic scales and do not require the technological precision of laser-controlled few-ion computing methods. Since the atoms are Bose condensed, they have the property of superfluidity and, therefore, have resistance-less current in which no energy is lost or heat is dissipated, similar to superconducting electronic devices. The vast knowledge of electronics may be leveraged to easily adapt to ultra-cold atomic atomtronic circuits.

Physicists have developed a new type of circuit that is little more than a puff of gas dancing in laser beams. By choreographing the atoms of the ultra-cold gas to flowas a current that can be controlled and switched on and off, the scientists have taken a step toward building the world’s first‘atomtronic’ device.

The research team used Bose-Einstein condensate to make atomtronic sensors. The team reports creating this gas by cooling sodium atoms suspended in magnetic felds. Researchers then trapped the atoms in a pair of crossed laser beams and further chilled the atoms to less than 10-billionths of a degree above absolute zero. The two beams also shaped the condensate that formed at these low temperatures into a flattened dough-nut with a radius of about 20 micrometre.

A second pair of lasers transferred energy to the dough-nut to start its rotation. Because atoms in the condensate behave as a single, coherent quantum particle, such a ring of the substance does not speed up or slow down gradually. It jumps between different speeds, much like a blender would, if it could change settings instantaneously.

The scientists chose the lowest setting for their ring; about one revolution every second. Because the condensate also happens to be frictionless, this ring should, in theory, rotate forever. Limited by technical diffiulties, the research team kept it going for about 40 seconds—the lifetime of their condensate.

Scientists believed that Bose-Einstein condensate could provide an extremely sensitive rotation sensor. They added a ‘weak link’ to their condensate ring—a barrier created by a blue laser that could speed up or shut down the flow. Theoretically, if the condensate were kept still and the barrier was attached to a rotating sensor, the barrier would cause a sudden jump in current at certain rotation speeds.

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Atomtronic devices
The atoms in the condensate flow as a current, which can be switched on and off like a normal circuit. Atomtronics uses atoms in strange quantum states to power devices or computer memory. This is different from spintronics, which stores information based on the spin of individual electrons, allowing each one to store two bits of data instead of one.

Computer scientists and particle physicists have made several advances in these fields in the past several months. Using atoms instead of electrons to process information could change the way we think about computing. In quantum computing we store a quantum state on an object, perform operations on the object and then read out the final state. If the system is not coherent, the initial stored information is lost.

Fig. 2: Atomtronic analogy to diode circuit
Fig. 2: Atomtronic analogy to diode circuit

Atoms trapped in optical lattices have been considered extensively for specific quantum computing schemes due to their inherent energy conserving characteristics. Therefore the dynamics of atomtronic devices would be coherent and potentially useful in quantum computing. It is also suggested that there is the possibility that atomtronics could be useful in obtaining sensitive measurements. It is thus concluded that atomtronic systems provide a nice test of fundamental concepts in condensed matter physics. While these ideas have been modeled, they are yet to be built.

Atomtronic diode
Atomtronic diode is a device that allows an atomic flux to flow across it in essentially only one direction. The atomtronic analogy of a diode is formed from the joining of p- and n-type semiconductor materials. Electrons are replaced by ultra-cold atoms, the battery is replaced by high and low chemical potential reservoirs, and the metallic crystal lattices (the microscopic medium that the electrons traverse) are replaced by an optical lattice. The atomtronic diode is achieved by energetically shifting one-half of the optical lattice with respect to the other.

The wires and atomtronic components are composed of optical lattices, and current refers to the number of atoms that pass a specifi point in a given amount of time. The desired function of an atomtronic transistor is to enable a weak atomtronic current to be amplified, or to switch on or off a much larger one. The team has also modeled an atomtronic transistor. The atomtronic version of transitor exhibits on/off switching behavior and acts as an amplifier.

By confiuring the optical lattice in a manner researchers show that it is possible to recover the characteristics of the conventional electronic transistor in the atomic world.

Limitations of atomtronics
Scientists are hoping to use the condensate in the way that superconductors have been used to make improved devices and sensors. Idea for a useful device was inspired by superconducting quantum interference devices, commonly known as SQUIDs. Scientists also believe that Bose-Einstein condensate could provide an extremely sensitive rotation sensor.

It is pointed out, however, that atomtronics probably won’t replace electronics as atoms are sluggish compared to electrons. This means it might be difficult to replace fast electronic devices with sluggish atomtronic devices.


The author is a professor in the department of physics at S.L.I.E.T., Longowal, Punjab

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