Why could a quantum computer theoretically do calculations in a few minutes that even the most powerful super computers would be unable to do in billions of years? Until 2007, it seemed that the only possible answer to this question would be to attribute the performance of a machine driven by qubits, the quantum bits, to the quantum entanglement, the strange and mysterious phenomenon that could exponentially increase data processing capacity. Particles, atoms or molecules described as being entangled are usually so strongly interconnected – physicists use the word correlated – that they are able to exchange information regardless of whether they are next to each other or millions of kilometers apart. Although entangling is powerful, it is also fragile and is only maintained in special situations, mainly in tightly controlled systems that do not interact with the external environment.
A new concept to assess correlations not covered by the laws of classical physics – namely, quantum discord – has gained momentum in the last few years, showing that it might be possible to build quantum devices based on components without any traces of entanglement. There is more. Atoms and particles with a certain level of discord are able to preserve their quantum properties at room temperature, in macroscopic systems, and in situations where there is ‘noise’, understood here as the influence of external medium on the system.
Deriving from a concept that resembles that of the theory of information, discord is a statistical measure used to establish whether there is something of a quantum nature in a physical system such as a group of electrons or molecules. Scientists have conducted a number of measures to discover the existence of typical quantum properties, such as the so-called wave-particle duality, able to establish a communication channel among some of the system’s components. This link might be the entanglement itself, the strongest known form of quantum connection (albeit difficult to be maintained) or other types of weaker quantum correlations. The exact nature of these weaker correlations is still unknown to researchers, but there is evidence that they can be longer-lasting than the entanglement and are powerful enough to transmit information.
Tiago Cirilo“Prior to the discord concept, many researchers thought that systems without entanglements could not be quantum systems,” says Roberto Serra, from the Federal University of ABC (UFABC). Serra is one of the Brazilian physicists who have dedicated themselves to this matter at the National Institute of Quantum Information, Science and Technology (INCT-IQ), a joint initiative of the National Council for Scientific and Technological Development (CNPq) and FAPESP. “We are showing that systems with some kind of discord (and no entanglement) can be robust and become the basis of applications in metrology and computer sciences.”
Therefore, quantum discord encompasses each and any correlation that is in discord (hence the name) with Newton’s Laws of Physics, a phenomenon visible in our daily lives. The quantity of discord in a system is described through a mathematical equation. “If the measure of quantum discord is different from zero, then the system has some kind of quantum characteristic,” explains physicist Felipe Foschini, from the Federal University of Ouro Preto (Ufop), who has published papers on the new concept.
Physicists from all over the world – with special mention to some recent studies conducted by Brazilian researchers – have been working on the discord concept and have found quantum features in systems previously viewed as being strictly classic , i.e., systems that seemed to be governed by Newtonian physics. A team of INCT-IQ scientists published two articles, one after the other, in the Physical Review Letters (PRL) journal in the second half of last year. Both articles described results of experiments that explore this new concept.
Tiago CirilloIn an article published on August 12, Serra and collaborators from the São Carlos Physics Institute of the University of São Paulo (IFSC-USP), the Brazilian Center for Research on Physics (CBPF), in Rio de Janeiro, and Embrapa, described how for the first time they directly measured discord in a quantum system at approximately 26 degrees Celsius, created by using nuclear magnetic resonance. This is an embryo of what might one day be a liquid-state quantum computer.
At the CBPF lab, the researchers coded two qubits in chloroform molecules, a colorless, dense, sweet compound nowadays used as a solvent and as raw material for the production of polymer precursors, such as Teflon. Strictly speaking, the researchers coded a quantum bit in the spin of the nucleus of a hydrogen atom and another in a carbon atom, by applying a magnetic field of 12 tesla – which is millions of times more powerful than the Earth’s magnetic field – to the system. Spin is a basic property of elementary particles, such as electrons and photons, and of the nuclei of atoms, and is usually represented by an arrow pointing upwards or downwards. “We use magnetic field pulses to manipulate the spin of the nucleus,” says physicist Diogo de Oliveira Soares Pinto, a member of the group led by professors Tito Bonagamba and Eduardo Azevedo from the University of São Paulo at São Carlos, who participated in the experiment. “It was impossible for entanglement to exist in the conditions in which we did this work.”
On September 30, the group published a second article in PRL. The article described another interesting result, again derived from observations of the two qubit system created in the chloroform molecules. The researchers measured sudden changes in the behavior of the discord that had been caused by contact with the environment. They noticed how the quantum effects of the system disappeared gradually, due to fluctuations and noises in the thermal environment. After some time, the interactions could de-structure the two qubits, causing a progressive loss of the system’s coherence.
Tiago CirilloIn this experiment, the physicists noticed that the discord seemed to be fairly resistant to environments that disturb the system. In the approximately five milliliters of chloroform used in the experiment, only one in every million molecules of the compound carried the qubits codes in the atoms. Although “diluted” in an almost entirely classical system, the quantum nature of the chloroform sample is preserved and could be useful for the development of applications. “Any communication process must keep control of the correlation forms of a system,” says the CBPF’s Ivan Oliveira, one of the co-authors of the two aforementioned studies. “We need to separate the classic and the quantum portion of the information.”
More recently, on February 10 of this year, the Brazilians published a third article on discord in PRL. This time, the group’s work involved an optical system, for which they created a simple and direct means of verifying the existence of discord in photons, which are light particles. The researchers coded two qubits by using horizontal or vertical polarization, one of the properties of photons. A system was developed to register, using a single measure, the existence of quantum correlations in the system, a strategy that is referred to as witnessing quantum discord. Normally, it is necessary to slice the system into various parts, in a manner that resembles a CAT scan, and conduct at least four measurements to discover if there is any quantum connection among the photons. “Now, with only one measure, we are able to detect the presence of discord,” explains Stephen Walborn, from the Federal University of Rio de Janeiro (UFRJ), who coauthored the study.
A concept that has been ignored for years
The concept of quantum discord was initially proposed in 2001 by two groups of physicists that had developed the concept independently. One group headed by Wojciech H. Zurek, of the Los Alamos National Laboratory in the United States. The second was headed by Vlatko Vedral, from England’s Oxford University. At first, the concept had no major impact on the scientific community. The concept was rather abstract for a field of study whose main point of interest had historically revolved around entanglement, the mysterious phenomenon that Albert Einstein described as a “long-distance phantom-like action.”
Tiago CirilloThe first experimental studies were conducted in 2007. They showed that systems at room temperature containing discord (and no entanglement) were able to transmit information by means of quantum bits, so many physicists re-read the papers that had been submitted by Zurek and Vedral six years before. This aroused a boom of interest in the topic. “Quantum discord has shed a new light on issues that have been debated for years,” says physicist Amir Caldeira, from the State University of Campinas (Unicamp), coordinator of the INCT-IQ and author of papers on discord.
According to Vlatko Vedral, not every system with quantum discord can be manipulated to generate applications in computer sciences or in other fields. “We must be careful when choosing the systems we are going to work on. This issue is still being addressed,” says the physicist from Oxford University. “To understand the difference between classical physics and quantum physics, to understand why a cat cannot be in two places at the same time, but atoms can, we have to be able to differentiate the states with discord and those without.” Currently, physicists only know that certain discord systems (with no entanglement), such as the chloroform molecules or photons, can process these sought-after quantum bits.
The qubit is the quantum analog of the classical bit, defined as the smallest unit in which information can be coded, stored, and transmitted in existing computers and telecommunication systems, optic fibers or wireless networks. However, there are significant differences between the two concepts. At a certain moment the classical bit, also referred to as binary digit, can only be found in one of two possible values or states: for example, 0 or 1. In computers nowadays, the 0 is represented by the interruption of the voltage in a circuit (off state) and 1 is represented by the release of the current (on state). A qubit is more than that. The qubit can simultaneously represent the values equivalent to 0 and 1. It can exist in overlapping states, a weird quantum property that provides parallel potential to calculations. “The qubits exponentially increase computing capacity,” says Roberto Serra. “To put it simply, we could say that two qubits are equivalent to 4 bits, 3 qubits to 8 bits, 4 qubits to 16 bits and so on.”
The overlapping of states is a typical ability of quantum systems (whether they are comprised of atoms, electrons, photons, or molecules) to behave concomitantly as a particle and a wave. This is the so-called particle-wave duality. The situation becomes less surreal when we imagine a pebble being thrown into a lake. The pebble causes tiny ripples on the surface of the water in the form of concentric circles that can, at the same time, go past two neighboring bridges on the shore of the lake. In this case, if one bridge represents the number 0 and the other bridge, the number 1, part of the wave is 0 and part is 1. The wave is 0 and 1 concurrently.
However, a quantum computer that would provide two answers to a problem would not be very valuable, because only one answer would be correct. A second quantum phenomenon comes into play at this point: wave interference. Going back to the lake example, after crossing the two bridges, wave 1 and wave 0 meet again. This interaction can be destructive, the waves can cancel each other out and the final result is 0. This interaction can also be constructive, when the waves combine and the final result is 1. The so-called quantum algorithms are mathematical instructions; they are like programs that increase the probability that the overlapping of states and the interaction of waves will lead to the right answer at the end of the data processing. Is this strange? Yes. Welcome to the quantum world.
The US$ 10 million black box
The size of a room, this machine with 128 entangled qubits calls itself the first commercial quantum computer
The date May 25, 2011 will certainly be remembered when the history of quantum computing is told. On this day, the Canadian company D-Wave Systems announced – to some skepticism voiced by the academic community – the sale of the first self-named quantum computer produced for commercial purposes. Instead of resorting to the silicon chips currently used in personal computers, the D-Wave One, the name of the machine, does calculations by exploring the quantum properties of a processor with 128 qubits. This processor contains a group of superconducting current rings at 30 milikelvin, a temperature that is close to absolute zero. The first unit of the computer was allegedly sold for US$ 10 million to U.S. aeronautics company Lockheed Martin. The company installed the equipment at the end of last year at the quantum computing center at the University of Southern California (USC) at the Marina Del Rey campus.
The D-Wave One is an enormous black box, literally. The processor, which measures a few centimeters, is protected from outside interference because it is sheltered in a closed compartment, which is twice as tall as an adult man and takes up 10 square meters. This container, which resembles an irregular cube, has refrigeration systems and a protection system to shield it from the influence of external magnetic fields. These systems are responsible for ensuring the best conditions for the processing of the qubits.
“The quantum chip is comprised of 128 equal superconducting rings, each one measuring 100 micrometers (one hundredth of a millimeter),” says theoretical physicist Frederico Brito, from the Federal University of Pernambuco (UFPE), who worked for D-Wave from May 2008 to July 2009. When the current turns counterclockwise in the rings, it represents an upward spin (or the 0 of classical computer science). When the current turns clockwise, it represents a downward spin (or 1). The device in each ring is referred to as the Josephson junction. It generates a quantum effect, such as tunneling and interference of waves, which increase the machine’s theoretical problem-solving capacity.
In the opinion of some physicists, the D-Wave One is also a black box in the metaphorical sense of the word. Few people know how the machine works or whether there is anything quantum-like about it. To clarify doubts and wear down the resistance of the academic community, the Canadian company published an article on May 12th last year, less than two weeks prior to the announcement that it had sold its first computer, in Nature, the prestigious British journal. In the article, the company’s scientists provide details on the technique used to generate the 128 qubits. The machine explores the so-called adiabatic quantum computing process.
In simple words, this type of computing makes a system run at its lowest possible energy level, the so-called ground state, at a temperature generally close to absolute zero. The next step is to promote change in the system at such a slow pace that these changes are able to maintain the device’s quantum properties without making it run at the next level of energy. In the case of the D-Wave computer, the changes consist in making the current change direction, from clockwise to counterclockwise, or vice versa.
Approximately 85% of the machine’s qubits are already operational, according to Daniel Lidar, director of the University of Southern California’s quantum computation center. “We still don’t know how powerful the processor is,” says Lidar. “We intend to study it very closely.” The D-Wave One was developed to provide the best solutions for certain types of problems, such as image recognition and protein folding.
1. National Institute of Science and Quantum Information Technology (nº 2008/57856-6); Modality Thematic Project; Coordinator Amir Caldeira – Unicamp; Investment R$ 1,384,811.24 (FAPESP) and R$ 5,700,000.00 (CNPq)
2. Quantum information and decoherence (nº 2005/04471-1); Modality
Young Investigators Program; Coordinator Roberto Serra – UFABC; Investment R$ 68,321.95 (FAPESP)
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AUCCAISE, R. et al. Environment-induced sudden transition in quantum discord dynamics. Physical Review Letters. v. 107, n. 14, p. 140403-1/ 140403-5. 30 Sept. 2011.
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