7-Oct-2022: Harnessing quantum entanglement for futuristic energy storage technology

Experiments with entangled photons, and establishment of pioneering quantum information science that received the Nobel Prize in physics this year, also saw a new theoretical concept by Indian scientists exploring connections between the laws of thermodynamics and Quantum Information Theory (QIT). This new concept could facilitate harnessing quantum entanglement for futuristic energy storage technology.

The scientists have theorised a concept called ‘ergotropy’ that represents the amount of extractable work from a system by keeping its entropy (measure of randomness of a system) constant. The idea if harnessed can open pathways for putting quantum batteries to use in a way that is much efficient than its classical counterpart.

They have proposed thermodynamic quantities that capture a signature in multipartite quantum systems called ‘genuine multipartite entanglement where several particles behave like a single unit even when they are separated. 

According to thermodynamics, thermal equilibrium states are completely passive states as no work can be extracted from such a state even if many copies are available. But the situation becomes more intriguing when the states are entangled. 

Local thermality or local passivity of such states does not always imply that the global state is thermal or passive, and hence useful form of energy can be extracted under global operations. From a composite quantum system ergotropic work, therefore, can be extracted by different means.

One can probe the individual parts locally to get useful energy which can further be stored in a battery for later uses. Probing can also be done on the whole composite system, resulting in extraction of more work. The difference between work extraction from individual parts and work extraction from the composite system is called ergotropic gap. 

Ergotropic gap can be enhanced if the parts of a composite quantum system are prepared in an entangled state. This in turn provides an experimentally efficient method to detect entanglement which has established useful resource for several protocols, such as, quantum teleportation, quantum super dense coding, and secure quantum key distribution whose implications deeply impacted physics and computer science.

Dr. Manik Banik, scientist, S. N. Bose National Centre for Basic Sciences, an autonomous research institute under Department of Science and Technology along with his colleagues Dr. Mir Alimuddin (a Chanakya Post-Doctoral Fellow) and Mr. Samgeeth Puliyil (a BSMS project student from IISER TVM) have turned their attention to genuine multipartite entangled systems which have more drastic manifestations. In their Letter titled “Thermodynamic signatures of genuinely multipartite entanglement” published in Physical Review Letters they have highlighted that genuinely entangled states that are again of different types can be detected with the help of ergotropic gap.

Particularly, they have shown that suitably defined functions of ergotropic gap --minimum ergotropic gap, average ergotropic gap, ergotropic fill, and ergotropic volume -- can serve as good measures of entanglement in multipartite systems. Importantly, their proposed entanglement quantifiers are defined in terms of energy instead of entropy, which in turn makes it possible to measure these quantities in laboratory.

Identification, characterization, and quantification of entanglement are of extreme practical relevance. When the laboratories will be able to harness the ergotropic gap, pathways will open for putting quantum batteries to use, that will be extremely efficient over the classical counterpart, and hence the consequences will be far reaching in terms of mitigating climate change.

13-Apr-2022: Quantum computers may help test fundamental physics providing universal programmable setup for quantum experiments

Going beyond the usually known use of quantum computers --- performing certain tasks at an exponentially faster rate than classical computers, scientists have for the first time used quantum computers for a novel purpose. They have used the new age computers to directly test the very foundations of the theory on which their working is based.

Quantum mechanics like any physical theory is based on experiments. This means that experiments are used to justify some axioms from which the full theory can be logically deduced. While a large section of the scientific community is invested in building devices towards quantum computing applications, a separate community is invested in precision tests of fundamental aspects of quantum theory itself.

A group of scientists from the Raman Research Institute (RRI), an autonomous institute of the Department of Science and Technology in a collaborative research have used quantum computers to perform some precision tests of the fundamental aspects of the quantum theory called Sorkin and Peres tests. The first is a test of the probabilistic aspect of quantum mechanics which helps calculate the chances of events happening while the second is a test of an aspect of the superposition principle, which expresses the fact that quantum objects may behave as waves -- throwing two stones in a pond gives a wave pattern which is the sum of two waves.

The collaborative work started through a discussion between Professor Urbasi Sinha of RRI Bangalore and conference delegate Prof. Lorenzo Macconne from University of Pavia, Italy during the Quantum Frontiers and Fundamentals (QFF 2020) conference hosted by RRI Bangalore in January 2020. Over the next two years Prof. Sinha, with long standing expertise and contributions in the domain of precision tests of quantum mechanics, along with her post doc explored the possibility of performing experiments on quantum computers with Prof. Macconne, an expert on quantum information theory.

The use of a quantum computer to perform tests of crucial quantum principles in the research published as rapid communication letter in the journal Physical Review Research has led to the natural emergence of an entirely new research direction for the physics community that brings together diverse research disciplines under one unifying umbrella.

As quantum computers are scalable quantum systems, this could provide a universal programmable setup for quantum experiments. A quantum circuit, which is like a low-level program for quantum computers, could be a Rosetta stone that allows translation of experiments from one physical system to another.

As a corollary, the scientists have also shown that quantum mechanics is true and the tests can be used as a benchmark to evaluate how well a quantum computer performs. “Our method provides a nice way to create well defined benchmarks for quantum computers so that we know exactly how error prone they are, by using the very foundations of quantum theory as the benchmarking tool,” said Professor Sinha.

30-Dec-2021: Long lived correlations between waves in atomic systems at ultralow temperatures can be exploited for efficient quantum computing

Correlations between waves in atomic systems or spin coherences are long-lived at ultralow temperatures, says a new study by scientists who have developed a new technique to measure it. A system with long-lived spin coherences is a better resource as quantum computer. It allows quantum operations and logic gates to be more efficiently implemented so that the system becomes a better quantum sensor compared to systems where coherence is short-lived.

This newly explored property of atomic systems at low temperature can be exploited for efficient quantum sensing and quantum information processing for application in quantum computation and secure communication. The newly discovered technique can help study the real-time dynamics of quantum phenomena such as quantum phase transitions in a non-invasive manner.

Spin is a fundamental quantum property of atoms and elementary particles such as electrons and protons. As atoms are cooled to lower temperatures, their quantum nature is manifested more prominently. However, while the spin degree of freedom is a highly discussed topic, especially in the context of quantum information processing, the dynamical measurements on spins at ultralow temperatures were not available. This is because most of the detection techniques in cold atom experiments are destructive and disturbs the atomic sample during detection.

A team of Scientists from Raman Research Institute, Bangalore, an autonomous institute of the Department of Science & Technology, Govt. of India, have measured the spin properties of atoms cooled to micro-Kelvin temperatures using the new method they have devised. Quantum properties dominate over every day classical observations at this temperature –- very near absolute zero temperature, and it is for the first time that spin dynamics have been detected at this temperature regime using polarization fluctuation measurements.

With the new technique, the scientists measured the properties of spins and lifetime of an atomic spin state with a million-fold improvement in detection sensitivity compared to the existing technology. They proved that spin coherence at this low temperature is long-lived.

In this work led by Sanjukta Roy, Dibyendu Roy, and Saptarishi Chaudhuri and co-authored by Ph.D. students Maheswar Swar and Subhajit Bhar from RRI increased signal strength of spin noise by a million-fold by using coherent laser drive. They made the spin noise spectroscopy technique usable for spectroscopists measuring systems where signal level is too low to detect. The research has been published in the journal Phys. Rev. Research. The work has been financially supported by funding from DST (Department of Science and Technology) and MeitY (Ministry of Electronics and Information Technology).

According to the RRI team, this work derives its original motivation from Nobel laureate Sir C V Raman’s seminal work on light scattering. They used laser cooling techniques to cool down the neutral atoms near absolute zero temperatures and used laser light to coherently drive quantum transitions in these cold atoms and a polarimetric detection technique to precisely detect the spin dynamics in these atoms. Eventually, they determined the lifetime of the spins in cold atoms and found them to be long-lived – nearly a millisecond, which is at least a thousand times more than the spin lifetime of the atoms at room temperature. The RRI team explained the observations with the help of a theoretical framework based on advanced quantum mechanical concepts.

“In this novel technique, the spin-dynamics of the atoms could be detected at the single-atom level instead of the bulk properties detected using available techniques,” the RRI team said, explaining the importance of the new technique. 

According to the team, this technology can be used to make devices that can precisely detect small magnetic fields, which has important applications in mining and prospecting.  The work also has important applications in biomedical imaging, where time-resolved measurements of small magnetic fields are required.

21-May-2020: Certifying Quantum Entanglement: A step towards Quantum Security

Scientists from S. N. Bose National Centre for Basic Sciences (SNBNCBS), Kolkata, an autonomous institute of the Department of Science and Technology have developed a novel protocol to find out whether a pair of electrons is in an entangled state so that they can be safely used as resources for facilitating quantum information processing tasks. The protocol has been developed through theoretical and experimental analysis.

Quantum entanglement is one of the peculiarities of quantum mechanics, which makes phenomena such as quantum teleportation and super-dense coding possible. It is the physical phenomenon that occurs when a pair or group of particles is generated, interact, in a way such that the quantum state of each particle of the pair or group cannot be described independently of the state of the others. Entangled states are key resources to facilitate many quantum information processing tasks and quantum cryptographic protocols.

However, entanglement is fragile and is easily lost during the transit of photons through the environment. Hence it is extremely important to know whether a pair of photons is entangled, in order to use them as resource. Verification of entanglement requires the use of measurement devices, but such devices may be hacked or compromised by eavesdroppers. Device-independent self-testing (DIST) is a method that can be used in order to overcome such a possibility.

This method enables the verification of entanglement in an unknown quantum state of two photons without having direct access to the state, or complete trust in the measurement devices. The theory relies on the application of the quantum uncertainty principle while implementing full device independence is a difficult task. In several practical situations, one of the parties may be fully trusted, whereas, the other may not be trusted like in the case of server-client relationship in banking transactions. For such situations, quantum information theory enables one-sided DIST (1sDIST).

In the protocol published in Physical Review A, Dr. Archan S. Majumdar from SNBNCBS and his team, the theoretical idea is based on applying the fine-grained uncertainty relation to perform quantum steering. This idea has been successfully implemented experimentally by his team in collaboration with a group in Beijing Computational Science Research Centre, and Key Laboratory of Quantum Information, Hefei. The experiment uses an all-optical set-up in which entangled pairs of photons are created by laser light on Beta barium borate (BBO) crystals, a nonlinear optical crystal, used as laser crystal. The team used Bob as the trusted party and Alice as untrusted, to verify that the pair of photons they share is entangled.

3-Mar-2020: New test with quantum coins & computers for quantum sensing

Researchers from Raman Research Institute (RRI), an autonomous institution under the Department of Science & Technology along with collaborator from the C.R. Rao Advanced Institute of Mathematics, Statistics and Computer Science have devised a new test for fairness of quantum coin or ‘qubit’ (the basic unit of information in a quantum computer) using entanglement theory.

This is a significant contribution to quantum state discrimination, an essential aspect of quantum information science which is expected to influence quantum sensing. The new test uses entanglement to test the fairness of the quantum coin.

Entanglement is a special type of correlation that exists in the quantum world with no classical counterpart. The researchers from RRI made use of this quantum resource to arrive at a test for fairness of a quantum coin (a qubit). Their strategy, which makes use of entanglement, enables better discrimination between quantum states. Such advantage is valuable in quantum sensors.

This work by Supurna Sinha, Joseph Samuel, Anirudh Reddy and Kumar Shivam from RRI along with collaborator Arpita Maitra, from the C.R. Rao Advanced Institute of Mathematics, Statistics and Computer Science  is appearing in Pramana, the Journal Of Physics and a related paper has appeared in the International Journal of Quantum Information.

This work is a significant contribution to the domain of quantum state discrimination, which is an essential aspect of quantum information science. It brings out the crucial role of entanglement in improving our ability to discriminate quantum states. In this work the researchers concretely implemented the theoretical idea on the simulation facility of the IBM quantum computer. They also carried out experiments on the IBM quantum computer which have brought out the shortcomings of the experimental hardware and their work is expected to pave the way towards improving the experimental devices (by reducing gate errors and noise due to decoherence) used in the IBM quantum computer facility.

By repeated trials, one can determine the fairness of a classical coin with a confidence which grows with the number of trials. A quantum coin can be in a superposition of heads and tails. Given a string of qubits representing a series of trials, one can measure them individually and determine the state with a certain confidence. The team has shown that there is an improved strategy which measures the qubits after entangling them, which leads to a greater confidence.

This strategy is demonstrated on the simulation facility of IBM quantum computers. Exploration of the issue of graininess of the quantum measurement process stemming from the inherent limitation of the resolution of a detector naturally led to the question of state estimation and discrimination of quantum states. The team pursued this question and arrived at the interesting result of how entanglement can be used as a valuable resource for state discrimination in the quantum domain.

In their exploration the researchers used a variety of tools: analytical techniques, numerics and computer simulation and experiments on the IBM quantum computing facility. All these tools were used collectively to arrive at an understanding of the role of entanglement in quantum state discrimination.

The domain of Quantum Information and Quantum Computing Technology is a growing area of research which is expected to influence Data Processing, which in turn, plays a central role in our lives in this Information Age. For instance, bank transactions, online shopping and so on crucially depend on the efficiency of information transfer. Thus the recent work on quantum state discrimination is expected to be valuable in people’s lives in the current era.