Manipulating Quantum Systems An Assessment of Atomic, Molecular, and Optical Physics in the United States (2020) / Chapter Skim
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4 Foundations of Quantum Information Science and Technology
4 Foundations of Quantum Information Science and Technology
Pages 104-145
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From page 104... ...
The final step of measurement provides the answer to the calculation. Theoretical research carried out over the past two decades indicates that largescale quantum computers may be capable of solving some otherwise intractable problems with far-reaching applications in, for example, cryptology, chemistry, materials science, and fundamental physical science.
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From page 105... ...
This is a key challenge that theoretical quantum computer science must address in the coming decade. It seems virtually certain that the small-scale quantum computers that are beginning to become available will allow numerous people from a variety of disciplines to start playing with them and coming up with new and unexpected ideas for how best to 1 National Academies of Sciences, Engineering, and Medicine, 2019, Quantum Computing: Progress and Prospects, The National Academies Press, Washington, DC.
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From page 106... ...
Programmable quantum systems of up to about 50 quantum bits have been realized using trapped ions, neutral atoms, and superconducting circuits. Ion trap quantum computers represent the gold standard of qubit coherence and quantum control, and were recently used to realize sophisticated algorithms for quantum chemistry.
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In addition, ideas from quantum informa tion theory and quantum entanglement theory may deepen our understanding of the quantum structure of spacetime. In the next decade, these experimental and theoretical methods will allow researchers to start implementing and testing novel quantum algorithms for di verse scientific applications, and to explore practical methods for quantum error correction and fault tolerance.
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From page 108... ...
Experiments with tens of trapped ions, atoms, superconducting qubits, and other systems have witnessed the presence of the strongest forms of entanglement, while other experiments with many particles (for instance, atomic ensembles with millions of atoms) have also detected some weak forms of entanglement.
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From page 109... ...
It is important to emphasize that recent advances in realizing quantum ma chines allow one to study the fundamental aspects of quantum entanglement in the laboratory. In particular, the experimental systems described in this chapter allow researchers to experimentally probe various aspects of quantum many-body
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Last, atomic systems provide natural quantum interfaces, or transducers, where "stationary qubits" stored in atomic quantum memory are interfaced with, for example, optical or microwave photons as "flying qubits," as required in building "on chip," intra-laboratory, or wide-area quantum networks. Below, recent highlights are presented demonstrating the progress, future promise, and challenges associated with quantum control of large-scale, many-body atomic systems.
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From page 111... ...
Last, AMO methods and techniques are employed for controlling electronic and spin degrees of freedom of atom-like impurities in solid-state systems, with applications ranging from implementation of quantum networks to nanoscale quantum sensing. Trapped Ion Quantum Computing For the past two decades, trapped atomic ions have been among the most ad vanced candidates for the implementation of quantum processors (see Figure 4.1)
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From page 112... ...
By applying external electromagnetic fields from nearby arrays of electrodes inside a vacuum chamber, individual atomic ions can be confined or trapped for indefinite periods of time The most popular atomic ion species used for quantum computing purposes are those with single valence electronic configurations such as Be+, Ca+, and Yb+, whose essential attributes are the required laser wavelengths for cooling and manipulation, the electronic structure of the atom -- primarily its nuclear spin and ancillary electronic energy levels -- and the atomic mass. When laser-cooled, a collection of trapped atomic ion qubits forms a crystal, and established techniques using lasers and microwaves allow the initialization and measurement of quantum states within trapped ion qubits with nearly per fect fidelity.
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From page 113... ...
While it appears possible to scale a trapped ion quantum computer to 30-100 qubits in a single crystal, it may prove difficult to push well beyond this level given technical limitations such as trap potential fluctuations and other slowly varying control parameters. However, there are many opportunities to scale trapped atomic ions using more modular approaches.
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From page 114... ...
The specializa tion of these platforms to a single task endows them with relatively good scaling properties compared to universal quantum computing devices such as quantum computers, not only regarding the number of particles or qubits, but also in the fidelity of quantum operations they can perform. Furthermore, imaging methods such as quantum gas microscopes for optical lattices or the spin readout of trapped ions and atomic arrays provide detailed access to the properties of the quantum states at single-particle resolution.
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From page 115... ...
Varia tional quantum simulation (VQS) can be understood as an optimization procedure in which the quantum system takes care of the classically difficult task of evaluating the cost function to be optimized, by sampling from highly entangled quantum states created by the quantum co-processor.
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From page 116... ...
, whose application in a variational optimization context could open interesting perspectives to address long-standing equilibrium problems in quantum chemistry, condensed-matter, and high-energy physics. Theoretical efforts should be directed at quantifying the computational power of PQS platforms in the context of solving optimization problems, and exploring how PQSs could function as modular building blocks for a future generation of quantum simulators and quantum computers.
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From page 117... ...
Last, recent theoretical work demonstrated that this approach is well suited for realization and testing of quantum algorithms for solving complex combinato rial optimization problems, paving the way toward exploring the first real-world applications of quantum computers.
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. These platforms hold promise for many potential applications, ranging from high-fidelity quantum information processing and quantum simulations to quantum metrology.
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In free space, these "atoms" spontaneously decay very rapidly by emitting microwave photons. However, unlike the case with optical photons, one can completely enclose these "atoms" inside a superconducting box that effectively acts like a nearly perfectly reflecting set of microwave mirrors fully surrounding the "atom." This makes it almost impossible for the "atom" to spontane ously decay (because the microwave photons keep getting reflected back and cannot escape)
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Figure 4.3 illustrates these quantum control and measurement capabilities. This powerful ability to control quantum states opens up a new regime of strong nonlinear quantum optics at the single-photon level.
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Recent experimental advances involving programmable quantum simulators already allow researchers to carry out quantum simulations with system sizes that cannot be handled classically, and to gain unprecedented insights into the physics of such systems. One example involves understanding of non-equilibrium quantum phases.
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Yao, and C Monroe, Observation of a discrete time crystal, Nature 543:217-220, 2017, https://doi.
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(a) 0 FSA Exact −2 log | �Z2 | ψ� |2 −4 −6 −8 −10 −20 −10 0 10 20 E FIGURE 4.4 Nonequilibrium quantum dynamics in 1D arrays of trapped neutral atoms.
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From page 124... ...
In the future, quantum simulators may also enable us to ad dress currently unsolvable problems in particle physics, including the real-time evolution of the hot quark-gluon plasma emerging from a heavy-ion collision or the deep interior of neutron stars. The phenomena in condensed-matter and high-energy physics the commit tee wants to address are described by gauge theories, and the challenge is thus the development of quantum simulators for gauge, and in particular, lattice-gauge theories -- that is, gauge theories discretized on a lattice.
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. Figure 4.3.1 illustrates the basic ingredients of Abelian lattice-gauge theories as quantum spin-ice taken from condensed-matter physics.
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There is a third method of quantum simulation of complex many-body problems: variational quantum simulation. Refer to the section "From Quantum Computers to Programmable Quantum Simulators" earlier in this chapter for a description of this technique.
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In the quantum simulation, electron and positron configurations on the lattice are encoded in spin configurations, and are propagated on the four-qubit ion-trap quantum computer for four Trotter steps (involving a total of 220 quantum gates)
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From page 128... ...
Based on the different variations of the phase estimation algorithm, mappings to Ising Hamiltonians, the variational quan tum eigensolver (VQE) , and other quantum simulation techniques, scientists have been able to obtain results with modest accuracy using up to six qubits on several experimental platforms (photonic quantum computer, NMR, ion trap, and super conducting-based qubits)
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BELL INEQUALITIES, QUANTUM COMMUNICATION, AND QUANTUM NETWORKS From Bell Inequalities to Quantum Communication Quantum theory predicts that reality is not nearly as simple as one might have imagined. In particular, it predicts that physical quantities observable in experi ments do not have values until they are measured.
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From page 130... ...
Renouncement of the corresponding worldview, known as "local realism" and advocated by Einstein, was considered surprising enough to demand experimental tests. In fact, situations in which Bell inequalities are predicted to be violated by quantum systems are so rare that specific experiments had to be designed to perform the tests.
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From page 131... ...
But we also know that RSA can be broken by Shor's algorithm on a quantum computer, or could be broken if an adversary would find a mathematical algorithm for a classical computer allow ing that individual to speed up the decomposition of a large number into its prime factors, or if the adversary had computers more powerful than ours. This may well happen in the future, which means that present encrypted messages, saved blindly today, could be deciphered a few years from now.
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Such memories will be essential in future quantum networks. Nevertheless, a major problem remains to be solved: how to create and main tain entanglement at large distance, say, more than 50 km.
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Recently, there have been significant advances in developing such hybrid quantum systems, based on trapped ions, neutral atoms, color defect centers, quantum dots, rare earth ions, superconducting devices, and so on. The outstanding challenge is to develop a hybrid quantum system that can si multaneously fulfill all these requirements with high fidelity and efficiency.
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From page 134... ...
Loss errors are suppressed by re peating this heralded procedure until the two adjacent stations receive the confirmation of certain successful detection patterns via two-way classical signaling, while simultaneously storing successfully entangled pairs. Alternatively, one may encode the logical qubit into a block of physical qubits that are sent through the lossy channel and use quantum error correction to restore the logical qubit with only one-way signaling.
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From page 135... ...
In the coming decade these approaches will likely result in realization and testing of medium-scale, functional quantum network prototypes that extend the range of quantum communication. QUANTUM INFORMATION SCIENCE FOR SENSING AND METROLOGY Realization of Spin Squeezing Many precision measurements in atomic physics are carried out in the form of a frequency measurement of the phase evolution speed between two atomic states (see also Chapter 6)
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From page 136... ...
Treutlein, Quantum metrology with nonclassical states of atomic ensembles, Reviews of Modern Physics 90:035005, 2018, https://doi.org/10.1103/RevModPhys.90.035005, copyright by the American Physical Society. unentangled coherent spin state (CSS)
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New Applications of Quantum Sensing In recent years, solid-state atom-like quantum systems have attracted intense interest as precision quantum sensors with wide-ranging applications in both the physical and life sciences. Most prominently, nitrogen-vacancy (NV)
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From page 138... ...
The resulting "on-chip" quantum diamond NMR/MRI could enable label-free sensing of biomark ers, single-cell metabolomics for quantitative cell biology and cell-based drug screening, and functional and structural MRI of biological tissues and organisms with subcellular resolution, thus providing a revolutionary new tool to chemical and biological scientists. FIGURE 4.4.1 Quantum diamond NMR spectroscopy of picoliter volume samples.
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From page 139... ...
How do we nearly perfectly control these systems with imperfect controllers? The quantum states that give quantum machines their extraordinary
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From page 140... ...
A third aspect is to connect quantum machines through quantum networks and to invent hybrid systems for transduction between different hardware platforms and signal modalities. For example, cryogenic superconducting microwave quantum circuits offer great advantages for universal control of complex photon states, while fiber optics offers the ability to transmit information over large distances at room temperature.
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From page 141... ...
Most importantly, advances in building quantum machines should allow researchers to implement and test novel quantum algorithms for diverse scientific applications, and to ex plore practical methods for quantum error correction and fault tolerance. They could enable the first realizations and tests of quantum networks with applications to long-distance quantum communication and nonlocal quantum sensing.
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From page 142... ...
The key distinguishing feature of AMO systems is a combination of an excellent degree of coherence and well-developed quantum control techniques that can be directly extended to medium-scale systems involving tens to hundreds of identical qubits without need for complex quantum error correction. In the coming decades, these systems and techniques should al low researchers to explore classically intractable problems in quantum dynamics, explore outstanding questions in physics of high-temperature superconductivity and in spin liquids, allow realization and testing of quantum optimization algo rithms, and investigate hardware-efficient approaches to quantum error correction.
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From page 143... ...
This approach opens up unique opportunities for realizing deep quantum circuits with coherent systems consisting of hundreds of qubits in two or three spatial dimensions, with applications ranging from testing quantum algorithms and exploring efficient approaches for quantum error corrections, to realization of quantum machine learning models and generation of large-scale entangled states for quantum me trology. In particular, programmable quantum simulators based on trapped ions and neutral atoms hold great promise for simulating complex systems ranging from spin liquids to lattice-gauge theories.
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From page 144... ...
In addition, these quantum networks provide a viable route for scaling up quantum processors to large-scale devices, by connecting small-scale quantum computers via quantum channels, thereby allowing quantum computation or simulation on quantum networks. It is one of the key features of atomic quantum hardware that quantum processors, involving local quantum memory and gate operations, representing the nodes of the quantum network, combine naturally with atom-photon quantum interfaces, enabling the needed conversion of "stationary" atomic to "flying" photonic qubits.
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From page 145... ...
Finding: The federal government has decided to pursue a "science first" policy for quantum information science. Recommendation: In support of the National Quantum Initiative, federal funding agencies should broadly support the basic research underlying quantum information science.
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