Quantum Refrigerator Keeps Qubits Cool

Despite the potential of quantum computing for drastically accelerating suitable real-time applications, response time analysis is still required to guarantee that quantum programs running on quantum computers satisfy application-specific timing requirements. This paper describes an inaugural project to determine whether a quantum program running on a quantum computer satisfies the timing constraints of a realtime application, that is, is quantum computing punctual and reliable in this time-sensitive application domain? Can this timing guarantee be formally verified? We leverage the existing work on the functional reactive programming (FRP) model to predict the worst-case response time (WCRT) of fault-tolerant classical computing systems since the timing analysis of re-executions for fault recovery plus transient-faults-induced wasted execution times is similar to determining the response time of FRP tasks. Ongoing work shows that accounting for wasted execution times due to errors in quantum computers resulting from quantum decoherence and state fidelity can be treated similarly and develops a mapping from quantum programs to FRP programs for efficient timing analysis.

Local Friendliness (LF) inequalities follow from seemingly reasonable assumptions about reality: (i) ``absoluteness of observed events'' (e.g., every observed event happens for all observers) and (ii) ``local agency'' (e.g., free choices can be made uncorrelated with other events outside their future light cone). Extended Wigner's Friend Scenario (EWFS) thought experiments show that textbook quantum mechanics violates these inequalities. Thus, experimental evidence of these violations would make these two assumptions incompatible. In [Nature Physics 16, 1199 (2020)], the authors experimentally implemented an EWFS, using a photonic qubit to play the role of each of the ``friends'' and measured violations of LF. One may question whether a photonic qubit is a physical system that counts as an ``observer'' and thereby question whether the experiment's outcome is significant. Intending to measure increasingly meaningful violations, we propose using a statistical measure called the ``branch factor'' to quantify the ``observerness'' of the system. We then encode the EWFS as a quantum circuit such that the components of the circuit that define the friend are quantum systems of increasing branch factor. We run this circuit on quantum simulators and hardware devices, observing LF violations as the system sizes scale. As errors in quantum computers reduce the significance of the violations, better quantum computers can produce better violations. Our results extend the state of the art in proof-of-concept experimental violations from branch factor 0.0 to branch factor 16.0. This is an initial result in an experimental program for measuring LF violations at increasingly meaningful branch factors using increasingly more powerful quantum processors and networks. We introduce this program as a fundamental science application for near-term and developing quantum technology.

Entanglement and decoherence are two intimately related phenomena that lie at the heart of quantum information. The EPR-Bell states are the most basic quantum systems, which form the complete entangled basis of the two-particle Hilbert space, and which are important in many quantum computation schemes. The bit flip, phase flip, and bit-phase flip noise channels represent all the possible errors in quantum computation. Thus, the quantum decoherence characteristic of maximally and partially Bell entangled states under different noisy channels is studied using the concurrence. Our work is useful for linear-optical quantum computation and quantum communication schemes whose building blocks are entangled state.

A generalization of the results of Rasetti and Zanardi concerning avoiding\nerrors in quantum computers by using states preserved by evolution is\npresented. The concept of dynamical symmetry is generalized from the level of\nclassical Lie algebras and groups to the level of dynamical symmetry based on\nquantum Lie algebras and quantum groups (in the sense of Woronowicz). A natural\nconnection is proved between states preserved by representations of a quantum\ngroup and states preserved by evolution with dynamical symmetry of the\nappropriate universal enveloping algebra. Illustrative examples are discussed.\n

Quantum computers are susceptible to errors. While quantum computers can be guarded against errors using error correction codes, near-term quantum computers will not have sufficient number of qubits to implement error correction and must perform their computation in the presence of errors. Qubit measurement is typically the most error-prone operation on a quantum computer, with measurement errors ranging from 8% to 30% reported on current machines. This goal of this paper is to mitigate measurement errors by exploiting the state-dependent bias of measurement errors. Experiments on the IBM-Q5 and IBM-Q14 machines show variation in measurement errors depending on the state being measured. For example, measuring an all-zero state on IBM-Q5 has a fidelity of 84%; however, the fidelity drops to 62% while measuring the all-one state. To improve measurement fidelity, we propose Invert-and-Measure, which transforms the system from a vulnerable state to a stronger state and then performs the measurement in the stronger state. We propose two designs for Invert-and-Measure. First, Static Invert-and-Measure (SIM), which executes two instances of the program, one with standard measurements and the other with inverted measurements and combines the results. Second, Adaptive Invert and Measure (AIM), which learns the relative bias of different states using runtime profiling and produces specialized inversions to increase the likelihood of obtaining the correct answer. Our evaluations, using IBM-Q5 and IBM-Q14, show that SIM improves the application reliability by up to 2X, and AIM by up to 3X.

The importance of reliability and fault tolerance is paramount in quantum computation. This paper proposes a Fault Tolerance Algorithms and Methodologies (FTAM) assessment technique for quantum circuits, by adopting the saboteur-based Simulated Fault Injection methodology from classical computation. By drawing the inspiration from classical computation, the HDLs were employed for performing fault injection, due to their capacity of behavioral and structural circuit description, including hierarchical features. The cornerstone of this approach is the adaptation of the available quantum computation error models (with the quantum computing features and constraints) to the classical, HDL framework of the simulated fault injection techniques. The experimental simulated fault injection campaign results are consistent with the analytical assessments -from a qualitative point of view - but at the same time they provide a much realistic description.

Fingerprinting of quantum computer devices is a new threat that poses a challenge to shared, cloud-based quantum computers. Fingerprinting can allow adversaries to map quantum computer infrastructures, uniquely identify cloud-based devices which otherwise have no public identifiers, and it can assist other adversarial attacks. This work shows idle tomography-based fingerprinting method based on crosstalk-induced errors in NISQ quantum computers. The device- and locality-specific fingerprinting results show prediction accuracy values of 99.1% and 95.3%, respectively.

Quantum machine learning recently gained prominence due to the computational ability of quantum computers in solving machine learning problems that are intractable on a classical computer. However, achieving a quantum advantage on present-day quantum computers remains an open challenge. In this work, we primarily focus on solving machine learning problems using a hybrid model based on both quantum and classical computers together for the classification task. We propose the quantum multi-class classifier (QMCC) as a variational circuit with a hybrid classical-quantum approach using quantum mechanical properties such as superposition and entanglement. A unitary operation on a single qubit for the state preparation is designed and also demonstrated using a real quantum computer on the IBMQX platform. The entire variational circuit for the classification task is implemented on a quantum simulator. We performed our quantum simulations on three benchmark datasets: Iris dataset, Banknote Authentication (BNA) dataset, and Wireless Indoor Localization (WIL) dataset for machine learning algorithms. Our simulation results show that the proposed QMCC model classified Iris dataset with an accuracy of 92.10%, BNA dataset with an accuracy of 89.50%, and WIL dataset with an accuracy of 91.73%. The proposed model can also be extended to multiple class classifiers.

Compiling high-level quantum programs to machines that are size constrained (i.e. limited number of quantum bits) and time constrained (i.e. limited number of quantum operations) is challenging. In this paper, we present SQUARE (Strategic QUantum Ancilla REuse), a compilation infrastructure that tackles allocation and reclamation of scratch qubits (called ancilla) in modular quantum programs. At its core, SQUARE strategically performs uncomputation to create opportunities for qubit reuse. Current Noisy Intermediate-Scale Quantum (NISQ) computers and forward-looking Fault-Tolerant (FT) quantum computers have fundamentally different constraints such as data locality, instruction parallelism, and communication overhead. Our heuristic-based ancilla-reuse algorithm balances these considerations and fits computations into resource-constrained NISQ or FT quantum machines, throttling parallelism when necessary. To precisely capture the workload of a program, we propose an improved metric, the "active quantum volume," and use this metric to evaluate the effectiveness of our algorithm. Furthermore, our results show that SQUARE improves the average success rate of NISQ applications by 1.47X. Surprisingly, the additional gates for uncomputation create ancilla with better locality, and result in substantially fewer swap gates and less gate noise overall. SQUARE also achieves an average reduction of 1.5X (and up to 9.6X) in active quantum volume for FT machines.

Clustering bike sharing stations using Quantum Machine Learning: A case study of Toronto, Canada

Finding ground states and low-lying excitations of a given Hamiltonian is one of the most important problems in many fields of physics. As a novel approach, quantum computing on Noisy Intermediate-Scale Quantum (NISQ) devices offers the prospect to efficiently perform such computations and may eventually outperform classical computers. However, current quantum devices still suffer from inherent quantum noise. In this work, we propose an error mitigation scheme suitable for parametric quantum circuits. This scheme is based on projecting a general quantum noise channel onto depolarization errors. Our method can efficiently reduce errors in quantum computations, which we demonstrate by carrying out simulations both on classical and IBM's quantum devices. In particular, we test the performance of the method by computing the mass gap of the transverse-field Ising model using the variational quantum eigensolver algorithm.

Surface code is an error-correcting method that can be applied to the implementation of a usable quantum computer. At present, a promising candidate for a usable quantum computer is based on superconductor-specifically transmon. Because errors in transmon-based quantum computers appear biasedly as Z type errors, tailored surface and XZZX codes have been developed to deal with the type errors. Even though these surface codes have been suggested for lattice structures, since transmons-based quantum computers, developed by IBM, have a heavy-hexagon structure, it is natural to ask how tailored surface code and XZZX code can be implemented on the heavy-hexagon structure. In this study, we provide a method for implementing tailored surface code and XZZX code on a heavy-hexagon structure. Even when there is no bias, we obtain $$ 0.231 \%$$ as the threshold of the tailored surface code, which is much better than $$ 0.21 \%$$ and $$ 0.209 \%$$ as the thresholds of the surface code and XZZX code, respectively. Furthermore, we can see that even though a decoder, which is not the best of the syndromes, is used, the thresholds of the tailored surface code and XZZX code increase as the bias of the Z error increases. Finally, we show that in the case of infinite bias, the threshold of the surface code is $$ 0.264\%$$ , but the thresholds of the tailored surface code and XZZX code are $$ 0.296 \% $$ and $$ 0.328 \%$$ respectively.

Quantum computing and machine learning provide an entirely new paradigm for advanced solutions in cybersecurity. The supercomputational power that quantum computing has will immediately be capable of solving tough cryptographic problems that classical computers find very hard to solve, thus providing extra-strength encryption and decryption techniques. In parallel, machine learning algorithms detect changing cyber threats; they identify complex patterns in very large datasets and provide real-time predictions of future vulnerabilities. This chapter will describe the synergies between quantum computing and ML, which might potentially lead to the development of mighty cybersecurity frames able to thwart attacks by sophisticated breaches such as those based on quantum principles. It takes up discussions regarding how QML models will change the entire mechanism of threat detection and response in a world like never before of unprecedented speed and accuracy.

As quantum computers are becoming real, they have the inherent potential to significantly impact many application domains. In this paper we outline the fundamentals about programming quantum computers and show that quantum programs are typically hybrid consisting of a mixture of classical parts and quantum parts. With the advent of quantum computers in the cloud, the cloud is a fine environment for performing quantum programs. The tool chain available for creating and running such programs is sketched. As an exemplary problem we discuss efforts to implement quantum programs that are hardware independent. A use case from quantum humanities is discussed, hinting which applications in this domain can already be used in the field of (quantum) machine learning. Finally, a collaborative platform for solving problems with quantum computers – that is currently under construction – is presented.

Holonomic quantum computation exploits a quantum state's non-trivial,\nmatrix-valued geometric phase (holonomy) to perform fault-tolerant computation.\nHolonomies arising from systems where the Hamiltonian traces a continuous path\nthrough parameter space have been well-researched. Discrete holonomies, on the\nother hand, where the state jumps from point to point in state space, have had\nlittle prior investigation. Using a sequence of incomplete projective\nmeasurements of the spin operator, we build an explicit approach to universal\nquantum computation. We show that quantum error correction codes integrate\nnaturally in our scheme, providing a model for measurement-based quantum\ncomputation that combines the passive error resilience of holonomic quantum\ncomputation and active error correction techniques. In the limit of dense\nmeasurements we recover known continuous-path holonomies.\n