How to test the “quantumness” of a quantum computer?

Abstract

Recent devices, using hundreds of superconducting quantum bits, claim to perform quantum computing. However, it is not an easy task to determine and quantify the degree of quantum coherence and control used by these devices. Namely, it is a difficult task to know with certainty whether or not a given device (e.g., the D-Wave One or D-Wave Two) is a quantum computer. Such a verification of quantum computing would be more accessible if we already had some kind of working quantum computer, to be able to compare the outputs of these various computing devices. Moreover, the verification process itself could strongly depend on whether the tested device is a standard (gate-based) or, e.g., an adiabatic quantum computer. Here we do not propose a technical solution to this quantum-computing "verification problem," but rather outline the problem in a way which would help both specialists and non-experts to see the scale of this difficult task, and indicate some possible paths toward its solution.Keywords: quantum computing, adiabatic quantum computing, quantum coherence, quantum annealing, D-Wave Systems, quantum simulations, quantum speed-up Suppose we are given a black box and told that this is a quantum computer. How do we test if it is capable of performing quantum computing? The standard approach would be to run a series of tests of few-qubit operations, and compare the outputs with results obtained using a standard (classical) computer simulating the quantum evolution. This is possible for small-size systems, likely comprising less than 40-50 qubits, given the current capabilities and fundamental limitations of classical computers (due to the exponential growth of computing resources needed to model large quantum systems [11]. But what if the system size is larger? Another challenging task would be to find out how to change the design of the "black-box computer" to improve its performance. Namely, if the black box computer is not working as a quantum computer, how to find this out and how to fix the problem?Now we are facing a similar situation. The development of an "adiabatic quantum annealer" by D-Wave Systems Inc. over the last few years did produce reactions ranging from excitement to skepticism [2][3][4][5][6][7][8][9]. Considerable interest was generated by:(1) the demonstration of quantum annealing in an 8-qubit register of the prototype processor [10]; (2) the sale of a 128-qubit quantum annealer "D-Wave One"to Lockheed Martin and its installation at the University of Southern California (2011), and its upgrade to a 512-qubit "D-Wave Two" (2013); (3) the subsequent evidence of quantum annealing in the working 108 qubits of this device [11,12]; (4) the realization of a quantum adiabatic algorithm on a nominally 512-qubit (439 qubits operational) device "D-Wave Two" outperforming at least some classical algorithms [13] and losing to some other classical algorithms Questions were raised concerning the relative speed of the D-Wave processors compared to classical optimization algorithms [7] and the quantum char...

Can We See or Compute our Way out of Plato's Cave?

“…The effort to produce a universal quantum computer over the last 15 years has not yet succeeded in creating one, due to serious technological obstacles. However, some kind of a quantum computing device, containing hundreds of quantum bits (qubits) was realized [5], even though the question of the degree of its "quantumness" remains open [6]. This is just one example of the field's enormous progress, especially in the area of superconducting qubits.…”

Propagation of fluctuations in the quantum Ising model

“…Introduction: The quantum Ising model has recently attracted additional attention as a standard generic model of quantum computers used to evaluate the behaviour of prototype devices [1][2][3]. In particular, its study would considerably expand our understanding of both fundamental and practical limitations of adiabatic quantum computers and quantum annealers [4], where the device is initiated in the strong transverse field, and then the spin-spin (qubit-qubit) coupling is gradually switched on. Entanglement between large number of spins on the intermediate stages of switching plays the key role in the system reaching its final ground state.…”

Effects of lasing in a one-dimensional quantum metamaterial

“…[1]. The achieved levels of control and global quantum coherence of these systems still fall short of the requirements of universal quantum computing, and better theoretical methods of their simulation and assessment are required [2]. Nevertheless, they are already sufficient or nearly sufficient for the realization of less demanding quantum devices, such as quantum metamaterials [3][4][5][6][7][8][9].…”