Experimental test of non-macrorealistic cat states in the cloud

Abstract

The Leggett–Garg inequality attempts to classify experimental outcomes as arising from one of two possible classes of physical theories: those described by macrorealism (which obey our intuition about how the macroscopic classical world behaves) and those that are not (e.g., quantum theory). The development of cloud-based quantum computing devices enables us to explore the limits of macrorealism. In particular, here we take advantage of the properties of the programmable nature of the IBM quantum experience to observe the violation of the Leggett–Garg inequality (in the form of a ‘quantum witness’) as a function of the number of constituent systems (qubits), while simultaneously maximizing the ‘disconnectivity’, a potential measure of macroscopicity, between constituents. Our results show that two- and four-qubit ‘cat states’ (which have large disconnectivity) are seen to violate the inequality, and hence can be classified as non-macrorealistic. In contrast, a six-qubit cat state does not violate the ‘quantum witness’ beyond a so-called clumsy invasive-measurement bound, and thus is compatible with ‘clumsy macrorealism’. As a comparison, we also consider un-entangled product states with n = 2, 3, 4 and 6 qubits, in which the disconnectivity is low.

Estimating Coherence Measures with Untrusted Devices

“…[ 98,99 ] Several types of multipartite quantumness such as entanglement and quantum discord between the systems might lead to the violation of classical constraints in macrorealism consequence, or detection of “coherence”. [ 24,25,100,101 ] It will be interesting to investigate the interplay between these quantum correlations and coherence and their detection via (semi) DI scenarios. We hope our result may shed light on the close relation between coherence and randomness generation and bring more possibilities in quantum cryptography.…”

Experimenting quantum phenomena on NISQ computers using high level quantum programming

“…Our data are quite off compared to theoretical and simulation results. This is not a surprising result, as the IBM's NISQ computers are known for noises and mostly limit to shallow circuits [9,10,[16][17][18][19][20][21]. Finding the origin for these errors is outside the scope of this work, as it requires in-depth information from the chip design, fabrication process, microwave setup, cryostat, and their inter-connections.…”

“…Designed as a universal computing platform and programmed using high-level language through cloud access, there is a range of remarkable works performed on real devices with few qubits. They include the demonstration of important quantum algorithms [9,10], simulation of quantum phenomena [11][12][13][14][15][16], or reproducing foundation quantum experiences [17][18][19][20][21].…”

Quantum simulation of parity–time symmetry breaking with a superconducting quantum processor

“…We achieve this by making use of the emergent technology of superconducting processors, on which significant technical progress has been shown in recent times by IBM [24]. Although still imperfect, these devices have already enabled important results such as quantum error correction [25], fault-tolerant gates [26], proofs of violation of Mermin [27] and Leggett-Garg [28] inequalities, demonstrations of non-local parity measurements [29,30], simulations of paradigmatic models in open quantum systems [31], the creation of highly entangled graph states [32], the determination of molecular ground-state energies [33], the implementation of quantum witnesses [34], and quantumenhanced solutions to large systems of linear equations [35]. The use of a superconducting quantum processor offers the possibility of extracting all relevant quantum correlations and of designing and programming efficiently the required gates, adapted to the particular topology of the machine.…”