Efficiently preparable states of light enable quantum computing by measurement

Giuseppe Castagnoli, Dalida Monti, Alexander V. Sergienko

The reason for the higher efficiency exhibited by some quantum algorithms over their classical counterparts is examined by considering the interplay between the reversible actions required to prepare the computer registers in an entangled state before measurement (the "initial actions"), and the final measurement action -- whereas measurement is interpreted in a new way, particularly suited to a problem solving context. This unification shows that the computation process, comprising the measurement outcome, is significantly influenced by both the initial actions and the need to satisfy the constraints set by the final measurement action. Reviewing the existing quantum algorithms in the light of this dual influence, yields new valuable insight in the nature of the quantum computation speed up.

Matthew Pysher

A. K. Chakraborty

Ting Zhang, Jinzhao Sun, Xiao-Xu Fang +3 more

A crucial subroutine for various quantum computing and communication algorithms is to efficiently extract different classical properties of quantum states. In a notable recent theoretical work by Huang, Kueng, and Preskill [Nat. Phys. 16, 1050 (2020)NPAHAX1745-247310.1038/s41567-020-0932-7], a thrifty scheme showed how to project the quantum state into classical shadows and simultaneously predict M different functions of a state with only O(log_{2}M) measurements, independent of the system size and saturating the information-theoretical limit. Here, we experimentally explore the feasibility of the scheme in the realistic scenario with a finite number of measurements and noisy operations. We prepare a four-qubit GHZ state and show how to estimate expectation values of multiple observables and Hamiltonians. We compare the measurement strategies with uniform, biased, and derandomized classical shadows to conventional ones that sequentially measure each state function exploiting either importance sampling or observable grouping. We next demonstrate the estimation of nonlinear functions using classical shadows and analyze the entanglement of the prepared quantum state. Our experiment verifies the efficacy of exploiting (derandomized) classical shadows and sheds light on efficient quantum computing with noisy intermediate-scale quantum hardware.

Ulf Leonhardt, Henryk Paul

Asuka Inoue, Takahiro Kashiwazaki, Taichi Yamashima +7 more

Measurement-based optical quantum computers (MBOQCs) using continuous-variables (CVs) of light are a candidate for large-scale fault-tolerant quantum computing [1]. This form of processing has a high carrier frequency of light, so it can also perform ultra-fast quantum computing. To achieve ultra-fast MBOQCs, broadband squeezed light generation and ultra-fast realtime amplitude measurement are required. So far, over-4.5-dB squeezed light generation, which is required for preparing a two-dimensional cluster state, has been demonstrated in THz bandwidth by using noise intensity measurement with a narrow bandwidth [2], [3]. However, the realtime amplitude measurement frequency is limited to several GHz. Up to now, the highest frequency is still limited to 9 GHz with 0.1-dB squeezing even though specific detectors have been designed [4]. This is because there is a tradeoff relationship between the bandwidth and the quantum efficiency of homodyne detectors. As a result, we cannot use a fast balanced homodyne detector such as a detector for high-speed optical communications with a large optical loss, which collapses the quantum states of light. To achieve ultra-fast realtime amplitude measurement, we need a new measurement method to overcome the tradeoff limitations.