Macroscopic superpositions require tremendous measurement devices

We consider fundamental limits on the detectable size of macroscopic quantum superpositions. We argue that a full quantum mechanical treatment of system plus measurement device is required, and that a (classical) reference frame for phase or direction needs to be established to certify the quantum state. When taking the size of such a classical reference frame into account, we show that to reliably distinguish a quantum superposition state from an incoherent mixture requires a measurement device that is quadratically bigger than the superposition state. Whereas for moderate system sizes such as generated in previous experiments this is not a stringent restriction, for macroscopic superpositions of the size of a cat the required effort quickly becomes intractable, requiring measurement devices of the size of the Earth. We illustrate our results using macroscopic superposition states of photons, spins, and position. Finally, we also show how this limitation can be circumvented by dealing with superpositions in relative degrees of freedom.

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Spacetime Quantum Reference Frames and superpositions of proper times

Quantum ◽

10.22331/q-2021-07-22-508 ◽

2021 ◽

Vol 5 ◽

pp. 508

Author(s):

Flaminia Giacomini

Keyword(s):

Hilbert Space ◽

Reference Frame ◽

Degrees Of Freedom ◽

Reference Frames ◽

Proper Time ◽

Relativistic Quantum ◽

Dynamical Evolution ◽

Quantum Superposition ◽

Local Frame ◽

Physical Systems

In general relativity, the description of spacetime relies on idealised rods and clocks, which identify a reference frame. In any concrete scenario, reference frames are associated to physical systems, which are ultimately quantum in nature. A relativistic description of the laws of physics hence needs to take into account such quantum reference frames (QRFs), through which spacetime can be given an operational meaning. Here, we introduce the notion of a spacetime quantum reference frame, associated to a quantum particle in spacetime. Such formulation has the advantage of treating space and time on equal footing, and of allowing us to describe the dynamical evolution of a set of quantum systems from the perspective of another quantum system, where the parameter in which the rest of the physical systems evolves coincides with the proper time of the particle taken as the QRF. Crucially, the proper times in two different QRFs are not related by a standard transformation, but they might be in a quantum superposition one with respect to the other.Concretely, we consider a system of N relativistic quantum particles in a weak gravitational field, and introduce a timeless formulation in which the global state of the N particles appears "frozen", but the dynamical evolution is recovered in terms of relational quantities. The position and momentum Hilbert space of the particles is used to fix the QRF via a transformation to the local frame of the particle such that the metric is locally inertial at the origin of the QRF. The internal Hilbert space corresponds to the clock space, which keeps the proper time in the local frame of the particle. Thanks to this fully relational construction we show how the remaining particles evolve dynamically in the relational variables from the perspective of the QRF. The construction proposed here includes the Page-Wootters mechanism for non interacting clocks when the external degrees of freedom are neglected. Finally, we find that a quantum superposition of gravitational redshifts and a quantum superposition of special-relativistic time dilations can be observed in the QRF.

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All-silicon metasurfaces for polarization multiplexed generation of terahertz photonic orbital angular momentum superposition states

Journal of Materials Chemistry C ◽

10.1039/d1tc00594d ◽

2021 ◽

Vol 9 (16) ◽

pp. 5478-5485

Author(s):

Jie Li ◽

Guocui Wang ◽

Chenglong Zheng ◽

Jitao Li ◽

Yue Yang ◽

...

Keyword(s):

Angular Momentum ◽

Orbital Angular Momentum ◽

Degrees Of Freedom ◽

Engineering Application ◽

Superposition State ◽

Superposition States

The superposition state of photonic orbital angular momentum (OAM) has more degrees of freedom than pure photonic orbital angular momentum, with rich physical implications and engineering application possibilities.

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324 Naturalistic Characterization of Sleep in Chronic Insomnia Using a Non-Contact Sleep Measurement Device

SLEEP ◽

10.1093/sleep/zsab072.323 ◽

2021 ◽

Vol 44 (Supplement_2) ◽

pp. A130-A130

Author(s):

Devon Hansen ◽

Mary Peterson ◽

Roy Raymann ◽

Hans Van Dongen ◽

Nathaniel Watson

Keyword(s):

Sleep Onset ◽

Contact Device ◽

Chronic Insomnia ◽

Poor Sleep ◽

Measurement Device ◽

Home Setting ◽

Time In Bed ◽

Sleep Measurement ◽

Measurement Devices

Abstract Introduction Individuals with insomnia report poor sleep quality and non-restorative sleep, and often exhibit irregular sleep patterns over days and weeks. First night effects and logistical challenges make it difficult to measure these sleep characteristics in the laboratory. Also, sensitivity to sleep disruption from obtrusive measurement devices confounds sleep measurements in people with insomnia in their naturalistic setting. Non-contact sleep measurement devices have the potential to address these issues and enable ecologically valid, longitudinal characterization of sleep in individuals with insomnia. Here we use a non-contact device – the SleepScore Max (SleepScore Labs) – to assess the sleep of individuals with chronic insomnia, compared to healthy sleeper controls, in their home setting. Methods As part of a larger study, 13 individuals with chronic insomnia (ages 25-60y, 7 males) and 8 healthy sleeper controls (ages 21-46y, 6 females) participated in an at-home sleep monitoring study. Enrollment criteria included an age range of 18-65y and, for the insomnia group, ICSD-3 criteria for chronic insomnia with no other clinically relevant illness. Participants used the non-contact sleep measurement device to record their sleep periods each night for 8 weeks. Sleep measurements were analyzed for group differences in both means (characterizing sleep overall) and within-subject standard deviations (characterizing sleep variability across nights), using mixed-effects regression controlling for systematic between-subject differences. Results Based on the non-contact sleep measurements, individuals with chronic insomnia exhibited greater variability in bedtime, time in bed, total sleep time, sleep latency, total wake time across time in bed, wakefulness after sleep onset, sleep interruptions, and estimated light sleep, compared to healthy sleeper controls (all F>5.7, P<0.05). No significant differences were found for group averages and for variability in estimated deep and REM sleep. Conclusion In this group of individuals with chronic insomnia, a non-contact device used to characterize sleep naturalistically captured enhanced variability across nights in multiple aspects of sleep stereotypical of sleep disturbances in chronic insomnia, differentiating the sample statistically significantly from healthy sleeper controls. Support (if any) NIH grant KL2TR002317; research devices provided by SleepScore Labs.

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Coherent electron displacement for quantum information processing using attosecond single cycle pulses

Scientific Reports ◽

10.1038/s41598-020-79004-8 ◽

2020 ◽

Vol 10 (1) ◽

Author(s):

Hicham Agueny

Keyword(s):

Quantum Information ◽

Quantum Information Processing ◽

Coherent Motion ◽

Superposition State ◽

Phase Information ◽

Quantum Superposition ◽

Single Cycle ◽

Precise Control ◽

Advanced Control ◽

A Chain

AbstractCoherent electron displacement is a conventional strategy for processing quantum information, as it enables to interconnect distinct sites in a network of atoms. The efficiency of the processing relies on the precise control of the mechanism, which has yet to be established. Here, we theoretically demonstrate a new route to drive the electron displacement on a timescale faster than that of the dynamical distortion of the electron wavepacket by utilizing attosecond single-cycle pulses. The characteristic feature of these pulses relies on a vast momentum transfer to an electron, leading to its displacement following a unidirectional path. The scenario is illustrated by revealing the spatiotemporal nature of the displaced wavepacket encoding a quantum superposition state. We map out the associated phase information and retrieve it over long distances from the origin. Moreover, we show that a sequence of such pulses applied to a chain of ions enables attosecond control of the directionality of the coherent motion of the electron wavepacket back and forth between the neighbouring sites. An extension to a two-electron spin state demonstrates the versatility of the use of these pulses. Our findings establish a promising route for advanced control of quantum states using attosecond single-cycle pulses, which pave the way towards ultrafast processing of quantum information as well as imaging.

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Preparation of macroscopic quantum superposition states of a cavity field via coupling to a superconducting charge qubit

Physical Review A ◽

10.1103/physreva.71.063820 ◽

2005 ◽

Vol 71 (6) ◽

Cited By ~ 69

Author(s):

Yu-xi Liu ◽

L. F. Wei ◽

Franco Nori

Keyword(s):

Cavity Field ◽

Quantum Superposition ◽

Charge Qubit ◽

Macroscopic Quantum ◽

Superposition States

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Macroscopic Quantum Superposition States in a Model of Photon-Supersonic Phonon Interaction

Chinese Physics Letters ◽

10.1088/0256-307x/17/7/014 ◽

2000 ◽

Vol 17 (7) ◽

pp. 504-506

Author(s):

Chai Jin-Hua ◽

Wang Yan-Bang ◽

Lu Yi-Qun

Keyword(s):

Quantum Superposition ◽

Phonon Interaction ◽

Macroscopic Quantum ◽

Superposition States

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Practical Reference-Frame-Independent Measurement-Device-Independent Quantum Key Distribution Systems Against the Worst Relative Rotation of Reference Frames

Communications in Theoretical Physics ◽

10.1088/0253-6102/70/4/379 ◽

2018 ◽

Vol 70 (4) ◽

pp. 379 ◽

Cited By ~ 1

Author(s):

Chun-Mei Zhang ◽

Jian-Rong Zhu ◽

Qin Wang

Keyword(s):

Reference Frame ◽

Quantum Key Distribution ◽

Distribution Systems ◽

Reference Frames ◽

Key Distribution ◽

Relative Rotation ◽

Independent Measurement ◽

Measurement Device ◽

Measurement Device Independent

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Machine Learning-Assisted Measurement Device-Independent Quantum Key Distribution on Reference Frame Calibration

Entropy ◽

10.3390/e23101242 ◽

2021 ◽

Vol 23 (10) ◽

pp. 1242

Author(s):

Sihao Zhang ◽

Jingyang Liu ◽

Guigen Zeng ◽

Chunhui Zhang ◽

Xingyu Zhou ◽

...

Keyword(s):

Machine Learning ◽

Real Time ◽

Reference Frame ◽

Quantum Key Distribution ◽

Short Term Memory ◽

Key Distribution ◽

Transmission Efficiency ◽

System Stability ◽

Measurement Device ◽

Measurement Device Independent

In most of the realistic measurement device-independent quantum key distribution (MDI-QKD) systems, efficient, real-time feedback controls are required to maintain system stability when facing disturbance from either external environment or imperfect internal components. Traditionally, people either use a “scanning-and-transmitting” program or insert an extra device to make a phase reference frame calibration for a stable high-visibility interference, resulting in higher system complexity and lower transmission efficiency. In this work, we build a machine learning-assisted MDI-QKD system, where a machine learning model—the long short-term memory (LSTM) network—is for the first time to apply onto the MDI-QKD system for reference frame calibrations. In this machine learning-assisted MDI-QKD system, one can predict out the phase drift between the two users in advance, and actively perform real-time phase compensations, dramatically increasing the key transmission efficiency. Furthermore, we carry out corresponding experimental demonstration over 100 km and 250 km commercial standard single-mode fibers, verifying the effectiveness of the approach.

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