18 research outputs found
Distrustful quantum steering
Quantum steering is an asymmetric form of quantum nonlocality where one can
trust the measurements of one of the parties. In this work, inspired by
practical considerations we investigate the scenario if one can not fully trust
their measurement devices but only up to some precision. We first find the
effect of such an imprecision on standard device-dependent quantum tomography.
We then utilise this result to compute the variation in the local bound of any
general steering inequality depending on the amount of trust one puts in one of
the party's measurement devices. This is particularly important as we show that
even a small distrust on Alice might cause the parties to observe steerability
even if the quantum state is unsteerable. Furthermore, this effect becomes more
relevant when observing higher dimensional quantum steering.Comment: 6 pages, 2 Figures. Close to published versio
Certification of randomness without seed randomness
The security of any cryptographic scheme relies on access to random number
generators. Device-independently certified random number generators provide
maximum security as one can discard the presence of an intruder by considering
only the statistics generated by these devices. Any of the known
device-independent schemes to certify randomness require an initial feed of
randomness into the devices, which can be called seed randomness. In this work,
we propose a one-sided device-independent scheme to certify two bits of
randomness without the initial seed randomness. For our purpose, we utilise the
framework of quantum networks with no inputs and two independent sources shared
among two parties with one of them being trusted. Along with it, we also
certify the maximally entangled state and the Bell basis measurement with the
untrusted party which is then used to certify the randomness generated from the
untrusted device.Comment: 7 pages, 1 Figur
Quantum steering without free will
Quantum networks with independent sources allow the observation of quantum
nonlocality without inputs. Consequently, the incompatibility of measurements
is not a necessity for observing quantum nonlocality when one has access to
independent sources. Here we investigate the minimal scenario without inputs
where one can observe any form of quantum nonlocality. We show that even two
parties with two sources that might be classically correlated can witness a
form of quantum nonlocality, in particular quantum steering, in networks
without inputs if one of the parties is trusted, that is, performs a fixed
known measurement. We term this effect as swap-steering. The scenario presented
in this work is minimal to observe such an effect. Consequently, a scenario
exists where one can observe quantum steering but not Bell non-locality. We
further construct a linear witness to observe swap-steering.Comment: 5 pages, 3 figure
Shared randomness allows violation of macroscopic realism using a single measurement
Macro-realistic description of systems is based majorly on two basic
intuitions about the classical world, namely, macrorealism per se, that is, the
system is always in a distinct state, and non-invasive measurements, that is,
measurements do not disturb the system. Given the assumption of no-signalling
in time, one utilizes Leggett-Garg inequalities to observe a violation of
macroscopic realism which requires at least three measurements. In this work,
we show that if one has access to shared randomness then one can observe a
violation of macroscopic realism using a single measurement even if no
signalling in time is satisfied. Interestingly, using the proposed scheme one
can also rule out a larger class of models, which we term "macroscopic
no-signalling" theories which can not violate the no-signalling in time
conditions. We further construct a witness to observe the violation of
macroscopic no-signalling.Comment: 4 pages, 2 Figures. Comments are welcome:
Certification of the maximally entangled state using non-projective measurements
In recent times, device-independent certification of quantum states has been
one of the intensively studied areas in quantum information. However, all such
schemes utilise projective measurements which are practically difficult to
generate. In this work, we consider the one-sided device-independent (1SDI)
scenario, and propose a self-testing scheme for the two-qubit maximally
entangled state using non-projective measurements, in particular, three
three-outcome extremal POVM's. We also analyse the robustness of our scheme
against white noise.Comment: 6 pages, 1 figure, comments are welcome