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