7 research outputs found
Scavenging quantum information: Multiple observations of quantum systems
Given an unknown state of a qudit that has already been measured optimally,
can one still extract any information about the original unknown state?
Clearly, after a maximally informative measurement, the state of the system
`collapses' into a post-measurement state from which the {\em{same}} observer
cannot obtain further information about the original state of the system.
However, the system still encodes a significant amount of information about the
original preparation for a second observer who is unaware of the actions of the
first one. We study how a series of independent observers can obtain, or
scavenge, information about the unknown state of a system (quantified by the
fidelity) when they sequentially measure it. We give closed-form expressions
for the estimation fidelity, when one or several qudits are available to carry
information about the single-qudit state, and study the `classical' limit when
an arbitrarily large number of observers can obtain (nearly) complete
information on the system. In addition to the case where all observers perform
most informative measurements we study the scenario where a finite number of
observers estimate the state with equal fidelity,regardless of their position
in the measurement sequence; and the scenario where all observers use identical
measurement apparata (up to a mutually unknown orientation) chosen so that a
particular observer's estimation fidelity is maximized.Comment: 17 pages, 1 figur
Recycling of quantum information: Multiple observations of quantum systems
Given a finite number of copies of an unknown qubit state that have already
been measured optimally, can one still extract any information about the
original unknown state? We give a positive answer to this question and quantify
the information obtainable by a given observer as a function of the number of
copies in the ensemble, and of the number of independent observers that, one
after the other, have independently measured the same ensemble of qubits before
him. The optimality of the protocol is proven and extensions to other states
and encodings are also studied. According to the general lore, the state after
a measurement has no information about the state before the measurement. Our
results manifestly show that this statement has to be taken with a grain of
salt, specially in situations where the quantum states encode confidential
information.Comment: 4 page
Elusive Diagnosis of Superficial Peroneal Nerve Entrapment Case report
Isolated neuropathy of the superficial peroneal nerve (SPN) is a relatively rare type of peripheral neuropathy. It is linked to the mechanical entrapment of the SPN in predisposed locations of its anatomical pathway. Associated clinical findings are insufficient lifting of the latero-dorsal part of the foot, stepping on the lateral border of the foot, and commonly, a strong pain localized in the nerve dermatome.Case report. We describe a case of a 14-year-old female patient with right leg pain lasting 24 months. Repeated neurological examinations with negative findings on electromyography (EMG) were performed. The patient underwent a Steindler surgery for a suspected diagnosis of a heel spur, without any improvement. Despite complex pharmacotherapy, chronic pain developed. The patient was unable to walk, being bound to a wheelchair. Amputation of her lower limb under the knee was also considered. SPN entrapment was diagnosed at a physical examination at EuroPainClinics. Decompression of the SPN under local anaesthesia was performed at the clinic.Results. The symptoms improved immediately after the procedure, and following 2 months of rehabilitation, the patient was completely symptom-free. Her clinical state remains unchanged until this day.Conclusions. SPN entrapment is not a common diagnosis in the group of pain syndromes. Regarding the lower limb, it is imperative to include it on the list of differential diagnoses in cases of pain and functional disorders of the lateral muscle groups of the calf and leg. In the case of SPN entrapment, EMG findings may be negative
Testing sequential quantum measurements: how can maximal knowledge be extracted?
The extraction of information from a quantum system unavoidably implies a
modification of the measured system itself. It has been demonstrated recently
that partial measurements can be carried out in order to extract only a portion
of the information encoded in a quantum system, at the cost of inducing a
limited amount of disturbance. Here we analyze experimentally the dynamics of
sequential partial measurements carried out on a quantum system, focusing on
the trade-off between the maximal information extractable and the disturbance.
In particular we consider two different regimes of measurement, demonstrating
that, by exploiting an adaptive strategy, an optimal trade-off between the two
quantities can be found, as observed in a single measurement process. Such
experimental result, achieved for two sequential measurements, can be extended
to N measurement processes.Comment: 5 pages, 3 figure