arXiv subjects

J. Oppenheim

Publications and source records attributed to J. Oppenheim.

9 recordsLinked to original sources

arXiv2004

The Universal Composable Security of Quantum Key Distribution

The existing unconditional security definitions of quantum key distribution (QKD) do not apply to joint attacks over QKD and the subsequent use of the resulting key. In this paper, we close this potential security gap by using a universal composability theorem for the quantum setting. We first derive a composable security definition for QKD. We then prove that the usual security definition of QKD still implies the composable security definition. Thus, a key produced in any QKD protocol that is unconditionally secure in the usual definition can indeed be safely used, a property of QKD that is hitherto unproven. We propose two other useful sufficient conditions for composability. As a simple application of our result, we show that keys generated by repeated runs of QKD degrade slowly.

quant-ph↗

arXiv2003

A probabilistic and information theoretic interpretation of quantum evolutions

In quantum mechanics, outcomes of measurements on a state have a probabilistic interpretation while the evolution of the state is treated deterministically. Here we show that one can also treat the evolution as being probabilistic in nature and one can measure `which unitary' happened. Likewise, one can give an information-theoretic interpretation to evolutions by defining the entropy of a completely positive map. This entropy gives the rate at which the informational content of the evolution can be compressed. One cannot compress this information and still have the evolution act on an unknown state, but we demonstrate a general scheme to do so probabilistically. This allows one to generalize super-dense coding to the sending of quantum information. One can also define the ``interaction-entanglement'' of a unitary, and concentrate this entanglement.

quant-ph↗

arXiv2002

Temporal Ordering in Quantum Mechanics

We examine the measurability of the temporal ordering of two events, as well as event coincidences. In classical mechanics, a measurement of the order-of-arrival of two particles is shown to be equivalent to a measurement involving only one particle (in higher dimensions). In quantum mechanics, we find that diffraction effects introduce a minimum inaccuracy to which the temporal order-of-arrival can be determined unambiguously. The minimum inaccuracy of the measurement is given by dt=1/E where E is the total kinetic energy of the two particles. Similar restrictions apply to the case of coincidence measurements. We show that these limitations are much weaker than limitations on measuring the time-of-arrival of a particle to a fixed location.

quant-ph↗

arXiv2001

A Thermodynamic Sector of Quantum Gravity

The connection between gravity and thermodynamics is explored. Examining a perfect fluid in gravitational equilibrium we find that the entropy is extremal only if Einstein's equations are satisfied. Conversely, one can derive part of Einstein's equations from ordinary thermodynamical considerations. This allows the theory of this system to be recast in such a way that a sector of general relativity is purely thermodynamical and should not be quantized.

gr-qc↗

arXiv1998

Time as an Observable

The role of time in quantum mechanics is discussed. The differences between ordinary observables and an observable which corresponds to the time of an event is examined. In particular, the time-of-arrival of a particle to a fixed location is not an ordinary quantum mechanical observable. While we can measure if the particle arrives, we argue that the time at which it arrives always has an inherent ambiguity. The minimum inaccuracy of time-of-arrival measurements is given by dt>1/E where E is the kinetic energy of the particle. The use of time-of-arrival operators, as well as current operators, is examined critically.

quant-ph↗

arXiv1998

Time-of-Arrival States

Although one can show formally that a time-of-arrival operator cannot exist, one can modify the low momentum behaviour of the operator slightly so that it is self-adjoint. We show that such a modification results in the difficulty that the eigenstates are drastically altered. In an eigenstate of the modified time-of-arrival operator, the particle, at the predicted time-of-arrival, is found far away from the point of arrival with probability 1/2.

quant-ph↗

arXiv1998

When does a Measurement or Event Occur

Within quantum mechanics it is possible to assign a probability to the chance that a measurement has been made at a specific time t. However, the interpretation of such a probability is far from clear. We argue that a recent measuring scheme of Rovelli's (quant-ph/9802020) yields probabilities which do not correspond to the conventional probabilities usually assigned in quantum mechanics. The same arguments also apply to attempts to use the probability current to measure the time at which a particle arrives at a given location.

quant-ph↗

arXiv1998

Minimum Inaccuracy for Traversal-Time

Using various model clocks it has been shown that the time-of-arrival cannot be measured more accurately than 1/E where E is the kinetic energy of a free particle. However, this result has never been proved. In this paper, we show that a violation of the above limitation for the transit-time, implies a violation of the Heisenberg uncertainty relation.

quant-ph↗

arXiv1998

Measurement of Time-of-Arrival in Quantum Mechanics

It is argued that the time-of-arrival cannot be precisely defined and measured in quantum mechanics. By constructing explicit toy models of a measurement, we show that for a free particle it cannot be measured more accurately then $Δt_A \sim 1/E_k$, where $E_k$ is the initial kinetic energy of the particle. With a better accuracy, particles reflect off the measuring device, and the resulting probability distribution becomes distorted. It is shown that a time-of-arrival operator cannot exist, and that approximate time-of-arrival operators do not correspond to the measurements considered here.

quant-ph↗