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Advanced quantum communication systems
Abstract
Quantum communication provides several examples of communication protocols which cannot be implemented securely using only classical communication. Currently, the most widely known of these is quantum cryptography, which allows secure key exchange between parties sharing a quantum channel subject to an eavesdropper. This thesis explores and extends the realm of quantum communication. Two new quantum communication protocols are described. The first is a new form of quantum cryptography---relativistic quantum cryptography---which increases communication efficiency by exploiting a relativistic bound on the power of an eavesdropper, in addition to the usual quantum mechanical restrictions intrinsic to quantum cryptography. By doing so, we have observed over 170% improvement in communication efficiency over a similar protocol not utilizing relativity. A second protocol, Quantum Orienteering, allows two cooperating parties to communicate a specific direction in space. This application shows the possibility of using joint measurements, or projections onto an entangled state, in order to extract the maximum useful information from quantum bits. For two-qubit communication, the maximal fidelity of communication using only separable operations is 73.6%, while joint measurements can improve the efficiency to 78.9%. In addition to implementing these protocols, we have improved several resources for quantum communication and quantum computing. Specifically, we have developed improved sources of polarization-entangled photons, a low-loss quantum memory for polarization qubits, and a quantum random number generator. These tools may be applied to a wide variety of future quantum and classical information systems.
... Quantum cryptography systems have been demonstrated operating at speeds of up to 1.25 GHz. True random number generators are not available that operate at this speed, so these systems must use pseudo-random number generators [3]. ...
For many years dissipative quantum maps were widely used as informative models of
quantum chaos. In this paper, a new scheme for generating good pseudo-random numbers
(PRNG), based on quantum logistic map is proposed. Note that the PRNG merely relies on
the equations used in the quantum chaotic map. The algorithm is not complex, which does
not impose high requirement on computer hardware and thus computation speed is fast. In
order to face the challenge of using the proposed PRNG in quantum cryptography and other
practical applications, the proposed PRNG is subjected to statistical tests using well-known
test suites such as NIST, DIEHARD, ENT and TestU01. The results of the statistical tests were
promising, as the proposed PRNG successfully passed all these tests. Moreover, the degree
of non-periodicity of the chaotic sequences of the quantum map is investigated through
the Scale index technique. The obtained result shows that, the sequence is more
non-periodic. From these results it can be concluded that, the new scheme can generate
a high percentage of usable pseudo-random numbers for simulation and other applications
in scientific computing.
... A reasonable assumption, and one by which our QRNG operates, is that by providing 10 "extra" bits of entropy (i.e., 266 random bits input), the hash function will be sufficiently saturated. It can be shown [21] that 266 random input bits corresponds to an output entropy of 255.9999, or a Shannon entropy per bit of 0.999996. The corresponding output min-entropy is 255.28 bits, or 0.9972 random bits per bit. ...
A quantum random number generator (QRNG) is one which re- lies on a physical process, extracting randomness from the inherent uncertainty in quantum mechanics. This is to be contrasted with current pseudo-random number generators (PRNG), which although useful, are in fact deterministic: they always yield the same output sequence given the same input seed. This is unacceptable for some applications, such as quantum cryptography, which promise uncon- ditional security. Unfortunately, the rate of QRNGs is still too slow for practical commercial quantum key distribution systems (which presently run at speeds over 1 GHz). Previous QRNGs have been implemented, with the most common relying on the behavior of a photon at a beam-splitter, producing a random bit dependent on which of the two paths in which the photon is detected. However, these are totally limited by detector saturation rates, typically in the low MHz range. We previously proposed that by instead using the time interval between detections, much more random information could be extracted per detection event. Specifically, instead of only one bit per detection, in principle one could extract as many bits as the measurement time resolution would allow. Over the past two years, we have been exploring this approach and have constructed a functional QRNG operating at rates up to 130 Mbit/s. The random output has been tested and has passed all common cryptographic random number tests.
We describe and compare two proposals for creating macroscopic superpositions using single-photon optomechanical systems. The realization of the proposed experiments poses major technological challenges, which we examine. Reaching the quantum ground state is essential for both schemes. We present experimental results on optical cooling, which provides a way to reach the quantum ground state for low frequency optomechanical resonators.
We present a scheme for achieving macroscopic quantum superpositions in optomechanical systems by using single photon postselection and detecting them with nested interferometers. This method relieves many of the challenges associated with previous optical schemes for measuring macroscopic superpositions and only requires the devices to be in the weak coupling regime. It requires only small improvements on currently achievable device parameters and allows the observation of decoherence on a time scale unconstrained by the system's optical decay time. Prospects for observing novel decoherence mechanisms are discussed.
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