quantum mechanics

Researchers take step toward creating quantum computers using entangled photons in optical fibers

Matt Daniels — Fri, 18 Apr 2008 09:04:00 -0700

Description

Researchers take step toward creating quantum computers using entangled photons in optical fibers

Prem Kumar, the AT&T Professor of Information Technology in the Department of Electrical Engineering and Computer Science and the director of the Center for Photonic Communication and Computing at Northwestern University's McCormick School of Engineering, and his research group recently demonstrated one of the basic building blocks for distributed quantum computing using entangled photons generated in optical fibers, and their research was published in the April 4 edition of Physical Review Letters.

From PhysOrg:

Researchers take step toward creating quantum computers using entangled photons in optical fibers

Kumar’s group, which uses photons as qubits, found that they can entangle two indistinguishable photons together in an optical fiber very efficiently by using the fiber’s inherent nonlinear response. They also found that no matter how far you separate the two photons in standard transmission fibers they remain entangled and are “mysteriously” connected to each other’s quantum state.

For this paper, Kumar and his team used the fiber-generated indistinguishable photons to implement the most basic quantum computer task – a controlled-NOT gate, which allows two photonic qubits to interact.

“This device that we demonstrated in the lab is a two-qubit device — nowhere near what’s needed for a quantum computer — so what can you do with it"” Kumar says. “It’s nice to demonstrate something useful to give a boost to the field, and there are some problems at hand that can be solved right now using what we have.”

Physics & Space Science

Source

http://www.physorg.com/news126883921.html

Increasing interest in Quantum Foundations

Sabine Hossenfelder — Fri, 21 Mar 2008 10:54:35 -0700

Description

The question how to unify Einstein's General Relativity with Quantum Theory is the 'holy grail' of today's theoretical physics. None of the pursued research programs have yet lead to any convincing outcomes. Current approaches focus on our difficulties with quantizing General Relativity.

An alternative solution would be that the problem why we are not making progress might be in that we don't understand quantization rather than not being able to figure out how to apply it. During the biggest part of the last century, quantum mechanics has been dealt with on the level of 'We don't understand it but it works, so let's use it', but this seems to become less and less satisfactory. The quantization of gravity (so far) does not work, but it would become important only in a regime where we have no evidence that our quantization procedure is appropriate.

This goes along with the stunning progress that has been made in the last decade in experimental possibilities on small quantum systems that allow us to closely examine quantum features like entanglement or the collapse of the wave-function. On the side of theoretical physics, the foundations of quantum theory as well as quantum information, are a way to attack longstanding problems and to open new points of view. But besides this, the questions about the foundations of quantum mechanics - like determinism, free will, or realism - is of a large public interest due to the philosophical implications. For both reasons the topic seems appealing and I therefore expect an increasing amount of research in the area of Foundations of Quantum Mechanics.

Source

A Framework for Practical Quantum Cryptography

Matt Daniels — Fri, 29 Feb 2008 07:55:51 -0800

NOTE: This content was aggregated from RSS feed. Original source is http://arxiv.org/abs/0802.4155

Quantum information storage is becoming feasible...

Quantum cryptography is the first quantum information task to reach the level
of mature technology, already fit for commercialization. It aims at the
creation of a secret key between authorized partners connected by a quantum
channel and a classical authenticated channel, whence the proper name of
Quantum Key Distribution (QKD). The security of the key can in principle be
guaranteed without putting any restriction on the eavesdropper's power.

The first two sections provide a concise up-to-date review of QKD, biased
toward the practical side. The rest of the paper presents the essential
theoretical tools and the main experimental platforms (discrete variables,
continuous variables and distributed-phase-reference protocols) in a synthetic
framework, which highlights similarities and differences and is open to include
future progress.

(arXiv:0802.4155v1 [quant-ph])

Physics & Space Science

A Quantum-Enhanced Prototype Gravitational-Wave Detector

Matt Daniels — Fri, 29 Feb 2008 07:55:51 -0800

NOTE: This content was aggregated from RSS feed. Original source is http://arxiv.org/abs/0802.4118

The quantum nature of the electromagnetic field imposes a fundamental limit
on the sensitivity of optical precision measurements such as spectroscopy,
microscopy, and interferometry. The so-called quantum limit is set by the
zero-point fluctuations of the electromagnetic field, which constrain the
precision with which optical signals can be measured. In the world of precision
measurement, laser-interferometric gravitational wave (GW) detectors are the
most sensitive position meters ever operated, capable of measuring distance
changes on the order of 10^-18 m RMS over kilometer separations caused by GWs
from astronomical sources. The sensitivity of currently operational and future
GW detectors is limited by quantum optical noise. Here we demonstrate a 44%
improvement in displacement sensitivity of a prototype GW detector with
suspended quasi-free mirrors at frequencies where the sensitivity is
shot-noise-limited, by injection of a squeezed state of light. This
demonstration is a critical step toward implementation of squeezing-enhancement
in large-scale GW detectors.

(arXiv:0802.4118v1 [quant-ph])

Physics & Space Science

Revisiting the security of quantum dialogue and bidirectional quantum secure direct communication. (arXiv:0801.2420v2 [quant-ph]

Matt Daniels — Sun, 17 Feb 2008 19:20:12 -0800

NOTE: This content was aggregated from RSS feed. Original source is http://arxiv.org/abs/0801.2420

From the perspective of information theory and cryptography, we analyze the
security of two quantum dialogue protocols and a bidirectional quantum secure
direct communication (QSDC) protocol, and point out that the transmitted
information would be partly leaked out in them. That is, any eavesdropper can
elicit some information about the secrets from the public annunciations of the
legal users. This phenomenon should have been strictly forbidden in a quantum
secure communication. In fact, this problem exists in quite a few recent
proposals and, therefore, it deserves more research attention in the following
related study.

Physics & Space Science

Teleportation using continuous variable quantum cloning machine. (arXiv:0802.2156v1)

Matt Daniels — Sun, 17 Feb 2008 19:20:11 -0800

NOTE: This content was aggregated from RSS feed. Original source is http://arxiv.org/abs/0802.2156

We show that an unknown quantum state in phase space can be teleported via
three-mode entanglement generated by continuous variable quantum cloning
machine (transformation). Further, proceeding with our teleportation protocol
we are able to improve the fidelity of teleportation obtained by Loock et.al.
[Phys.Rev.Lett. 84, 3482(2000)]. Also we study here the entanglement between
the two output copies from cloning machine.

Signals Round 3

Quantum information processing with single photons and atomic ensembles in microwave coplanar waveguide resonators

Matt Daniels — Sun, 17 Feb 2008 19:20:11 -0800

NOTE: This content was aggregated from RSS feed. Original source is http://arxiv.org/abs/0802.2154

We show that pairs of atoms optically excited to the Rydberg states can
strongly interact with each other via effective long-range dipole-dipole or van
der Waals interactions mediated by their non-resonant coupling to a common
microwave field mode of a superconducting coplanar waveguide cavity. These
cavity mediated interactions can be employed to generate single photons and to
realize in a scalable configuration a universal phase gate between pairs of
single photon pulses propagating or stored in atomic ensembles in the regime of
electromagnetically induced transparency.

Signals Round 3

A manifold of possible physics-laws in a universe where the planck constant and speed of light parameters vary. (arXiv:0802.2122

Matt Daniels — Sun, 17 Feb 2008 19:20:08 -0800

NOTE: This content was aggregated from RSS feed. Original source is http://arxiv.org/abs/0802.2122

I assume a universe whereby the speed of light and the planck constant are
not constants but instead parameters that vary locally in time-and space. When
describing motion, I am able to derive a modified path integral description at
the quantum level, which offers a natural extension of quantum mechanics. At
the microscopic level, this path integral intuitively describes a physics with
many quantum realities thus leading to a novel concept of manifold of physics,
which can be looked at as a novel action principle. This paradigm reflects the
notion that the observed laws of physics on any given scale are determined by
the underlying distribution of the fundamental parameters (i.e Quantum
Mechanics is just one point on this manifold), thus leading to many possible
physical-law based behaviors. By choosing a Gaussian distribution of the
parameters, a quadratic action term appears in the path-integral, which in
turns leads to a complex classical action (and by continuation a new
description for inertia) at the classical level. In the accompanying manuscript
the classical doublet equation of motion is applied to the Newtonian
gravitation field, and a MOND-like, dark-energy-like, and pioneer-anomaly-like
solutions are derived.

Signals Round 3

Quantum Computing Breakthroughs

Alex Soojung-Kim Pang — Tue, 23 Oct 2007 11:10:30 -0700

Description

Working prototypes of quantum computers may be demonstrated by 2040, making a whole new range of computationally intensive tasks possible.

Simon Bone and Matias Castro of the Imperial College in London offer this concise explanation of quantum computing in their work A Brief History of Quantum Computing:

'In the classical model of a computer, the most fundamental building block, the bit, can only exist in one of two distinct states, a 0 or a 1. In a quantum computer the rules are changed. Not only can a "quantum bit," usually referred to as a "qubit," exist in the classical 0 and 1 states, it can also be in a coherent superposition of both. When a qubit is in this state it can be thought of as existing in two universes, as a 0 in one universe and as a 1 in the other. An operation on such a qubit effectively acts on both values at the same time.'

Nobel physicist Richard Feynman and Charles Benett of IBM, among others, made significant early contributions to understanding the computational use of a quantum bit (qubit) from the physical properties of matter. Great progress is continuing worldwide at laboratories such as the Centre for Quantum Computation, a collaboration between Oxford and Cambridge Universities.

Implementation of quantum computing would make certain types of computation extremely fast -- potentially trillions of times faster than today -- and secure, using encryption techniques that are unbreakable because of the almost unimaginable number of instructions that can potentially be executed simultaneously. In quantum computing, a whole range of computationally intensive tasks that were previously impossible -- including image understanding, real-time speech recognition, generation of unbreakable codes, and extreme compression of data and media -- will become common."

This will be enabled by:

"Growing demand for data and media encryption because of mushrooming worldwide need for data privacy and protection of intellectual property
Growing research investment in nanotechnologies for many applications
Continued funding of research leading toward quantum computing devices by governments and universities in the UK, North America, Europe, Israel, and Asia, as well as commercial enterprises worldwide
Continuing research progress in mathematics, physics, nanoscale materials sciences, and computer software programming"

Early indicators include:

"Forecast by researchers at UC Berkeley that revenue-producing products will likely be available using carbon nanotubes in 2020 for quantum computation in 2040-2060
Recent major progress in developing quantum computing hardware by researchers from the US National Institute of Standards and Technology (NIST), along with colleagues from New Zealand and Germany"

What to watch:

One bit of information is encoded into a single atom (expected by 2017 if Moore's law continues to hold).
Enormously difficult theoretical breakthroughs are made.

Signals

Convergence on a Theory of Everything

Alex Soojung-Kim Pang — Tue, 23 Oct 2007 11:10:29 -0700

Description

Experimental physicists may finally converge on a single underlying theory that describes all the fundamental workings of the universe, from subatomic particles ruled by quantum mechanics to the gravitational forces so elegantly explained by Einstein's general theory of relativity.

Our universe is made up of building blocks much smaller than atoms. These particles, such as electrons, leptons, and quarks, are governed by three forces: electromagnetism and strong and weak nuclear forces. What about gravity, though? That's the question that Einstein asked for the last thirty years of his life, and physicists are still unable to answer it.

The goal is a Grand Unified Theory, a 'theory of everything' that ties together all of these phenomena in a single equation or expression that explains the nature and behaviour of all matter. Building such a theory, Einstein suggested, would be like 'reading the mind of God'. This theory of everything could illuminate some of the biggest mysteries at the heart of physics, from the origins of space and time to the secrets of black holes to the cause of the universe's accelerating expansion.

In the last two decades, many scientists have converged on a branch of physics called string theory as the most likely way to explain it all. The basic idea is that at the heart of every particle is a tiny vibrating string that's a millionth of a billionth of a billionth of a billionth of a centimeter long. Unlike the three-dimensional world that we perceive, the strings vibrate in ten dimensions. Every kind of particle and force corresponds to a particular vibrational pattern of a string. Michio Kaku, a physicist at City University of New York and cofounder of string field theory, explains it this way: 'Much as pulling on a rubber band changes its vibration frequency, altering a string's mode of vibration transforms an electron into a neutrino, a quark, or another particle. As they vibrate, they force space and time to curl around them, giving rise to gravity in exactly the manner that Einsitein described in his theory of relativity.'

The problem is that so far, there is no experimental proof that string theory is correct. However, massive efforts are now under way to develop technology and instruments that could aid these scientific detectives in their quest for the one true Grand Unified Theory."

This will be enabled by:

"Big Science' projects such as satellites and particle accelerators for experimental physics
Encouragement of interdisciplinary collaboration between science and engineering, and within those fields, such as collaboration within science among astrophysics, mathematics, and particle physics"

Early indicators include:

"Calculation by Harvard scientists in 1995 of the 'information' held in a black hole, thought to be a massive tangle of strings
Theoretical discovery that the topology of space is not smooth as general relativity predicts but actually can tear and repair itself
Massive university and institutional programs to support string theory research"

What to watch:

"US NASA and ESA launch the Laser Interferometer Space Antenna before 2010 to detect gravity waves that, if found, may have been caused by the vibration of strings shortly after the Big Bang.
The Large Hadron Collider particle accelerator comes online near Geneva, Switzerland, in two years, with the goal for string theorists being to discover unknown massive particles produced by strings vibrating at very high 'pitches'.
Laboratory gravity tests conducted by more than a dozen teams indicate gravity leaking from three dimensions into the higher dimensions predicted by string theory.
The multiuniversity Cyrogenic Dark Matter Search captures particles of dark matter, predicted by string theory, in deep underground mines shielded from the Earth's atmosphere. Other teams in Japan and Europe also find dark matter in their own dark matter hunts."