Quantum Technology Based Gravitational Wave Detectors
Igor Pikovski, ITAMP, Harvard-Smithsonian Center for Astrophysics
The recent detection of gravitational waves has paved the way for gravitational wave astronomy. Here we propose and discuss two novel schemes for gravitational wave detection. In the first part, we discuss the prospects of using optical lattice clocks for a space-based detector. We show that using quantum control techniques, optical clocks can be tuned to be sensitive to a wide range of sub-Hz gravitational wave frequencies. We compare such a clock detector to other existing and proposed techniques, showing that it can be a complementary method for space-based gravitational wave detection. In the second part, we discuss the potential of using superfluid optomechanical systems as tunable, ground-based gravitational wave sensors at high frequencies.
Experimental Tests of Gravitational Decoherence
Nathan McMahon, University of Queensland
Quantum mechanics and general relativity are the two most major achievements in physics, both tested to high levels of accuracy. However as the theory currently stands these two ideas are fundamentally incompatible. While understanding the full theory is a significant project, by taking the Newtonian limit we can begin to test gravitational interactions within the quantum regime.
Recently an approach to this problem has been proposed by Kefri, Taylor and Milburn[3] where gravitational interactions can only carry classical information, and therefore are described by classical channels rather than quantum ones. Results from this model indicate that there can be no entanglement generated by purely gravitational interactions [4], and if shown to be true certain classes of quantum technologies and protocols may turn out to be infeasible, such as in the recent proposal by Johnsson, Brennen and Twamley [5] for the measurement of the gravitational field using superconducting qubits.
This model is just one in a class of such models where the gravitational interactions fundamentally cause spontaneous collapse, such as the Penrose and Diosi models [1, 2], and leads to significant testable differences distinguishing it from the original quantum theory. Considering optomechanical systems, we find that gravitational decoherence appears as a pure temperature shift in the bath modes. Therefore any prior signals of the decoherence would simply appear as a external source of heating in any prior experiments not testing for gravitational decoherence.
Our work has considered methods to distinguish the signature of gravitational decoherence from other sources of heating and the viability of such methods. In particular we will discuss a proposal for an experimental test for gravitational decoherence, which appears to be feasible with current technologies. Tests for the gravitational decoherence predicted by the model would be fascinating beyond furthering knowledge of possibilities of technologies and protocols and, if gravitational decoherence is shown to be present, would raise questions which may provide new directions for quantum foundations and relativistic quantum information.
References
- [1] On gravity’s role in quantum state reduction, Roger Penrose, General relativity and Gravitation 28(5):581- 600, (1996).
- [2] The gravity-related decoherence master equation from hybrid mechanics, Lajos Diosi, J.Phys.Conf.Ser. 306 012006 -(9), (2011).
- [3] A classical channel model for gravitational decoherence, D. Kafri, J.M. Taylor and G. J. Milburn, New J. Physics 16, 065020, (2014).
- [4] Bounds on quantum communication via Newtonian gravity, D. Kafri, G. J. Milburn, J. M. Taylor, New J. Physics 17, 015006, (2015).
- [5] Macroscopic superpositions and gravimetry with quantum magnetomechanics, MT Johnsson, GK Brennen, J Twamley, ar iv:1412.6864, (2014).
Quantum Interference Along Satellite-Ground Channels
Francesco Vedovate, Department of Information Engineering, University of Padova
One of the main challenges in Quantum Mechanics is establishing if fundamental bounds to quantum interference exist: for instance, can it be measured by observers in relative motion and at arbitrary large distance? In order to answer such kind of questions, it is necessary to observe quantum interference in unexplored conditions. We experimentally demonstrated interference at single-photon level with visibility up to 67% along three different satellite-ground channels with path length up to 5000 km. We exploited a superposition of two photon temporal modes reflected by a rapidly moving Low Earth Orbit satellite thousand kilometers away and observed the single-photon interference at a ground station (MLRO Observatory of Italian Space Agency - Matera - Italy). The relative velocity of the satellite respect to the ground introduces a varying modulation in the interference pattern which can be predicted by special relativistic calculation. These results attest the viability of the use of photon temporal modes for tests of quantum foundations in gravitational fields in the context of quantum optics experiments in Space, as recently proposed in the optical version of the Colella-Overhauser-Werner experiment which turns up to be feasible with current technology.
Symmetric Couplings Limit Relativistic Quantum Communication
Robert Jonsson, University of Waterloo
The transmission of quantum information via relativistic quantum fields is of central interest in relativistic quantum information. In this talk we discuss how symmetric couplings, e.g. isotropic couplings, between signaling device and field put severe limitations on the possibility to transmit quantum information in a wide range of scenarios. For example, one-directed communication scenarios, i.e., where the receiver cannot signal back to the sender, result in zero quantum capacity.
This observation highlights fundamental limits of relativistic quantum communication, and may lead to implementations of relativistic bit commitment using relativistic quantum fields.
One case of one-directed communication is given by the recently discovered timelike and energyless signals in 1+1 dimensional spacetimes. In 1+1 dimensional spacetimes the propagation of information can decouple from the flow of energy in a massless scalar field: Whereas energy propagates strictly lightlike, the field's amplitude carries signals arbitrary far into the sender's future lightcone. In this talk, we review how this phenomenon arises from the properties of left-moving and right-moving field modes, independent of any particular type of signalling device.
The possibility to reach a large number of receivers in a sender's future lightcone appears to run afoul of the No-Cloning Theorem, and thus raises the question of how much quantum information timelike and energyless signals can carry. To address this question, we study signalling between harmonic oscillators coupling to a scalar massless field inside a cavity. This model allows us to distinguish classical and quantum features of the signals, and to treat them non-perturbatively.
Biexciton Scattering by an Impurity with Internal Entanglement
Fumika Suzuki, University of British Columbia
The study of achieving superposition of a macroscopic object/composite system is of particular interest for the investigation of the quantum-to-classical transition and decoherence mechanisms these days. To create superposition of a composite system, it is often convenient to split its wave function into two or more components by equipment. In [1], they studied a composite system of two particles tied to each other by a binding potential which is scattered by a mirror so that its wave function is separated into transmitted and reflected parts. It was observed that even if it is not immersed in a heat bath, internal degrees of freedom of the system itself carries which-way information and causes loss of coherence.
On the other hand, there is increasing interest in experiments with ultracold atoms and molecules trapped in optical lattices in condensed matter physics. Rotational excitation of polar molecules trapped in an optical lattice induces rotational excitons, and it was showed that tunable non-linear interactions between those excitons can form Frenkel biexciton. A biexciton, unlike a single exciton, possesses its internal structure which can be entangled due to its interaction with an impurity. Here, we study the effect of this internal entanglement on the behaviour of wave function of biexciton scattering by an impurity in a one-dimensional optical lattice. The study can give insight into superposition experiments with a composite system.
[1] F. Queisser and W. G. Unruh, arXiv:1503.08814 (2015).
Effect of Relativistic Motion On Witnessing Non-Classicality of Quantum States
Agata Checinska, Institute of Theoretical Physics, University of Warsaw
We show that the operational definition of non-classicality of a quantum state depends on the motion of the observer. We use the relativistic Unruh-DeWitt detector model to witness non-classicality of the probed field state. It turns out that the witness based on the properties of the P-representation of the quantum state depends on the trajectory of the detector. Inertial and non-inertial motion of the device have qualitatively different impact on the performance of the witness.
Spontaneous Emission from an Analogue Event Horizon in a Dispersive Dielectric
Maxime Jacquet, University of St Andrews
Quantum fluctuations at the horizon of black holes causes their evaporation via thermal emission of bosons and fermions (1): black holes are not black but emit Hawking radiation. Sadly this effect is too feeble to ever be detected. Fortunately, laboratory-based systems can recreate the physics of waves in Lorentzian geometries (2), allowing experimental tests of Hawking’s theory.
In particular, light in dispersive media can be made to interact with itself to create an optical analogue to the event horizon, from which spontaneous emission of light from the vacuum can be observed (3). To date, analogue Hawking radiation has however remained elusive, mostly because the extreme conditions at the horizon render the analytical prediction the exact parameters of the radiation for an actual experiment difficult to make.
We reveal the properties of spontaneous emission from the vacuum at a moving refractive index step in a dispersive dielectric by expanding on an analytical model for light-matter interaction (4). We establish the conditions for event horizons as a function of the speed and height of the step in the medium, and study the various configurations of modes of the field in the vicinity of the step with and without analogue horizons. We then analytically calculate the emission spectra from all modes of positive and negative frequency in the laboratory frame (5). We find that, as a result of the various mode configurations, the spectrum is highly structured into intervals with black hole-, white hole-, and no horizon.
The emission spectrum in the laboratory frame is found to be a combination of emissions corresponding to different frequencies in the frame moving with the pulse, leading to a characteristic shape. In particular, the existence of a peak in the ultraviolet, associated with emission into a mode with negative frequency, is an interesting feature of our spectrum. We also calculate the scaling of the total flux and the flux spectral density with step height. Future work will look into quantum correlations between the negative frequency modes and positive frequency modes, the ultimate observable in optical experiments aiming at verifying Hawking’s theory.
1. S. Hawking. 1974, Nature
2. Unruh, W.G. 1981, Phys. Rev. Lett
3. T. Philbin, et al. 2008, Science
4. S. Finazzi and I. Carusotto. 2013, Phys. Rev. A 5. M. Jacquet and F. Koenig. 2015, Phys. Rev. A
GUP Parameter from Quantum Corrections to the Newtonian Potential
Fabio Scardigli, AUM Kuwait & Politecnico Milano
We propose a technique to compute the deformation parameter of the generalized uncertainty principle (GUP) by using the leading quantum corrections to the Newtonian potential. The calculation gives, to this order, an unambiguous numerical result. The physical meaning of this value is discussed, and compared with analogous previous results, and with known bounds on the GUP deformation parameter.
Biorthogonal Quantum Mechanics in Spacetime
Margaret Hawton, Lakehead University
The commutation relations satisfied by the Klein Gordon field operator and its canonically conjugate momentum imply the relativistic biorthogonality relation h (x) j (y)i = (~=2) (x y) [1]. This is a special case of the Klein Gordon scalar product used to normalize modes in Birrell and Davies [2]. According to the mathematical formalism of biorthogonal quantum mechanics [3, 4] this implies a configuration space completeness relation and a positive definite probability density for a transition to a position eigenket. In addition, biorthogonality leads to the remarkable conclusion that a Glauber detector measures number density, not energy density. By replacing the scalar field with the electromagnetic vector potential the biorthogonal formalism can be extended to photon beams with spin and orbital angular momentum [5].
[1] M. Hawton and V. Debierre, arXiv:151206067.
[2] N. D. Birrell and P. C. W. Davies, Quantum Fields in Curved Space (Cambridge, 1984). [3] A. Mostafazadeh, Int. J. Mod. Phys. A21, 2553 (2006).
[4] D. C. Brody, J. Math. Phys. A: Math. Theor. 47, 035305 (2014).
[5] M. Hawton and W. E. Baylis, Phys. Rev. A, 71, 033816 (2005).