A quantum clock must satisfy two basic constraints. The first is a bound on the time resolution of the clock given by the difference between its maximum and minimum energy eigenvalues. The second follows from Holevo’s bound on how much classical information can be encoded in a quantum system. We show that asymptotically, as the dimension of the Hilbert space of the clock tends to infinity, both constraints can be satisfied simultaneously. The experimental realization of such an optimal quantum clock using trapped ions is discussed.
We provide a brief discussion regarding relativistic limits on the discretization and temporal resolution of time values in a quantum clock. Our clock is characterized by a time observable chosen to be the complement of a bounded and discrete Hamiltonian that can have an equally spaced or a generic spectrum. In the first case, the time observable can be described by a Hermitian operator, and we find a limit in the discretization for the time eigenvalues. Nevertheless, in both cases, the time observable can be described by a POVM, and, by increasing the number of time states, we show how the bound on the minimum time quantum can be reduced and identify the conditions under which the clock values can be treated as continuous. Finally, we find a limit for the temporal resolution of our time observable when the clock is used (together with light signals) in a relativistic framework for the measurement of spacetime distances.
A clock is, from an information-theoretic perspective, a system that emits information about time.One may therefore ask whether the theory of information imposes any constraints on the maximum precision of clocks.Here we show a quantum-over-classical advantage for clocks or, more precisely, the task of generating information about what time it is.The argument is based on information-theoretic considerations: we analyze how the accuracy of a clock scales with its size, measured in terms of the number of bits that could be stored in it.We find that a quantum clock can achieve a quadratically improved accuracy compared to a purely classical one of the same size.
Abstract At the intersection of quantum theory and relativity lies the possibility of a clock experiencing a superposition of proper times. We consider quantum clocks constructed from the internal degrees of relativistic particles that move through curved spacetime. The probability that one clock reads a given proper time conditioned on another clock reading a different proper time is derived. From this conditional probability distribution, it is shown that when the center-of-mass of these clocks move in localized momentum wave packets they observe classical time dilation. We then illustrate a quantum correction to the time dilation observed by a clock moving in a superposition of localized momentum wave packets that has the potential to be observed in experiment. The Helstrom-Holevo lower bound is used to derive a proper time-energy/mass uncertainty relation.
This note clarifies a fundamental but often-misunderstood limitation of quantum computation: regardless of the exponentially large Hilbert space of an
N-qubit register, the amount of classical information that can be extracted from a single run is bounded by the quantum-to-classical measurement interface. Using the accessible information framework and the Holevo bound, we show that for a system of dimension
d (in particular
d=2N), the accessible classical information per run satisfies
Iacc≤logd, i.e. at most
N bits for
N qubits (log base 2). We emphasize that quantum advantage is therefore not about “reading out” exponentially many classical bits, but about how interference structures the distribution of
O(N) measured bits so that they encode useful answers. Finally, we discuss why temporal/phase coordination becomes an architectural requirement in *distributed* quantum computing and quantum networks: without a shared phase/time reference, operations are symmetry-restricted and measurement events across nodes may fail to compose reliably. This motivates cycle-anchored phase-window coordination and reference-frame/asymmetry viewpoints as practical foundations for scalable distributed systems. Keywordsquantum measurement, Holevo bound, accessible information, readout bottleneck, distributed quantum computing, quantum networks, phase coordination,
U(1) asymmetry.
Quantum systems promise to revolutionize information processing science and technology [1-3]. The preservation of quantum coherence, the defining property of qubits, fundamentally constrains the performance of quantum information processing with quantum memories [4]. While trapped atomic ions theoretically support million-year coherence based on spontaneous emission [5-7], experimental demonstrations have reached far less, only about an hour [8-13]. Here we combine clock-state qubits with decoherence-free subspace (DFS) encoding to achieve coherence exceeding ten hours. Using correlation-based phase tracking in 171Yb+ ion pairs sympathetically cooled by 138Ba+ ion, we demonstrate this without magnetic shielding or enhanced microwave phase stabilization that previously limited coherence times. DFS encoding references the qubit phase to the inter-ion energy difference to reject microwave phase noise and common-mode magnetic fluctuations, while clock states provide environmental insensitivity. Throughout measurements extended to 1600 seconds, we observe minimal coherence decay, with exponential fits yielding a coherence time of (3.77 +/- 1.09) x 10^4 seconds. Our results establish DFS encoding as a form of passive error correction that eliminates technical noise constraints, unlocking the million-year coherence potential of atomic ions for scalable quantum information processing.